How Tube Guitar Amplifiers Work
By Rob Robinette
Have comments, corrections or suggestions? Send them to robinette at comcast dot net.
This webpage is available in .pdf form: How_Tube_Guitar_Amps_Work_by_Rob_Robinette.pdf
Have you ever looked at the guts of a guitar amplifier and wondered what all those parts do? Well, I'll walk you through the signal flow and discuss the components in this very simple but great sounding 1950's Fender 5F1 Champ guitar amplifier. Once you understand the simple 5F1 you'll be able to understand more complicated amps. 5F1 was Fender's internal model code for the 1950's tweed Champ.
5F1 Champ Amplifier
Control on top, Circuit Board inside, tubes on bottom: V1 Preamp Tube on right, V2 Power Tube in center, V3 Rectifier Tube on left. The Power Transformer and Output Transformer are attached to the other side of the chassis.
We'll start with the amplifier layout diagram. If things get too cluttered you can refer back up to this clean diagram. The guitar input jacks are at the upper right, the circuit board is in the center, the power transformer (PT) is on the left and the tubes and speaker jack are at the bottom. The output transformer (OT) is not shown.
5F1 Champ Guitar Amplifier Layout Diagram
I added component numbers to this layout that match the schematic diagram below. Compare this to the picture of the chassis above. Click the image to see the full size or download the pdf here.
Annotated Layout With Signal Flow and Component Function
Tracing the signal path on this layout diagram and the schematic below will help you understand how this amp works.
Signal Flow Overview
Signal flow is shown above and below using orange arrows. Signal from the guitar enters at upper right guitar Input Jack 1 or 2 and flows down to the circuit board and then to the preamp tube V1A at bottom right where the signal goes through its first stage of amplification. The signal then goes up to the circuit board and on to the volume control at top center, then back down to tube V1B (the second half of the first tube) for its second stage of amplification. From there the audio signal goes back to the circuit board then down to the power tube V2 for the third stage of amplification. V2's output goes out the blue wire to the output transformer (not shown) for a current boost, then from the output transformer via the green wire to the speaker jack (to the right of V2) and on to the speaker. I have added component numbers to the layout diagram above that match the schematic below.
Annotated Weber 5F1 Schematic
The signal flow seems much simpler on the amp's schematic. Component numbers match the Layout diagram above. V1A is one half of tube V1, V1B is the other half. Voltages shown are approximate.
Common amp electronic component symbols.
Table of Contents
The next few paragraphs will help you visualize the flow of electrons through simple circuits. Learning to visualize the flow of individual electrons was a breakthrough for me in understating tube amplifier electronics.
Electric guitars generate an alternating current (AC) audio signal. The guitar's pickups are small electric generators. Pickups have magnets (poles) that magnetize the metal guitar strings. The movement of the magnetic field surrounding the magnetized strings generates electricity in the pickup's coil. The coil is simply a thin insulated wire wrapped around a spool and when a magnetic field cuts through a coil of wire it generates an electric voltage (electronic pressure) and current (electron flow) in the coil's wire.
The black and white wires leaving the pickup are the two ends of one long coil wire. A humbucker pickup is simply two of these coils connected end-to-end (in series).
Standard Guitar Circuit
The Pickup on the left is an inductive coil that generates the guitar signal. The Tone Control is a variable RC (resistance capacitance) low-pass filter which bleeds high frequencies to ground. The Volume Pot is wired as a potentiometer (variable voltage divider). The Volume Pot bleeds guitar signal to ground to lower the guitar's output volume.
I've been asked many times, "What is voltage?" It's pretty simple really. Like electrical charges repel the same way like magnetic poles repel. So if you cram a bunch of negatively charged electrons together onto the metal plates of a battery their negative charges repel each other--they want elbow room. The tighter they are packed together the higher the voltage. If you connect a wire across the battery terminals the jammed together electrons see the wire as a pipe with lots of room so they flow down the wire. When you have an 'excess' of electrons you have negative voltage. When you have a 'scarcity' of electrons you have a positive voltage.
A quick note about 'Conventional Current Flow.' In electric circuits negatively charged electrons actually flow from the negative '-' battery terminal to the positive '+' terminal. That's right, the electricity in your car flows from the battery's - terminal through the ground wire, through the car's body, through the radio's ground wire to the radio and then through the positive power wire back to the battery. The problem is that Benjamin Franklin guessed wrong on the direction of electrical flow so conventionally we think of electricity as flowing from + to -. With tubes you need to actually consider how the electrons are really moving in order to understand them.
When I think about a wire with very high positive voltage on it I imagine the wire as an empty pipe with very few electrons in it with lots of 'elbow room' so electrons really want to flow into that wire. Touch that wire to a ground and the electrons hanging out there will rush in to fill the void of electrons on the high voltage wire. Voltage is the force of electrons wanting to move from a conductor crowded with electrons to a conductor with fewer electrons and more elbow room. Current is the measure of how many electrons are flowing through a conductor--the more electrons flowing, the higher the current.
Keep this in mind when thinking about amp circuits, high voltage is an extreme scarcity of electrons and ground represents an unlimited supply of electrons. An AC guitar audio signal on a wire alternates between negative and positive voltage. A negative signal voltage packs electrons closer together (excess of electrons = negative voltage). The positive half of the AC guitar signal pulls electrons apart and creates a scarcity of electrons. Voltage = electron pressure.
As a guitar string vibrates it moves one way and generates a positive voltage in the pickup coil, then as the string reverses direction the voltage is reversed and a negative voltage is generated. This occurs with every vibration of the string so an alternating current (positive-negative-positive-negative. . .) makes up the audio signal put out by the guitar. I'll repeat that because it's a very important concept, as the guitar string moves one direction over the guitar pickup coil it generates a negative voltage (excess electrons), then as the string reverses direction the voltage reverses too and a positive voltage is generated (scarcity of electrons) and this repeats with every vibration of the string creating an Alternating Current (AC) electrical signal. This tiny little AC signal is what the guitar amp will amplify until it's strong enough to move a speaker cone in and out. The speaker cone alternates in and out with the alternating current from the guitar's pickup coil. For every guitar string movement there is a corresponding speaker cone movement.
If you graph a guitar audio signal the pitch of the guitar string's sound is expressed as wave spacing (frequency) and loudness is expressed as wave height (amplitude). A high frequency sound will have tight wave spacing and a low frequency sound will have wide wave spacing. In the graph below the high E string is on the left and the low E is on the right. A quiet sound will have short waves and a loud sound will have tall waves.
The direct relationship between string movement and electricity generated in the pickup coil is the key to understanding guitar amplification. Our guitar amplifier will make the electric audio waves taller to boost their loudness.
When multiple strings are played the multiple electric waves are summed into complex waves. See my short youtube video to see guitar audio on an oscilloscope.
Guitar AC Voltage Audio Signal
Fender Stratocaster connected directly to an oscilloscope. Each 'wave' on the oscilloscope is caused by one string vibration. The highest voltage (peak of wave) is created by the fastest string speed when it's moving directly over the pickup. The zero voltage point (center of graph) is when the string stops moving and reverses its direction. The left side of the graph shows a high open E string pluck followed by a pluck of the low open E. Voltage is on the left scale and time runs along the bottom. The top half of the signal is positive voltage and the bottom half is negative. The signal's voltage and current alternate between positive and negative. The tight wave spacing on the left is an indication of high frequency and pitch. The wave height is an indication of power and loudness. The high open E + low open E strings' signal on the right is a summation of the two strings' signals combined into a complex wave--the high open E wave is 'written' onto the large low open E wave. The speaker cone will move just like this graph. When the graph goes high with a positive voltage the speaker cone moves outward, when the graph goes low with a negative voltage the cone moves inward.
Amplifiers have large capacitors that store enough electricity to kill even when the amplifier is unplugged. If you open an amplifier you MUST verify no voltage remains in the capacitors before working inside it.
Guitar Audio Signal Input
The guitar cable's tip conductor connects to the input jack's "T" tip terminal. The cable's sleeve connects to the "G" ground terminal. The guitar signal travels down the wire and through grid stopper resistor R3.
The guitar's alternating current audio signal enters the amplifier at guitar input jack 1 or 2. 1 is the Hi input and 2 is the Lo, -6dB quieter input. Resistor R1 on jack 1 is the 'input resistor.' It provides a return path for the electricity generated in the guitar pickup coil. It also adds 1,000,000 ohms of input impedance to boost the signal voltage from the guitar. [Bonus info: R1 also functions as the 'grid leak' resistor for Tube V1A's grid. A grid leak drains off unwanted DC voltage to keep the tube's control grid near 0 DC volts.] The '1M' written on R1 is its rating of 1 megaohm. See more on impedance here and see this web page for grad school level information on how Fender multiple input jacks and jumpering channels works.
The signal moves from the guitar jacks down the yellow wires to resistors R2 or R3, which are 'grid stopper' resistors. They help stabilize the amplifier by removing much of the audio signal above human hearing. The "68K" written on the resistor refers to its resistance value of 68,000 ohms or 68 kilohms. [Bonus info: R2 and R3 also act as 'mixing resistors' and prevent interaction between two simultaneous inputs like two guitars or a guitar and microphone. Sometimes you will see resistor values written as 1K5 which simply means 1.5 kilohms.]
Signal from resistor R3 travels down the wire to tube V1A's grid (pin 2) then out the plate to coupling capacitor C1. Tube V1 is split into two identical halves, A & B.
After going through grid stopper resistor R3 the audio signal flows down the wire to the preamp tube's control grid, which is the entry to the 'A' half of the preamp tube (V1A). It's called V1A because tubes were called 'Valves' and this is tube number 1 and we're using the 'A' half of the tube. 12AX7 is the type of tube which happens to be the most popular preamp tube in use and it's really two tubes in one. See How Tubes Work for more info. The preamp tube amplifies the guitar audio signal then sends it out pin 1 (plate) up the yellow wire to capacitor C1, which is a 'coupling capacitor' or 'cap.' Coupling caps are sometimes also called 'blocking caps' because they block DC voltage but allow the AC guitar signal to pass. The 0.022µF written on the cap is it's rating of 0.022 micro Farads (0.000,000,022 Farads).
How capacitors block DC but let AC pass: Caps are actually made with sandwiched conductive plates but I like to visualize them as having a stretchable rubber membrane inside that blocks the flow of electricity. When voltage is applied to a capacitor the 'rubber membrane' stretches and bulges as electrons try to flow through it. The higher the voltage the more the membrane bulges. If you quickly reverse the capacitor's voltage polarity it will go from bulging one way to bulging the other way. This is what a small AC signal does--it stretches the 'membrane' back and forth as the voltage alternates which allows electrons on both sides of the capacitor to move back and forth (alternate) but a constant DC voltage that is trying to flow in one direction will be blocked by the membrane.
