How Tube Amplifiers Work
By Rob Robinette
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. Although this page discusses Guitar tube amps everything here applies to audio stereo tube amplifiers too with the goal of distortion prevention in audio amps being the biggest difference.
5F1 Champ Amplifier
Volume 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.
WARNING: A tube amplifier chassis contains lethal high voltage even when unplugged--sometimes over 700 volts AC and 500 volts DC. If you have not been trained to work with high voltage then have an amp technician service your amp. See more tube amplifier safety info here.
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 but "OT In" are the output transformer primary wires and "OT Out" are the secondary wires.
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. I have had questions about the grounding scheme shown in this layout. Grounding the V2 power tube grid leak (R9) and the power tube cathode resistor (R8) and cathode bypass cap (C6) to the first filter capacitor's ground (C3) is best practice and should result in a quieter amp. Click the image to download the pdf.
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 (orange arrows above, fat red line below). 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 in red 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. Click the image for the full size schematic. Click here for the clean schematic.
These are pretty much all the symbols you'll need to know to read tube amp schematics.
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 guitar 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 a wire coil that generates the guitar signal. The Tone Control bleeds high frequencies to ground. The Volume Pot is wired as a 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 one other--they want elbow room. The tighter they are packed together the higher the pressure so I like to think of negative voltage as electron pressure.
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 -. People say electric current flows + to - (conventional current flow) but electrons flow - to +. With tube electronics it's easier to think in terms of how the electrons are really moving in order to understand them.
If you connect a wire across a battery's terminals the jammed together electrons in the negative terminal see the wire as a pipe with lots of room so they flow down the wire. When you have an 'excess' of electrons tightly packed together you have negative voltage. When you have a 'scarcity' of electrons, or electrons are pulled apart from one another, you have a positive voltage.
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. Ground or earth represents an unlimited supply of electrons at zero volts (or neutral voltage). Touch that high voltage wire wire to a ground and the electrons hanging out there will rush in to fill the void of electrons in the 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.
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 string vibration 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 electron pressure (voltage) and electron flow (current) 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 is why guitar audio signals are AC, or alternating current--as the strings alternate their direction of travel the signal voltage alternates between + and -. 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.
An AC guitar audio signal on a wire alternates between positive and negative 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. Remember, voltage = electron pressure.
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 simply 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. Every little twitch on that line makes the speaker cone twitch. 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 sets the amp's input impedance to 1,000,000 ohms (1M) 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.
I recommend you now read How Tubes Work and come back here when finished.
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 0.022uF written on the cap is it's rating of 0.022 microfarads (0.000,000,022 Farads). In some old documents you'll see "micro-micro" or "uu" which means pico Farad.
Use this chart to help convert capacitor size such as: .1uF = 100nF and 1nF = 1000pF.
Capacitors are made of two conductive plates separated by an insulator or dielectric