If you are familiar with hydraulics a coupling capacitor is like a piston in a hydraulic line. Small alternating pressure changes will make the piston move back and forth so fluid is moved on both sides of the piston -- this is how small alternating current signals move through a capacitor.
Capacitors are made of two conductive plates separated by an insulator or dielectric. Common dielectrics are mica, polypropylene, ceramic, paper and even oil.
High voltage DC (direct current) power used by the tube is brought in through resistor R5, which is a 'load resistor.' We'll discuss the function of the load resistor later. The wire between tube pin 1 (plate) and R5 carries up to 250 volts DC. That wire carries both the AC audio signal out and the high voltage DC power the tube needs in. Coupling capacitor C1 allows the AC audio signal to pass through but blocks the DC on the wire and keeps it out of the volume pot.
Volume and Output Stage Driver
Signal flows from capacitor C1 to the Volume pot then down the orange wire to tube V1B's grid (pin 7) then out the plate (pin 6) to capacitor C2, then to a fork in the road--resistor R9 one way and the other way down the yellow wire to the power tube V2.
After going through capacitor C1 the audio signal flows up the yellow wire to the volume potentiometer (pot) which acts as a variable voltage divider. Volume knob left = more signal bled to ground and lower volume. Volume knob right = less signal bled to ground and higher volume. [Bonus info: The volume pot also functions as V1B's grid leak resistor] For more info see voltage dividers and potentiometers.
The signal then flows from the volume pot down the orange wire all the way to tube V1B's pin 7 (grid). V1B is the second half of the preamp tube. This second gain stage is called the output stage driver because it boosts the signal to the level needed by the power tube. The audio signal leaves tube V1B via pin 6 (plate) and flows up the yellow wire to capacitor C2, another coupling cap that blocks DC. High voltage DC is fed to the tube via load resistor R6. After C2 the signal flows down the yellow wire to the power tube's pin 5 (grid). Resistor R9 has a dual function. It adds input impedance to the power tube amplifier circuit and acts as the tube's 'grid leak' resistor which keeps the grid at 0 volts DC.
Power Tube to Output Transformer to Speaker Jack
Signal leaves V2's pin 3 and flows to the Output Transformer's primary winding then out the secondary winding to the speaker jack.
The power tube, V2 is sometimes referred to as the output tube. V2 is the final stage of amplification and its purpose is to amplify for power (voltage x current) where V1A and V1B were focused on voltage amplification. The signal enters the power tube at pin 5 (grid) and leaves via pin 3 (plate). It then goes to the output transformer (OT) which is mounted on the backside of the chassis and is not shown on the layout diagram.
Like we saw with the guitar's pickup, magnetism can be used to generate electricity in a coil. You can also do the reverse and pass electricity through a coil and generate magnetism. The amplifier's output transformer uses both of these principles to pass alternating current (AC) from its primary (input) winding to the iron core as magnetic flux and on to the secondary (output) winding as alternating current.
The output transformer's windings are really just two wire coils wrapped around an iron core. The input, or primary winding uses electric current flowing through it to generate a magnetic field or flux. This magnetic field fluctuates with the AC signal voltage and is captured by the transformer's iron core. The captured magnetic flux flowing through the core generates a voltage and current in the secondary winding. You can alter the voltage and current from primary to secondary by changing the ratio of coil wraps from primary coil to secondary.
Current flowing into the primary winding (above left) induces magnetic flux flow around the transformer iron core which in turn induces an electric voltage and current in the secondary winding. Put fewer wire wraps on the secondary (output) winding and its voltage will decrease (step down) but its current will increase. Most guitar amp transformers are of the 'double window' type (bottom of left diagram) and made with laminated iron magnetic cores. 5F1 transformers are shown on the right. The Power Transformer is lying on its side while the Output Transformer is standing vertically. Positioning transformers 90º out of phase with one another like this reduces interference hum.
Example: The primary winding has 200 wraps of wire in its coil and the secondary has 100 wraps. If a 10 volt alternating current is applied to the primary winding the secondary will generate 1/2 of the input or 5 volts. The current will change proportionally in the opposite direction. If 1 ampere of AC current is applied to the primary the secondary will generate 2 amps. This is what an amplifier's output transformer does, it steps down the signal's voltage but steps up the current because the speaker's voice coil needs current to generate a magnetic field to move the speaker cone.
The output transformer's primary takes in a high voltage, low current signal (high impedance signal) and puts out a low voltage, high current signal (low impedance signal) through the green wire to the speaker jack and on to the speaker. The alternating current audio signal flows through the speaker's voice coil which generates a magnetic field. The voice coil is simply a single wire wrapped into a coil as shown below. The magnetic field created by the voice coil is either attracted to or repelled by the speaker's magnet. Positive voltage generates a repulsive magnetic force and the speaker coil and cone moves outward away from the speaker magnet, negative voltage generates an attractive magnetic force and pulls the speaker cone inward. The speaker cone alternates between moving outward and inward as the signal voltage alternates between positive and negative. For every guitar string movement there is a corresponding speaker cone movement.
Speaker Voice Coil is an Electromagnet
Electric current flowing through the speaker's voice coil generates a magnetic field. When the electric current in the voice coil reverses, the magnetic field also reverses causing attraction and repulsion to the speaker magnet.
This magnetic attraction and repulsion moves the voice coil and speaker cone back and forth to create air pressure waves that our ears perceive as sound--the sweet sound of electric guitar. When the speaker cone moves outward a positive air pressure wave is created and when the cone moves inward a negative (low pressure) wave trough is generated. These air pressure waves move our ear drums in and out. The ear drum movement is translated into neuron activity which is sent to the brain where pleasure is created, thus electric guitar + amp = pleasure ;)
Bonus Info: You can determine the ohm rating of a guitar speaker by measuring the DC ohms (resistance) between the speaker terminals (while disconnected from the amp) and then multiply by 1.2. Example: You measure 6.5 ohms: 6.5 x 1.2 = 7.8 ohms = 8 ohm speaker.
The 'voice coil' is an electromagnet that interacts with the speaker magnet. The 'spider' supports the voice coil but allows it to move in and out freely. This excellent DIY speaker recone video shows speaker parts and function in detail.
Bonus Info: How Microphones Work
Dynamic microphones work exactly in reverse of how a speaker works. They have a diaphragm like a speaker cone that gets moved by sound (air pressure waves). The diaphragm moves a coil of wire wrapped around a magnet. The coil moving through the magnetic field creates electricity in the coil wire--an alternating current signal voltage.
When the microphone diaphragm moves the coil, electricity is generated.
When a singer sings a note her vocal chords vibrate like a guitar string. The movement of the vocal chords create air pressure waves that strike the microphone diaphragm and cause it to move. Speaking of diaphragms, our ear drums are diaphragms that when moved by sound waves cause neurons to fire to communicate with our brain. Yea, our ears are biological dynamic microphones.
When a positive, high pressure sound wave hits the microphone diaphragm it is pushed inward and a positive electrical current and voltage are created. When the low pressure wave trough hits the diaphragm it is pulled outward and a negative current and voltage are created in the coil. The microphone creates an alternating current voltage signal similar to an electric guitar AC voltage signal.
The 5F1 amplifier uses negative feedback (NFB) to reduce distortion, increase headroom and improve stability but a drawback is it also reduces overall amplifier gain. Negative feedback works by taking the speaker output voltage and feeding it back into the amp's signal stream before the driver or phase inverter circuit. A feedback resistor reduces the voltage to a suitable level before it joins the amp's signal stream. It's negative feedback because the signal is out of phase so when it's injected into the amp's signal stream it reduces the amp's signal voltage.
A green wire running from the 5F1's speaker jack carries the amplified audio signal through resistor R13 and injects the feedback at V1B's pin 8 (cathode). Resistor R13 is the Feedback Resistor and controls the level of feedback voltage passed to the cathode. Adding a switch to the NFB circuit is a common modification. Removing feedback makes an amp more aggressive with earlier break up and distortion at lower volume levels.
So the main purpose of a guitar amplifier is to take the tiny AC electrical signal generated by the guitar's pickup coil and make it strong enough to push and pull a speaker cone. The guitar amp is also used to shape the tone and control signal distortion giving us the clean, mellow sound of jazz guitar or the animal growl of hard rock. Distortion is an important part of guitar amplifier design and this is the primary difference between guitar and audio amplifiers. Audio amps are usually designed for absolute minimum distortion.
Now that we've covered the signal flow I'll go back and cover the other amplifier components that I didn't mention. Wall plug power of 120 volts AC (USA or 100, 220 or 240 volts AC in other countries) runs to the fuse F1. Fender guitar amp fuses are MDL type "slow blow" or "time delay" 1/4 inch (6mm) wide by 1 1/4 inch (30mm) long. The fuse is a 2 amp slow blow fuse. Slow blow means it won't blow instantaneously when the turn-on power surge runs through it. Sustained current greater than 2 amps is required to blow the fuse. Next the power flows to the power switch S1, which is located on the volume pot.
120 AC volts RMS (average) wall power equals 169.7 volts peak (Vp) and 339.4 volts peak-to-peak (Vpp).
Bonus Info: Alternating Current (AC) voltage is normally given in volts RMS (root-mean-square), which is a form of voltage averaging equal to a DC voltage. Always consider AC voltage as RMS unless it is specified as peak Vp or peak-to-peak Vpp. Multimeters show voltage in RMS. Our standard 120 volts AC wall power is RMS. You can convert RMS voltage to peak voltage by multiplying RMS by 1.414, so 120 volts AC at the wall is actually 169.7 volts measured from the + wave peak to 0. You can convert peak voltage to RMS by multiplying by 0.707. To convert RMS voltage to peak-to-peak voltage you multiply RMS by 2.828, so 120 volts AC at the wall is actually 339.4 volts measured from the + wave peak to the - wave peak. You can convert peak-to-peak to RMS by multiplying by 0.354.
AC voltage in the United States runs at 60 cycles per second, which is called Hertz (Hz). 120 volt wall power goes from zero to +169.7 volts, then down through zero to -169.7 volts, then back up to zero 60 times per second. This is why AC electrical noise picked up by guitar amps is often described as a 60 Hertz hum. Why is our wall power 60 cycles per second? Because power company electrical generators in the US turn at 60 revolutions per second (3600 revolutions per minute or RPM).
Bonus Bonus Info: Visualizing Alternating Current. One way to visualize how AC electricity flows is to think of the amplifier's power system as a rope and pulley system. Think of the wall power plug and the amplifier's power transformer as pulleys. A loop of rope representing the hot and neutral wires would be wrapped tightly around the wall power and transformer pulleys.
The power company's AC generator is like a hand grabbing the power rope (hot wire) and pushing it forward a few feet then stopping the rope movement and pulling the rope back, then pushing the rope again, then pulling it in this alternating pattern (doing one push-pull cycle 60 times per second). Electrons actually alternate their movement forward and backward, reversing course through AC wires and circuits like this rope movement.
Bonus Bonus Bonus Info: 240 volt circuits in the United States use two 120 volt hot wires instead of 120 volt's single hot wire and ground (called neutral). For 240 volts one wire pushes at +120 volts while the other pulls at -120 volts (like using two hands, one hand on each of the two ropes in the analogy above), then they alternate the pushing and pulling at the same 60 cycles per second for 240 volts of power. Alright, back to the amplifier. . .
After the amp's fuse and On/Off switch the 120v AC RMS runs to the power transformer (PT), through its primary winding, then back to the wall plug via the white Neutral wire. The white Neutral wire is a ground wire and is connected to the same ground as the Safety Ground wire at the building's electrical service entrance.
The 5F1's power transformer high voltage winding is rated at 325-0-325v. This means the transformer has a grounded 0 volt center tap and simultaneously puts out +325 volts AC RMS on one secondary winding wire and -325v on the other for a 650 volt AC RMS wire-to-wire voltage (650 volts AC RMS = 1,838 volts peak-to-peak :O ). Yes, that's high voltage that can kill you. At this very high voltage the 5F1 power transformer only needs to be rated for a paltry 70 milliamps (0.070 amp) max of AC current to run the amp circuit.
The power transformer has three secondary windings. The first winding as discussed above steps the 120v RMS AC wall power up to 650 volts RMS AC. Two other small secondary windings step the 120v AC down to 6.3 volts RMS AC and 5 volts RMS AC. Notice all voltages in transformer secondaries are always AC because a transformer can't pass DC from primary to secondary. The 6.3 volts are used to power the pilot light and heat the preamp and power tubes' heater filaments which heat the tubes' cathodes. The 5 volts are used to directly heat the rectifier tube's cathode.
Bonus Info: When I first learned that the power transformer primary coil was made up of one long wire that directly connects the 120v hot wire to the neutral (ground) wire I wondered why it didn't short out. The reason is the primary and secondary coils are coupled together by the transformer's iron core. Alternating current in the primary coil creates a magnetic field or flux that is captured by the core. That flux in the core creates an AC voltage in the secondary coil. The load (impedance) placed on the secondary winding by the amplifier is transferred through the core to the primary coil. That impedance keeps the primary coil from "shorting out."
Power Cord Wiring
Modern U.S. wall cords and sockets have a narrow blade for Hot (black wire 120v), a wide blade for Neutral (white wire ground), and a round or 'D' shaped prong for the chassis Safety Ground (green wire ground). Power cord wire colors are sometimes non-standard so use a multimeter to identify Hot and Neutral. Europeans sometimes use the letters E: Earth (safety ground), L: Line (hot) and N: Neutral to describe the three plug wires.
The 650 volts AC power from the power transformer is fed directly into V3, the rectifier tube. V3 is a full wave dual plate rectifier tube that converts alternating current (AC) into direct current (DC). The power transformer and rectifier work together as an electron pump which pulls electrons out of the amp circuit creating a positive voltage (electron scarcity = positive voltage). The amplifier's electronics need DC to amplify. The amp is powered by DC but the guitar signal moving through the amp is AC.
The flow of power starts at the power transformer at far left. 650V AC powers the V3 rectifier tube. V3 puts out 360V of DC. Note the yellow wires running to V1's pins 1 & 6 carry both high voltage DC power into the tube and the AC signal out (orange arrows). This diagram shows "conventional" power flow but the actual electrons flow in the opposite direction.
360 volts of DC flows out of V3's pin 8 (cathode) and is referred to as B+ or B+1 voltage (from old Battery Positive designation). Tube rectifiers are popular in guitar amps due to their dynamic power sag which adds to the amp's playing dynamics and note "bloom." Audio stereo tube amps usually use solid state rectifiers to reduce voltage sag that would be seen as distortion to the Hi Fi listener.
The B+ DC voltage flows to the output transformer's primary winding and to the circuit board's three large filter/reservoir capacitors, C3, C4 and C5 and two voltage dropping resistors, R10 & R11. These resistors and capacitors form RC (resistance capacitance) low pass filters that take the lumpy, pulsing DC output of the rectifier tube and smooth it out--the smoother the better. Any waves or ripples left over in the DC power would be added to our audio signal and heard as hum in the preamp and power tubes. These big capacitors also function as current reservoirs that help feed the amp during high demand. The hydraulic equivalent of a filter capacitor is a hydraulic accumulator. The '16µF 475V' written on the cap is its rating of 16 micro Farads and 475 volts. The '10K 2W' written on resistor R10 is its rating of 10,000 ohms and 2 watts.
The voltage dropping resistors separate the amp's power into three power supply nodes, B+1, B+2 and B+3. The 360 volts DC from the rectifier is B+1 then it's stepped down to 325 volts DC (B+2) then to 250 volts DC (B+3). The 360 volts DC B+1 direct from the rectifier is fed to the output transformer's primary input which flows on to the power tube plates. The 325 volts DC B+2 is connected to the power tube pin 4--the screen grid. The 250 volts DC B+3 is used to power the preamp tube. The filter capacitors and voltage dropping resistors also decouple the three B+ power nodes to prevent interaction, feedback and oscillation between the preamp and power tube. With no guitar signal present the 'idle' voltage at the V1 preamp tube's plate pins 1 and 6 will be around 170 volts DC after flowing through the Load Resistors R5 and R7.
Well that's it for the 5F1 Champ. It's a great sounding but simple guitar amp. The signal flow is very similar to most other Fender amps, they just have more parts. Really understanding the 5F1 will help you understand other more complex amps.
Fender Original 5F1 Layout and Schematic
5E3 Tweed Deluxe Annotated Schematic
The 5E3 Deluxe is the most common tube amp kit available. It uses two 6V6GT power tubes in a Class AB push-pull configuration. Click the image to view the full size (readable) annotated schematic.
5E3 Layout with Signal Flow and Annotations
Notice how convoluted the signal path is compared to the schematic. Input jacks are at top right and the speaker jack is bottom center. Click the image to view the full size (readable) annotated layout.
The Pinnacle of tweed Amps, the 1959 5F6-A Bassman with Annotations
My 5F6-A Bassman amp is the sweetest amp I have ever heard. See this for an explanation of How the Bassman Works.
Let's start with the tube's glass shell which holds a vacuum. A vacuum is needed to keep the electrodes from burning or oxidizing--the same reason a vacuum is needed in a light bulb (a light bulb is actually a single electrode tube!). The vacuum also keeps the electrons in the tube from colliding with air molecules. In the tube diagram below the DC power source on the right applies high voltage DC (around 250 volts for the 12AX7 preamp tube) to the Load Resistor which causes a voltage drop on the tube's 'plate.' The tube's cathode is connected to ground so there is up to 250 volts between the plate and cathode. The positive charge on the plate would love to pull the negatively charged electrons from the grounded cathode but the electrons cannot jump the gap between them -- yet.
A quick note about 'Conventional Current Flow.' In electric circuits negatively charged electrons actually flow from the negative '-' battery terminal to the positive '+' terminal. That's right, the electricity in your car flows from the battery's - terminal through the ground wire, through the car's body, through the radio's ground wire to the radio and then through the positive power wire back to the battery. The problem is that Benjamin Franklin guessed wrong on the direction of electrical flow so conventionally we think of electricity as flowing from + to -. With tubes you need to actually consider how the electrons are really moving in order to understand them.
Edison discovered while working on the light bulb that if you put a wire in a vacuum tube and heat the wire (light bulb filament) electrons would "boil off" into the vacuum. The kinetic energy of the hot, vibrating electrons in the hot wire actually knock electrons off the wire into the tube vacuum. He also discovered that since opposite electrical charges are attracted to one another he could put a positively charged 'plate' inside the tube to collect the free electrons and create a current from the hot filament (cathode) across the gap to the plate. This is the 'Edison Effect,' which acts as a one-way electronic valve--heat the filament and electrons flow, remove the heat and the flow stops. It's a one-way valve because the plate is not heated so electrons can't flow from the plate to the filament. This is why vacuum tubes are referred to as 'valves.'
Standard Vacuum Tube Preamp Circuit
Guitar signal voltage enters the tube on the left at the Grid and exits at the Plate. This tube is a 'triode' which means it has three electrodes, the grid, plate and cathode. Electrons flow from the cathode through the grid to the plate.
Later someone discovered you could put an electrically charged metal screen, or 'grid,' between the cathode and plate to block the electron flow. Because like charges repel, a negative charge on the grid (lots of excess electrons on the grid) repels the negatively charged free electrons trying to flow from the cathode through the grid to the plate. But a positive charge on the grid (scarcity of electrons) allows the free electrons to pass through the grid. If you fluctuate the electric charge on the 'control grid' you fluctuate the current flow between the cathode and plate. This effect is used as a signal amplifier by applying a low level guitar signal to the grid which controls the much larger flow of free electrons between the cathode and plate (the plate is also called the anode--next time you want to insult someone call them an 'anode' :D ). With 400 volts on the plate the electrons move at about 24 million miles per hour from cathode to plate (yea, wow).
Three Minute Long "How Tubes Work" Video
The tube rectifier shown in this video is a single plate, half wave rectifier but guitar amps use dual plate, full wave rectifiers that create 120Hz pulse DC.
In the Champ amp tube V1A is the first preamp and V1B functions as the output stage driver. V1A and V1B are triodes because they have three electrodes, a grid, plate and cathode. Some simple tubes like half-wave rectifiers have only a cathode and plate and are called diodes (two electrodes). A tube with four electrodes is called a tetrode. The Champ's power tube, V2 is a pentode with five electrodes. I guess you could say a light bulb is a uniode with just one electrode ;)
Notice in the Vacuum Tube diagram above we have both a filament and a cathode. In most tubes the cathode is indirectly heated by a filament wire. In the layout diagram at the top of this webpage the power transformer sends 6.3 volts of alternating current (AC) through the twisted green wires to power the pilot light and the V1 and V2 filament heaters. The rectifier tube is different as its cathode is directly heated by 5 volts AC from the yellow twisted wires. The wires are twisted to minimize the electronic noise generated by the AC flowing through the wires.
Referring to the above Vacuum Tube diagram again, the guitar audio signal enters on the left at 'input voltage.' The signal flows to the tube's 'control grid' shown as the dashed line inside the tube. As the audio signal voltage fluctuates the electrical charge of the grid fluctuates too. As the grid voltage fluctuates its blocking power fluctuates so the electron flow between the hot cathode and the plate fluctuates to the guitar signal. The voltage fluctuations on the plate are the amplified guitar signal. When the control grid voltage goes positive (fewer electrons) more current is allowed to pass through the grid from cathode to plate. When the grid goes negative (more electrons) the extra electrons on the grid repel the electrons that are trying to get through the grid to the plate because like electrical charges repel.
At this point the tube is acting as a current amplifier. Small voltage changes on the tube's control grid are amplified into large current changes on the tube's plate. Remember voltage is 'electronic pressure' and current is the amount of electrons flowing. We will discuss later how the tube's load resistor transforms the circuit from a current amplifier to a voltage amplifier.
Grid leak resistors 'leak' off unwanted DC voltage created when free electrons from the cathode hit the grid. Ridding the grid of these electrons prevents a grid voltage buildup which would affect the grid-to-cathode bias voltage.
An important concept to understand with tube electronics is that the plate brings in high voltage DC to power the tube and simultaneously carries the amplified AC audio signal out. The AC audio signal 'rides on top' of the DC. This is called 'DC offset.'
AC Audio Signal With DC Offset
The AC audio signal rides on top of the tube's high voltage DC.
How the Load Resistor Works
Once again referring to the Vacuum Tube diagram above 'R' is a load resistor. The load resistor changes the amplifier circuit from a current amplifier to a voltage amplifier. The load resistor allows a small flow of current from the tube's plate to make a large change in voltage thus creating a voltage amplifier. The load resistor restricts the flow from the 250v DC power supply to the tube's plate. When the tube flows negatively charged electrons to the positively charged plate the voltage drops across the load resistor and between the load resistor and plate. If you slow the flow of electrons from cathode to plate the voltage drop across the load resistor will decrease causing a voltage rise on the plate. Stop the flow completely and the voltage on both sides of the load resistor will equalize at 250 volts. If there's no current flowing there will be no voltage drop across the load resistor.
One way to describe how a tube is used as a voltage amplifier is with a water hose analogy. Think of the high voltage power as water faucet pressure and the wire running from the power supply to the tube plate as a water hose. You can simulate the load resistor by clamping the hose partially shut. Water flows through the clamp but at a reduced pressure. The water pressure is the voltage, the water flow is the current and the clamp is the load resistor resistance.
You have high pressure from the spigot and when water is flowing the clamp reduces the pressure beyond the clamp. There is a pressure drop across the hose clamp. Now put your thumb over the end of the hose--your thumb simulates the preamp tube's control grid. Close off the flow with your finger and you have no water flowing and the water pressure will rise and equalize on both sides of the clamp so there's no pressure drop across the clamp. This simulates the control grid stopping the electron flow from the cathode to the plate--no flow = no voltage drop across the load resistor.
Now allow some water to flow by releasing thumb pressure and the pressure will rapidly drop in the hose between your finger and the clamp. This simulates the control grid allowing electrons to flow to the plate. Add more thumb pressure to decrease the water spray and the hose pressure increases, allow more to flow by your thumb and the pressure drops. The partially clamped hose causes more pressure drop when small amounts of water are released. Without the restriction of the clamp much more water must be released to get the same pressure drop. The load resistor causes much more voltage drop when small amounts of current are released by the control grid. Without the load resistor much more current would have to flow between the cathode and plate to get the same voltage drop.
The tube's control grid 'releases pressure' (lowers voltage) by flowing negatively charged electrons onto the positively charged plate--this drops the voltage in the wire between the plate and load resistor. When the control grid slows the flow of electrons the voltage rises in the wire. These voltage fluctuations are the amplified guitar audio signal.
Triode Vacuum Tubes
Single triode vacuum tube on the left, note its tubular structure. The 12AX7 middle and right is a dual triode tube--two tubes in one. Pin 1 connects to the Plate (output), Pin 2 the Grid (input), Pin 3 the Cathode (source of electrons). Pin 6 connects to the 'B' triode's Plate, Pin 7 the Grid, Pin 8 the Cathode. Pins 4, 5 & 9 connect to the heater filaments. The halo like 'Getter' at the top of the tube held the material used to create the 'Getter Flash' (mirror-like substance on top inside of tube). The Getter Flash absorbs gas molecules to maintain the tube's vacuum. If the Getter Flash turns from silver to ash white you know the tube has been contaminated with too much gas from a vacuum leak.
6V6GT Beam Tetrode Power Tube
The 6V6GT is known as a beam tetrode but in reality it has five electrodes so it's should be called a beam pentode: cathode, plate, control grid (g1), screen grid (g2) and beam confining electrode . The screen grid provides a constant, positive voltage to strongly attract electrons from the cathode. It also isolates the control grid from the plate which reduces parasitic capacitance between them which increases the tube's gain and stability. Some amps have a switch to run a tetrode or pentode tube in lower power 'triode mode' by tying the screen grid and plate together which allows the screen grid's voltage to fluctuate with the plate. Non-beam power tubes such as the EL34 don't have a beam confining electrode but have a suppressor grid (g3) which is tied directly to the cathode and keeps high speed electrons from bouncing off the plate upon impact.
6V6GT Beam Tetrode
A beam tetrode's "beam confining electrode" is tied to the cathode and directs electron flow into a beam and onto the plate. The beam confining electrode is the tubes fifth electrode. The heater wires are shoved up inside the cathode, which is placed inside Grid NO.1 (control grid), which is placed inside Grid NO. 2 (screen), which is placed inside the beam confining electrode, which is placed inside the plate.
Tubes are delicate because of the very close and precise spacing between its electrodes: the cathode, grid(s) and plate. If the tube's support structure allows any movement of the electrodes the tube can become microphonic. Tapping an operating tube with a pencil will cause a microphonic tube's loose electrodes to vibrate and the changing distance between tightly spaced electrodes will affect the signal flow and be heard as ringing. Also the glowing hot filament wires can break if subject to shock. That's why you should allow your tubes to cool for a minute or two before moving your amp.
EL37 True Pentode Cathode and Grids
Take a look at this page with excellent detail shots of an EL37 true pentode's insides which clearly shows the control, screen and suppressor grids.
So to review, the filament heats the cathode, the cathode gives off electrons, the plate and screen attract the free electrons but the grid controls the flow--the control grid is the valve that controls the flow of electrons through the tube. Place a guitar signal voltage on the control grid and the guitar signal will control the flow and the tube's output will be the amplified guitar signal.
See Tube Guitar Amplifier Overdrive for specific information on how overdrive distortion is created.
Tube Bias Circuits
5F1 Cathode Bias
The 5F1's V1 and V2 use "common cathode biasing", also referred to as "self biasing" or just "cathode biased." V1A's bias voltage is set by cathode resistor R4 which is connected to V1A's cathode (pin 3). V1B's bias is set by R6. V2's bias is set by R8. Capacitor C6 is a cathode bypass cap that helps decrease local feedback and increase V2's gain. Although not shown on the original 5F1 Champ Fender schematic and layout, many Champs came from the factory with the C7 cathode bypass capacitor shown at extreme right. The bypass cap boosted the amp's gain. For Champs without the C7 bypass cap adding one is a common modification.
For the tube's control grid to control the flow of electrons between the cathode and plate there must be a voltage difference between the cathode and control grid. The voltage difference is what repels the electrons to control their flow. The cathode is 'boiling off' negatively charged electrons and a more negatively charged control grid can keep them in place because like charges repel. This voltage difference between the cathode and control grid is called tube 'bias.' Common cathode tubes use the voltage drop across a bias resistor placed between the cathode and ground to generate the bias voltage.
The much more powerful 5E3P amplifier shown below uses an "adjustable fixed bias" system to supply the bias voltage. It is called "fixed bias" because a steady bias voltage is applied to the tube control grid. A cathode biased amp's bias voltage will fluctuate (it's not fixed) with the input signal. A fixed bias amp applies a negative voltage (usually between -35 to -50 volts DC) to the power tubes' control grids and the cathodes are connected directly to ground at zero volts (there is no cathode resistor). Setting a higher % of maximum dissipation is considered a hotter bias. The power tube grid voltage is always negative on fixed bias amps and a hotter bias will have the grid voltage closer to zero. A hot bias for the 5F6A Bassman would be around -44V DC. Setting a lower % of max dissipation is considered a cooler bias and the grid voltage will be a larger negative number like -50V. See this for info on How to Measure and Adjust Bias.
Adjustable Fixed Bias System
Power Transformer on left supplies 50 volts AC to the Rectifier Diode. The AC power flows into the diode's negative terminal (cathode) so 50 volts of pulsing negative DC flows out. The 1K resistor and large 50uF Filter Capacitor form an RC (resistance capacitance) low pass filter to smooth out the DC pulses. The Bias Adjustment Pot adjusts the amount of negative DC that flows to the Power Tubes' Control Grids. The 27K resistor sets the maximum hot bias, reduce it for hotter max bias, increase it for cooler.
The Rectifier Tube V3
The rectifier tube, V3 is a one-way valve that combined with the power transformer acts as an electron pump to convert 650 volts RMS AC into 360 volts DC. This high voltage AC from the transformer is often called HT (high tension) voltage. Note that when you measure between ground and either of V3's AC input wires you'll see 325 AC volts, but if you measure between the two AC input wires you'll see 650 AC volts. An amplifier's rectifier pulls electrons out of the amp circuit to create positive voltage (electron scarcity = positive voltage). V3 is different from the preamp and power tubes in that It has two plates, two cathodes, no grid and its heater filaments are directly connected to the cathode to keep the heater-to-cathode voltage low.
650 volts of alternating current (AC) from the power transformer is connected to the rectifier's pins 4 and 6 which connect to the two plates. As the positive half of the AC wave (+325V) charges pin 4's plate positively, pin 6's plate is charged negatively. The pin 4 plate's positive charge attracts electron flow from the cathode generating a positive DC current on the wire attached to the cathode's pin 8 (pulling negatively charged electrons out of the B+ wire connected to pin 8 creates a positive voltage on the wire). Nothing happens to the negatively charged pin 6 plate.
Then as the negative half of the AC wave enters the tube (-325v), pin 6's plate is charged positively and attracts electron flow from the cathode while pin 4's plate is charged negatively and does nothing. Therefore both halves of the AC wave are converted to DC which makes V3's 5Y3GT tube a 'full wave' rectifier.
High voltage AC flows onto the plates connected to pins 4 & 6. High voltage DC current flows from the cathode out pin 8. Note the 5Y3GT on the right has a directly heated combined cathode/filament where the GZ34 on the left has a separate cathode and filament that are electrically connected.
Both positive and negative voltage is used to create pulsing DC in a full wave rectifier like the 5Y3 and GZ34.
Because the cathodes are directly heated by the power transformer's 5v AC supply the cathode has both 5v AC and high voltage B+ DC on it at the same time. This is why the 5v heater supply doesn't have a center tap like the 6.3v supply--the B+ DC voltage would be shorted to ground through a center tap. This 5v AC is on the B+ output wire but is filtered out at the first filter capacitor.
Most fixed bias amplifiers use a single diode to rectify the 50v DC fixed bias voltage. The single diode functions as a half wave rectifier and generates a very lumpy DC voltage that must be filtered by a resistor and relatively large capacitor (RC filter).
The cathode and filament are combined.
The Internals of the JJ GZ34S Rectifier Tube
The GZ34S's two big metal plates are hollow in the center so the two cathodes (hollow tubes) will fit up inside them. High voltage AC is brought in from the Power Transformer through Pins 4 and 6 to charge the plates. The fit is tight but the cathode and plate do not touch each other. The hollow cathodes have filament heater wires running their entire length inside them. The hot cathodes emit negatively charged electrons. The free electrons are pulled to the positively charged plates. Notice the thick power conductors that connect both cathodes to the output pin 8. Electrons are pulled through these conductors from the B+ wire attached to pin 8. Removing electrons from the B+ wire creates a positive charge in the wire. Conventionally we think of positive DC power flowing from the cathode and out pin 8 to the Standby switch when in reality the electrons are flowing the opposite direction.
The plate horizontal and vertical supports hold the plates and cathodes in place. The 'Getter Halo' (not shown) at the top of the tube's only function was to hold the 'getter flash' until it was flashed onto the inside of the tube (the silver coating on the top of the tube). This silver coating absorbs oxygen molecules to keep the tube's vacuum oxygen free.
Double folded cathode heater filaments are coated with thin electrical insulation and shoved up inside the hollow cathode.
All About Rectifiers
Amplifier rectifiers can use tubes or solid state diodes to rectify alternating current (AC) into direct current (DC). They do this by acting as one-way valves that only allow electrons to flow in one direction.
Solid state rectifiers (silicone diodes) are known as sounding stiff and punchy because they don't create as much voltage drop and sag as tube rectifiers which can have over 60 volts of voltage drop across them.
Voltage drop is caused by the internal resistance of the power transformer and rectifier. It lowers the amp's B+ voltage and can soften the dynamics of an amplifier where the higher voltage provided by solid state rectifiers can make an amp sound punchier, louder and offer up a tighter lower end and overdrive tone. Solid state rectifiers typically drop only 2 volts and a GZ34 tube rectifier drops around 10. The 5Y3 offers up around 60 volts of drop.
Voltage sag is the dynamic voltage drop across the rectifier that increases with current demand and creates output volume compression. Output volume compression occurs because high current demand during loud notes lowers the amps B+ voltage and maximum volume is decreased. Conversely, low demand, quiet passages create less voltage sag and generate greater amplification. This makes loud notes quieter and quiet notes louder which equals compression. Class A and AB amps differ in how much voltage sag is generated. Since Class A amplifiers idle near max current there are less current demand fluctuations and therefore less voltage sag. Voltage drop and sag help contribute to the warm, round, tubey sound of vintage amplifiers. The combination of the 5E3 Deluxe's small power transformer, 5Y3 rectifier tube and Class AB operation lead to metric shit tons of voltage sag.
A single diode or single plate rectifier tube is a half wave rectifier because only half of the AC wave is converted into DC voltage. Many fixed bias power supplies use a single diode half wave rectifier to generate the power tube bias voltage. Silicone diodes are similar to single plate rectifier tubes in that both have one cathode and one anode (plate is another name for a tube's anode). The term "diode" means two electrodes (cathode and anode). Tubes with two electrodes are also called diodes.
Half Wave Rectification In Amp's Bias Circuit
50V AC is tapped off the power transformer and sent to a single diode. It acts as a one-way valve to convert the 60Hz AC into 60Hz pulsing negative DC voltage. The DC out voltage is negative because of the polarity of the diode (turning the diode around would provide positive DC voltage). With the diode's cathode connected to the AC power source, negative DC is created. The bias circuit uses the power transformer's grounded center tap as the current return path.
Half Wave Rectification
60Hz AC in and 60Hz pulsing negative DC out. Half wave rectification is inefficient because it only converts half of the AC wave. It generates very lumpy DC power which is smoothed using a resistor and large capacitors that form a resistance-capacitance (RC) low pass filter.
Two diodes, or a standard dual plate rectifier tube such as the 5Y3 or GZ34, are conventional full wave rectifiers. They are "full wave" rectifiers because both the positive and negative AC wave is converted into DC voltage. Conventional rectifiers require a power transformer center tap to provide a DC current return path from the amp circuit back to the transformer.
Conventional Diode Rectification
Conventional two diode rectifier showing current flow during the positive half of the AC wave. The power transformer's center tap provides the current return path from the amp circuit back to the transformer. The center tap is grounded at zero volts.
Conventional rectifier during the negative half of the AC wave.
Conventional Tube Rectification
A standard dual plate rectifier tube like the 5Y3 works in exactly the same manner as the above two diode rectifier. That's why tube rectifiers are always paired with power transformers with center taps--the center taps are required to provide a current return path from the amp circuit back to the transformer.
Full Wave Rectification
Full Wave Conventional and Bridge rectifiers convert both the positive and negative AC wave (full wave) into DC voltage so they create 120Hz pulsing DC. Compare this graph with the "Half Wave Rectification" graphic above showing 60Hz pulsing DC. Knowing that power line and bias voltage are at 60Hz and high voltage rectified DC is at 120Hz helps with tracking down the source of amplifier hum.
Four diodes can be used to create a full wave bridge rectifier which does not need a power transformer center tap. A bridge rectifier is very efficient and extracts almost twice the voltage from an AC supply as a conventional rectifier.
4 Diode Bridge Rectifier Current Flow During Positive Half of AC Wave
All four diodes in a bridge rectifier act as one-way valves that allow current to flow in only one direction. The two diodes on the left side form the 'bridge' from the amp circuit back to the power transformer so a center tap is not required. As the outflow of current (shown with orange arrows) is 'pushed' by the transformer, the return path (shown with blue arrows) is simultaneously 'pulled' by the transformer's negative voltage so a bridge rectifier can extract twice the voltage of a conventional two diode rectifier which only 'pushes' because the transformer center tap is grounded at zero volts and does not pull.
Bridge Rectifier Current Flow During Negative Half of AC Wave
Note: All diagrams in this All About Rectifiers section show conventional current flow when in reality to create a positive voltage electrons must be removed from a conductor (+ voltage = scarcity of electrons, - voltage = excess of electrons).
All rectifiers must have a DC current return path back to the power transformer because the rectified DC flows in only one direction--away from the transformer. The transformer center tap or rectifier bridge provides the current return path back to the transformer.
Although a bridge rectifier will extract twice the voltage from a transformer compared to a conventional rectifier, a bridge rectifier will only supply 62% the rated current at that higher voltage. If you tape off the center tap of a power transformer and replace a conventional rectifier with a bridge rectifier it will generate twice the voltage but the winding wires would have to be of a heavier gauge to generate the same current.
Concise Rectifier Design Guide by Hammond
Note the FULL WAVE Capacitor Input Load at bottom left and FULL WAVE BRIDGE Capacitor Input Load at mid right. These are the two most common designs for tube amplifiers.
Why Center Tapped Transformers Don't Need a Bridge Rectifier
This bothered me for a long time until I did enough research to understand the current flow through 2 diode (conventional) and 4 diode (bridge) rectifiers.
Why does a power transformer with a center tap allow rectification with just two diodes versus four needed with no center tap? Because the transformer center tap provides the path for returning current to the transformer. If you use a transformer without a center tap then a 4 diode bridge rectifier is needed to provide the DC current return path from the amp circuit back to the transformer.
Conventional Rectifier and No Center Tap
With no center tap there's no return path to the transformer so this will not work.
Hybrid Rectifier With No Center Tap
To use a transformer with no center tap with a tube rectifier you can install the 'bridge half' of a bridge rectifier to provide a current return path. Two 1N4007 diodes running from the tube plate pins to ground will do the trick. Diode polarity is important, install the diodes with their stripes (cathodes) to the tube plates.
Hybrid Rectifier With No Center Tap + Backup Diodes
The two diodes connected to the tube plates are soft fail protection 'backup' diodes that will prevent AC from entering the amp if the tube fails as a short circuit. This is the equivalent of a bridge rectifier feeding a tube rectifier.
The 6.3v filament heater circuit is different from the high voltage circuit in that the 6.3v circuit can function fine without a center tap. The 6.3v center tap is not used as a current return path, its only used as a zero volt reference to balance the voltage of the two heater wires. Keeping the two wire's voltages the same helps with twisted wire hum cancellation. Since the heater circuit is AC and is not converted to DC both 6.3v wires act as "send" and "return" wires. In these purely AC wires the electrons flow back and forth in both lines so a separate return line isn't needed.
Amplifier Grounding Schemes
The key to understanding grounding schemes is to realize all the DC current flowing through the amp must return to the power transformer. For amp's using a conventional rectifier and power transformer center tap the DC current must return to the center tap. For amps using a bridge rectifier the current must return to the rectifier bridge (see the Rectifier section for more detail on this).
The Split Grounding Bus Bar
Splitting the power amp and preamp grounds is currently (no pun intended) the most popular grounding scheme used by amateur amp builders. By far most of the amp's B+ current is used up by the power tube plates so we want a nice "loop" from power transformer, to rectifier, to B+1 filter cap, through the output transformer to power tube plates, to power tube cathode & resistor, to power transformer center tap. That loop needs a ground reference connection to reduce hum so we bolt the center tap to the chassis but no current should flow into the chassis--it flows through the loop from the transformer, through the amp circuit and back to the transformer.
Note the split "Power Amp Ground Bus" and "Preamp Ground Bus" in the 5E3 Deluxe layout. The power amp is connected to the chassis at the upper right power transformer bolt. The preamp is connected to the chassis at the upper right input jack.
Since the power tube screens are part of the power amp we usually connect the B+2 (power tube screens) filter cap to the same power amp chassis ground point--but again the current is flowing in that loop back to the power transformer, not into the chassis.
The B+3 (preamp) filter cap provides power to the rest of the amp and its current must also make its way back to the power transformer center tap by way of the ground connection at the input jack. So the preamp's DC current must flow through the chassis from the input jack to the power transformer's center tap. We do it this way to try to prevent interaction between the high current power amp and low current preamp but eddy currents in the steel or aluminum chassis can add noise and hum.
Actual DC Electron Flow Through the 5E3
The actual flow of electrons is opposite the direction of conventional current because Benjamin Franklin guessed wrong on the direction of electrical flow.
Gray arrows show the actual DC electron flow through the amp (flows negative to positive) which is opposite of "conventional" current direction. Electrons flow from the power transformer center tap to chassis ground to all the tube cathodes. DC electron flow is very slow and takes about 170 hours to make a 600 inch loop around an amp circuit which includes the length of the power transformer secondary winding.
Actual DC Electron Flow With Single Point Bus Ground
This single point ground bus is only grounded at the upper right input jack. The other three input jacks and speaker jacks must be isolated from the chassis and connected to the ground bus. DC electrons flow in a circular motion around the amp.
Electron flow starts with the black Hot and white Neutral power cord wires at far left. Electrons alternate moving back and forth under AC voltage. They really don't move so much as vibrate because they only move about .00001 inch each AC cycle. These vibrating electrons in the power transformer primary winding induce AC electron vibration in the transformer secondary. The high AC voltage from the power transformer secondary tries to push and pull electrons to the rectifier tube but the rectifier acts as a one-way valve that only allows the vibrating electrons to move in one direction so electrons are pulled out of the rectifier's cathode into the transformer.
So the transformer and rectifier act as an electron pump that pulls electrons out of the B+ wire connected to the rectifier's cathode at pin 8. Pulling electrons out of the B+ wire creates a positive DC voltage (scarcity of electrons = positive voltage). Although the power transformer puts out an AC voltage, the only current flowing through the high voltage secondary is DC--the electrons flowing from the rectifier to the transformer center tap. For amps with bridge rectifiers, which do not need a transformer center tap, there will be AC current flowing in the high voltage secondary. See the Rectifier section for more info on how conventional and bridge rectifiers work.
The electrons pulled out of the B+ wire flow through the power transformer's high voltage secondary leads and out its grounded center tap to the ground bus bar. The center tap is the source of all DC electrons that flow through the amp (the guitar signal is AC). Most of the amp's electrons will flow from the ground bus through the power tube cathode resistor and on to the power tube cathodes. Then they flow out the plates and through the output transformer primary then to the B+1 power node, standby switch and back to the rectifier tube for a complete "lap" around the amp. During the amp's operation electrons are constantly making this circular trek around the amp. Electrons captured by the power tube screen flow to the B+2 node and on to the rectifier. Electrons also flow through the preamp tubes' cathodes and out their plates to the B+3 wire and back to the rectifier.
Big Picture Amp Power
The electric company's generators (spinning at 60Hz = 60 revolutions per second = 3600 revolutions per minute) push and pull electrons (alternating current or AC) through wires to your wall socket. You connect your amp's power transformer primary winding to the wall socket and electrons are pushed and pulled through the winding. The transformer's iron core captures the magnetic field (flux) generated by the primary winding and induces a stepped up higher voltage AC in the secondary winding. The secondary winding is connected to the 5Y3 rectifier's two plates. Every AC half cycle one plate is charged positive (scarcity of electrons) while the other plate is charged negatively (excess electrons). The negatively charged plate does nothing while the positively charged plate pulls electrons from the cathode and the B+ supply wire attached to it (removing electrons from the B+ wire creates a positive voltage).
During the next AC half cycle voltage on the two plates is reversed. One plate is charged positive and pulls electrons from the cathode while the other plate does nothing. Because the 5Y3 has two plates and pulls electrons during both halves of the AC cycle it is a 'full wave' rectifier and because it only 'pulls' electrons they flow in only one direction which makes the current DC (direct current). A rectifier with just one plate and cathode would only pull electrons during half the AC cycle so it would be a half wave rectifier. The single diode in bias power supplies is a half wave rectifier.
Fender Tweed 1959 5F6-A Bassman Schematic With Signal Path and Component Function
The 5F6A Bassman is a truly classic amp. Major differences from the 5E3 Deluxe include a tone stack with bass, treble and mid controls and a tone stack buffer (V2B) just before the tone stack. The long tail pair phase inverter adds gain and helps drive the big 5881 fixed bias power tubes into full distortion. The amp also came with much larger value filter/reservoir caps to drive the lows from a bass guitar. Click on image to see the full size (readable) graphic.
How Cathode Bypass Capacitors Work
Cathode Bypass Cap C5
Cap C5 at the bottom center bypasses the R13 cathode resistor.
Adding a bypass capacitor to a tube's cathode resistor is a common way to boost gain. Without a bypass capacitor electrons flow from ground through the R13 cathode resistor to the tube's cathode (3), through the grid (2) and out the plate (1). As electrons are pulled through the cathode resistor a positive voltage is formed on the cathode which is used as the tube's bias voltage. In the circuit above (5E3 Deluxe second gain stage) about 1.7 volts DC will be found on the cathode.
The larger the positive signal voltage is on the grid the larger the flow of electrons through the cathode resistor and the larger the voltage on the cathode. These increases in cathode voltage and therefore bias voltage acts as a negative feedback mechanism because a higher bias voltage leads to a reduction in bias current. This bias voltage fluctuation helps keep the flow of electrons through the tube in check. This is why cathode biased tubes are called "self biasing."
5E3 Deluxe V1A bypass cap.
Adding a bypass capacitor around the cathode resistor reduces this negative feedback because the bypass capacitor acts as an electron reservoir. When the tube's grid goes positive and flows more electrons through the tube it's much easier to pull electrons from the bypass cap reservoir than pull them through the cathode resistor. The freer flow leads to more electrons flowing through the tube so more gain is generated.
When the grid goes negative and slows the flow of electrons through the tube the capacitor is recharged and is ready for the next positive grid electron demand.
How the Bassman's Long Tail Pair Phase Inverter Works
The Long Tail Pair (LTP) Phase Inverter (also called the cathode-coupled phase inverter) is the most popular phase inverter in guitar amplifiers due to its large output voltage swing and sweet overdrive tone. Unlike the non-amplifying cathodyne phase inverter in the 5E3 Deluxe the LTP phase inverter not only creates a dual mirror image signal stream but it also acts as a gain stage boosting the signal by about half of what two normal triode gain stages would. This added gain gives its output more voltage swing to drive big bottle power tubes to a fully distorted state. The LTP is a true differential amplifier and uses both halves of a triode (usually a 12AX7 or 12AT7).
Signal flow shown with red arrows. Component numbers on the schematic and layout match. The signal enters the phase inverter at V3A's grid and flows out both its plate (inverted signal) and cathode (non-inverted signal). The cathode signal flows to V3B's cathode where it is amplified.
For simplicity I will refer to the upper V3A triode as the "upper triode" and V3B as the "lower."
The upper triode in the schematic above has a dual function. It acts as a normal gain stage by outputting an inverted signal at its plate but also acts like a cathode follower by outputting a non-inverted signal at its cathode.
In the schematic above the AC input signal flows through coupling capacitor C19. Cap C19 blocks the upper triode's 32.5v of DC grid voltage out of the tone stack. The signal then flows onto the upper grid while the lower grid is held at a constant DC voltage and all AC signal is sent to ground through shunt capacitor C20.
The upper and lower cathodes are tied together. All of the lower triode's input signal flows from the upper cathode. With the lower grid held constant, signal voltage fluctuations on the lower cathode alter the electron flow from it to the plate which creates an amplified signal on the lower cathode's plate.
R36 is the tail resistor that creates the relatively high voltage (34v DC in the Bassman) needed for the cathode follower function of the upper cathode. It also supplies a near constant current flow shared between the two cathodes--as current increases through the upper cathode the current decreases through the lower and vice versa.
R34 is a standard bias resistor and creates a 1.5 volt difference between both tubes' grid and cathode.
R37 and R38 are simply grid leak resistors which leak off DC grid current to maintain a steady DC bias voltage between the grid and cathode.
The plate load resistors R39 and R40 are different values to balance the difference in gain between V3A and V3B.
The negative feedback signal from the output transformer's 2 ohm speaker tap is injected into the LTP phase inverter in two places: the lower grid through C20 and between the R36 tail resistor and presence pot which leads to the cathode. The negative feedback signal on the lower grid counteracts the signal on the upper grid resulting in negative feedback attenuation. Injecting the NFB signal at R36 helps balance the feedback signal between the upper and lower triodes.
The Presence control (R35 and C21) removes a variable amount of high frequency from the negative feedback signal. Reducing negative feedback has the effect of boosting output so reducing the high frequencies in the negative feedback signal boosts high frequency output (C21 shunts the high frequency AC NFB signal voltage to ground). Increasing C21's capacitance value will lower the cutoff frequency and bypass a larger range of frequencies to ground therefore boosting a larger frequency range at the speaker output.
Capacitor C22 suppresses oscillations above audio frequencies between the two triodes' plates to help stabilize the circuit.
Function Detail: When a positive voltage signal arrives at the upper grid the reduction of blocking negative electrons on the grid allows electrons to flow from its cathode, through the grid, to its plate. The electrons flowing onto the plate lowers the plate voltage--this is the inverted and amplified output signal. As electrons leave the upper cathode a positive voltage is created on the cathode (scarcity of electrons = positive voltage) caused by the voltage drop across the cathode resistor R34. This positive signal voltage is also present in the lower cathode because the two cathodes are directly connected. Since the lower grid is held constant at 0 volts AC, any change in its cathode voltage will create a voltage difference between the grid and cathode. This voltage difference changes the flow of electrons from the cathode, through the grid to the plate. As the lower cathode goes positive (scarcity of electrons) fewer electrons will flow from it through the grid to the plate. The reduction of electrons flowing onto the plate raises the plate voltage--this is the non-inverted and amplified output signal.
Bassman 5F6-A LTP PI Voltages
Note the voltage difference between the tail resistor junction of 32.5v and the cathodes at 34v equaling a normal bias for a 12AX7 of 1.5v. The voltage difference between the grids (22 and 23v) and the resistor intersection (32.5v) is measurement error caused by voltage probe circuit loading. If the voltage difference were real the tube would be in cutoff.
How Spring Reverb Works
Old school spring reverb literally uses springs to delay and replicate a signal. Reverb simulates the reflected sound from a room's interior. In the schematic below the audio signal enters on the upper left and flows through a 500pF Reverb Filter capacitor which filters out most of the guitar signal's low frequencies. The low frequencies are too long and turn to mud when reverb is applied. The filtered signal then gets boosted by the Reverb Driver amplifier. The Reverb Driver is needed to generate the power to physically move the reverb springs.
After the Reverb Driver the amplified dry signal is then sent through the Reverb Transformer which trades high voltage for current. Amplified current is needed to drive the reverb tank input transducer. The tank's Input Transducer is simply an electromagnet used to move the spring. The amplified audio signal flows through the input transducer's coil which generates a magnetic force. The magnetism generated in the coil is alternately attracted to and repulsed by the transducer's magnet which makes it and the attached springs move. The movement travels down the Springs and causes movement of the Output Transducer magnet at the other end. The moving magnet's magnetic field cuts through the transducer's output coil which generates the 'wet' reverb signal voltage. In other words, the Input Transducer transforms electrical energy into mechanical movement. The Output Transducer transforms mechanical movement into electrical energy. The weak wet signal generated by the transducer output coil is then amplified by the Recovery Amplifier and flows through the Reverb Level pot and back to the amplifier.
'65 Princeton Reverb Spring Reverb Circuit
Guitar signal enters at upper left and gets amplified by the Reverb Driver (both stages of a 12AT7 tube in parallel) then flows into the Reverb Transformer which trades high voltage for current. The amplified current is sent to the Reverb Tank's Input Transducer (see tank detail below). The weak 'wet' signal from the tank's Output Transducer is amplified by the Reverb Recovery amp and passed through the Reverb Level control and back to the guitar amplifier.
Reverb Tank Detail
Signal enters tank on left and exits on the right. The Input Transducer's input coil moves the transducer magnet which moves the spring which moves the output transducer magnet which generates the reverb signal voltage in the output coil.
A reverb tank is simply made up of two transducers connected by two or more springs. The longer the tank the longer the reverb reflections.
The time it takes for the spring movement to travel from input transducer to output is the reverb delay. Multiple springs with slightly different makeup add multiple delays simulating sound reflections from multiple room features. The original spring movement doesn't actually stop at the output transducer. A diminished 'wave' is reflected back along the spring toward the input transducer, bounces off it and returns in weakened form to the output transducer generating multiple diminishing reverb 'reflections.'
AB763 Signal Tremolo Circuit
Tremolo circuit from the Fender blackface AB763 Deluxe Reverb. Tremolo is applied to the guitar signal at the phase inverter input instead of the power tube bias like the preceding small power tube tremolo circuit. V5A is the oscillator on the left and V5B is the tremolo driver on the right.
The AB763 tremolo circuit acts as an automatic wavering volume control and acts directly on the guitar audio signal. The guitar signal is tapped just before the 220k channel mixing resistor and phase inverter. The tremolo circuit bleeds the guitar audio signal to ground by a light dependant resistor (LDR) in the opto-isolator (or opto-coupler, often called the 'roach' because it looks like a bug). The opto-isolator is made up of a neon light bulb and a light dependant resistor. [We could call the neon bulb V10 because it is a single electrode tube ;) ] When the neon bulb is dim the LDR is at high resistance and less guitar signal is bled to ground. When the neon bulb is bright the LDR resistance is low and more signal is bled to ground.
The triode on the left is the Oscillator and its grid is connected to its plate through three oscillator caps that cause the plate voltage to oscillate. It's oscillation flows to the grid of the triode on the right, the Tremolo Driver. The Tremolo Driver amplifies the oscillation to drive the neon light bulb. The oscillating current from the driver tube plate runs through the neon light bulb causing it to oscillate in brightness from bright to dim. As the brightness of the neon bulb changes the output volume of the amp changes with it.
The Intensity control (lower right in schematic above) alters the amount of the guitar signal that is sent to the light dependant resistor (more signal sent to the LDR = more intense tremolo). The Speed control (upper center) alters the resistance in the first RC (resistor capacitor) pair which changes the RC's capacitor charge time which changes the tremolo circuits oscillation frequency (more resistance = slower charging = slower tremolo). The Tremolo Pedal control provides the ground needed for the oscillator to operate so if no tremolo pedal is plugged in the tremolo is off.
Note how the -55 volt power tube bias voltage is used as the oscillator tube's grid leak to jump start the tremolo oscillation when the foot switch is closed (turned on). An open foot switch (off) puts the oscillator grid at -55 volts which shuts down the oscillator by stopping all flow through the triode. A closed foot switch instantly removes the -55 volts from the oscillator grid and this sudden change in voltage instantly jump starts the oscillation.
Some Fender amps use "bias wiggle" tremolo. They work by wavering the power tube bias which wavers the power tube output.
For more information on how this tremolo circuit operates see Clark Huckaby's explanation.
How Ultra-Linear Output Transformers Work
If you power the power tube screen grids (g2) from the output transformer center tap (B+) the tubes function as normal pentodes since the screen voltage will be fixed at or near B+.
But if you power the screens with the ends of the output transformer winding (same voltage as the power tube plates) the tube functions as a triode because the plates' and screens' voltage will fluctuate together.
The ultra-linear output transformer taps are part way between the center tap and winding ends so the power tubes function as triode-pentode hybrids. The screen voltage will fluctuate with the plates but with much less amplitude. This results in more linear amplification (less clean signal distortion) and is used extensively in stereo audio tube amplifiers.
Ultra-linear is such a fundamental change to how a power tube operates that it does affect both the amp's clean and overdrive tone. Some Fender silver face amps use ultra-linear and they tend to be loud, clean and excellent pedal platforms.
This diagram from Richard Kuehnel's Vacuum Tube Circuit Design: Guitar Amplifier Power Amps shows the ultra-linear taps connected to the power tube screens:
The problem is ultra-linear operation reduces power tube distortion which usually sounds better than preamp distortion. This shifts the balance of preamp and poweramp distortion toward the preamp which is just not a good thing.
You can simply ignore the ultra-linear taps on an output transformer by disconnecting and insulating them and then powering the screen grids like normal guitar amps with an RC filter or choke between the center tap power node and screen grid node.
You can also put the screen grid connection on a switch and power them from either the ultra-linear taps or from their own power node. Use ultra-linear mode for clean headroom and to handle high output effects pedals and use the normal mode for sweet power tube distortion.
Understanding impedance was one of the last things I figured out about guitar and amp electronics.
Impedance, whose symbol is Z, is only a factor in alternating current (AC) circuits so in a guitar amplifier impedance applies to the guitar audio signal voltage and not the DC power supply.
Impedance is made up of three things that impede or restrict AC current flow: resistance, inductance and capacitance but its easier if you just think of impedance as "AC resistance." We'll leave inductance and capacitance to the end of this discussion to simplify things. The term "impedance" can apply to both signals and circuits.
A low impedance signal has relatively low voltage but high current.
A low impedance circuit is a heavy load circuit that allows AC current to flow easily.
A high impedance signal has high voltage but low current. Current flow is restricted in high impedance circuits.
A high impedance circuit is a low load circuit that restricts AC current flow.
You can think of a low impedance signal as a slow moving, deep river and a high impedance signal as a shallow, fast moving stream. I also visualize a high impedance signal as "thin" and a low impedance signal as being "thick."
Let's take a look at a guitar circuit:
Simple Guitar Circuit
The Pickup on the left generates the guitar signal. The Volume Pot bleeds guitar signal to ground to lower the guitar's output volume.
In the circuit below we short the guitar output jack and turn the volume pot all the way up so there's very little to restrict electron flow from one pickup coil wire, around the circuit and back to the pickup. This is a very low impedance circuit with lots of current flow and little restriction to generate electron pressure or voltage:
Low Impedance Circuit
Electrons easily flow from the pickup coil, around the circuit, back to the pickup so we have high current and low voltage. The guitar's AC output audio signal will have very little amplitude but lots of current will flow which makes it a low impedance signal.
In the circuit below we turn the volume pot all the way down so the electron flow from the pickup has to flow through the pot's 500 kilohms of resistance. The circuit is now a high impedance circuit. A small amount of current flows because of the volume pot's restriction. This restriction causes the electrons to stack up at the pot which generates electron pressure or voltage across the pot:
High Impedance Circuit
Electrons stack up trying to flow through the volume pot's 500k of resistance which reduces current but increases the voltage across the volume pot. The guitar's AC audio signal at the pot's input will have high amplitude but little current flow which makes it a high impedance signal.
You've probably heard how a tone stack can "load down" an upstream preamp circuit. The preamp tube's AC output audio signal is a high impedance signal with lots of voltage swing but very little current driving those voltage swings. If the following circuit has a low impedance it will "eat up" or load down the audio signal's current. The low impedance circuit allows too much current to sink to ground so the amplitude of the audio signal voltage swings are reduced and attenuated. That's what happens in the tone stack when you turn the tone controls down. The tone stack becomes a low impedance circuit and too much AC signal current can be bled to ground lowering the amp's output volume. User's don't want their volume to change when they change a tone control setting.
As I said in the beginning, impedance is made up of three things that impede or restrict AC current flow: resistance, inductance and capacitance.
Inductance is generated by AC signals flowing through coils so a guitar's pickups and an amp's chokes and transformers add impedance to its circuits.
Capacitance in a circuit acts as a drag on AC signals because the signal looses energy moving electrons on and off the opposing capacitor "plate." The "plate" can be the actual plate of a capacitor, the ground shield in a guitar cable or a nearby wire in an amp. There's even capacitance between the electrodes and grids in a tube.
Tube amp gain stages typically put out high impedance signals (high voltage, low current, thin).
Input and Output Impedance Bridging
It took me a long time to understand why input and output impedance matters in amplifier circuits. Here's my layman's explanation:
The maximum power-transfer theorem says to transfer the maximum amount of power from a source (guitar) to a load (amplifier), the load impedance should match the source impedance (a.k.a. Impedance Matching). Remember power = volts x current.
Impedance Matching is used for maximum power in the power amp circuit where the power tube output impedance is matched to the speaker impedance through the output transformer. But at the amp's guitar signal input we're not concerned with maximum power transfer, we want maximum voltage transfer because a guitar amplifier amplifies a signal voltage--the voltage changes are the audio signal. We really don't care about the current generated by a guitar's pickup coil, it's the coil's voltage changes that will be amplified by the preamp tubes. Since we favor voltage over current we can use an impedance mismatch to trade current for voltage. This intentional impedance mismatch reduces the current from the guitar but boosts the signal voltage the amplifier receives.
Guitar Output Impedance & Amplifier Input Impedance
Guitar circuit on left, amplifier input on the right. The current generated by the guitar pickup coil must be allowed to loop back to the guitar pickup. The "Amplifier Input Load Impedance" is typically a 1 megaohm input resistor mounted on the amp's input jack.
In the diagram above the guitar's pickup coil generates a signal voltage. The guitar's output impedance is made up of the resistance of the pickup coil, volume and tone controls + the guitar circuit's capacitance and inductance. Since we can't control the output impedance of the guitar we can maximize the signal voltage by making the amplifier input impedance as large as possible, preferably 10 or more times greater than the guitar's output impedance (Rule of 10).
A high input impedance reduces the current flowing through the amplifier but increases the signal voltage level. It also reduces distortion because the guitar's coil can output less current. This is called high impedance bridging and is used extensively in electronic audio circuits. The Input Resistor R1 on the 5F1 amplifier's guitar Jack 1 adds input impedance to boost the signal voltage from the guitar.
The same principal applies between amplifier stages. The output stage driver tube V1B sends the guitar signal voltage to the power tube V2. Low output impedance from the driver tube and high input impedance from the power tube boosts the signal voltage. This is what resistor R9 (power tube grid leak) does, it sets a high input impedance to the driver tube V2. Back to Signal Input.
Impedance Mismatch Between the Power Tube and Speaker
As mentioned earlier to get the most power out of the amplifier the power tube and speaker should match their impedance. But what happens when they don't match?
If your output transformer is designed to match your power tubes to an 8 ohm speaker and you connect a 4 ohm speaker (4 ohm load and 50% decrease) then the impedance as seen by the power tube plate decreases by 50% too. Less impedance will cause plate current to increase. The power tubes are stressed by this increased plate current so the power tube lifespan can be shortened. This is especially true of Class A amps because they idle near max current flow. Since the plate current idles near maximum, the entire power supply also runs at maximum output so the rectifier tube and power transformer will run hotter. Power filtering effectiveness is reduced as current demand increases so hum and noise may increase, especially in Class A amps. Increased hum can cause ghost notes which are caused by hum interacting with musical notes to create false harmonic tones.
Since the power tube and transformer are not coupled at maximum efficiency some of the plate current turns into heat inside the output transformer so the output transformer runs hotter.
Many vintage Fender tube combo amps have an aux speaker jack that's wired in parallel with the built in speaker. Plugging in a speaker rated at the same ohms as the built in speaker will cut the speaker load in half for a "one step" low mismatch. Fender designed their output transformers to handle this and is considered safe.
But there are two possible sonic improvements with a power tube and speaker low mismatch. Sweet sounding second harmonic distortion in the power tube increases and not-so-great sounding third harmonic distortion decreases dramatically. This is why it's worth experimenting with different speaker loads, you may like the tone you get.
If you connect a 16 ohm speaker to your 8 ohm output transformer the impedance as seen by the power tube plate increases and plate current decreases which can lengthen the lifespan of your power tubes, especially in Class A amps. This decrease in plate current will decrease demands on the power transformer and it will run cooler. Power filtering effectiveness is increased as current demand decreases so hum and noise should decrease, especially in Class A amps. Decreased hum can help prevent ghost notes.
Since the power tube and transformer are not coupled at maximum efficiency the amp's power output is reduced by about 12% and some of the plate current will turn into heat inside the output transformer so the output transformer will run hotter.
We get the opposite of the sonic improvements of a low impedance mismatch: Sweet sounding second harmonic distortion in the power tube decreases and nasty sounding third harmonic distortion increases.
The main problem with a high impedance mismatch is flyback voltage generated in the output transformer which can damage the power tubes and the output transformer itself. The flyback voltage spikes can cause the power tube to arc between pins or burn through the thin lacquer wire insulation used in the transformer windings. This is normally not a concern when going "one step" away from a match such as running a 4 ohm output transformer with an 8 ohm speaker unless the output transformer is cheaply made or really old. But running the 4 ohm transformer with a 16 ohm speaker can generate very high flyback voltages when running the amp hard near max volume.
If your power and/or output transformers run hot with a matched output transformer and speaker load then mismatching them is more of a risk. The bottom line is the greater the low impedance mismatch the harder your amp works, and the greater the high impedance mismatch the more likely you are to burn out your output transformer and/or power tubes. A low mismatch is typically safer than a high mismatch.
How Humbucking Guitar Pickups Work
Humbucking pickups are really two single coil pickups connected together in a way that cancels noise and doubles the pickup output voltage. Humbuckers use common mode noise rejection to reduce hum and other electromagnetic noise picked up from surrounding sources like fluorescent lights. Any noise that gets into both pickup coils is nullified when the two signals from the two coils are combined because the noise signals from the two coils are of opposite phase.
In other words the positive noise signal is added to the negative noise signal and they wipe each other out. We get a "positive" coil and "negative" coil by simply winding the wire in the opposite direction.
Why doesn't the guitar string signal get nullified like the noise? Because of the opposite magnetic polarity of the pickups' pole pieces. One pickup has North up magnets and the other has a South up magnets. This flips the signal polarity of one coil so the string signals are in phase, not out of phase like the noise. The string signals from the two coils are added together instead of being subtracted and nullified.
Just like two humbucker pickup coils, two separate pickups can be paired together to work as humbuckers and reduce noise if the two pickup coils are connected the right way.
You need two things to get hum cancelation in a humbucker pickup or between two single coil pickups: Opposite electrical current flow in the two coils--one coil's string generated current flows clockwise, the other coil flows counter-clockwise. The two pickups must also have opposite magnetic polarity (North+South).
The two coils of the bridge humbucker circulate electric current generated by the strings in opposite directions. The black arrows show the direction of current flow around the pickup coils. The same is true when you select switch position 2 which pairs the right bridge humbucker coil with the mid pickup and switch position 4 which pairs the mid and neck pickups. The magnetic poles of the left humbucker coil are South up and the right coil is North up. In switch position 2 the North bridge coil is paired with the South mid pickup. In switch position 4 the South mid pickup is paired with the North neck pickup. 5-Way switch positions are: 1 Bridge Humbucker, 2 North Humbucker coil + South Mid Pickup, 3 Mid Pickup, 4 South Mid Pickup + North Neck Pickup, 5 Neck Pickup.
It is pretty obvious when you pair two coils out of electrical phase (both coils' electrical flow running in the same direction)--the guitar tone is weak and tinny because the two string signals are subtracting and nullifying each other. The solution is easy, just swap one of the coil's two hookup wires to reverse its electrical flow.
It's much harder to detect by ear when a pair of coils are in phase electrically but out of phase magnetically--they will sound normal when paired but there will be no hum cancelation so you may notice some 60Hz electrical hum. The easiest way to check the magnetic polarity of two pickups is to carefully move the two pickups close together--top pole to top pole. If they magnetically repel each other they are the same polarity and will not hum cancel. If they attract each other they are opposite polarity and will hum cancel when paired. You can also purchase a $9 Schatten Design Magnetic Polarity Tester from Stewmac which makes checking installed pickups a breeze.
Many pickup manufacturers will change the magnetic polarity of their pickups for no charge. I had to do this on my Telecaster to get the mid and neck pickups to hum cancel. Note that if you change the magnetic polarity of a pickup the current flow direction will also reverse so you will have to swap its hookup wires to keep the same electrical polarity. Be aware that some aftermarket pickups have the opposite magnetic polarity as modern Fender pickups so when purchasing pickups that will be paired for humbucking make sure you know what magnetic polarity you're ordering.
Normal Humbucker Magnetic Polarity
Note the magnet on the bottom in contact with the Screw and Slug poles. Note how the bar magnet, slugs and screws all have a North and South pole.
Transistor Equivalent Circuits
Here's two transistor circuits that look and function very much like tube amp circuits. The first is the transistor common emitter amplifier:
Tube triode amp circuit on the left, transistor common emitter amp on the right. Remove transistor resistor R2 and this circuit looks just like a tube amplifier stage. The two circuits serve the exact same purpose.
The big difference between the tube and transistor amp circuit is the addition of resistor R2. Resistors R1 and R2 form a voltage divider which sets the circuit bias. Resistors RC and RE determine the amplifier gain and set the max current. Capacitor CE is a standard bypass cap to boost gain. Capacitors C1 and C2 are standard coupling caps.
Here's the transistor emitter follower. It bears an uncanny resemblance to a tube cathode follower:
Triode tube cathode follower on the left, transistor emitter follower on the right. Remove transistor resistor R1 and this circuit looks just like a tube cathode follower. The two circuits serve the exact same purpose.
Again, the big difference between the tube and transistor amp circuit is the addition of resistor R1. Resistors R1 and R2 form a voltage divider which sets the circuit bias. A transistor emitter follower can be used as an easy to implement tone stack buffer or effects loop buffer because they don't need a cathode heater.
How Voltage Dividers and Pots (potentiometers) Work
When I first got into electronics I had a hard time understanding voltage dividers but they are really pretty simple. Understanding them is important because a tube amp is full of them. Even tone controls and tone stacks can be thought of as voltage dividers.
Note how there is 100 volts at the top of the resistors and 0 volts at the bottom. The upper resistor is R1 and the lower is R2.
A voltage divider literally divides voltage. Take a look at the diagram above. We have two 500k resistors with 100 volts at the top and 0 volts (ground) at the bottom. If we tap the resistors at the top we get 100 volts, tap the resistors at the bottom and we get 0 volts. Now if we tap the resistors at the middle, half way down at 500k, we will get half the voltage, 50 volts. Tap higher and the voltage moves toward 100v, tap lower and the voltage goes down toward 0 volts.
Note the equation at the bottom of the diagram above. We can calculate the voltage out of the voltage divider by multiplying the Voltage in * R2 / (R1 + R2). For our voltage above we get: 100v * 500,000 / (500,000 + 500,000) = 100v * .5 = 50 volts out. The R2 / (R1 + R2) determines the ratio of the upper to lower resistor. If you make the lower resistor smaller more voltage will be bled to ground so you will have less voltage out.
If we made the top resistor R1 250k and the lower resistor R2 750k we know less voltage would be bled to ground so the output voltage would be higher: 100v * 750,000 / (250,000 + 750,000) = 100v * .75 = 75 volts out.
A potentiometer (pot) is simply a variable voltage divider.
Now take a look at the pot diagram above. Look familiar? It's simply a variable voltage divider. If we turn the voltage tap, or wiper, to the top of the pot we get 100 volts, turn the wiper to the bottom and we get 0 volts. In the diagram the wiper is at 50% so there's 500k above the wiper and 500k below the wiper so the input voltage is cut in half. Now we can tap the voltage by placing the wiper anywhere between the 100 volts at the input, to the 0 volts at the ground connection.
We can use the same equation as the voltage divider above to calculate the output voltage. "R1" would be the pot resistance on the input side of the wiper and "R2" would be the resistance on the ground side. For example we turn the 1M linear pot 1/4 turn up from "off" we know that 1/4 of 1M = 250k. That 250k is movement up from off or 0 volts so R2 = 250k and R1 would equal 750k. Voltage out = 100v * 250,000 / (250,000 + 750,000) = 100v * .25 = 25 volts out.
Ohm's Law Pie Chart
It's much easier for me to remember this stuff if I use A, V, R and W for Amps, Volts Resistance and Watts in these equations. For example the lower left Watts section shows three ways to calculate power: Volts * Amps or Amps squared * Resistance or Volts squared / Resistance.
Rob Hull's Easy to Use Resistor Color Code Reader
Note the 3-band resistor on top and 4-band on the bottom.
Common amp electronic component symbols.
This webpage is available in .pdf form: How_Tube_Guitar_Amps_Work_by_Rob_Robinette.pdf
By Rob Robinette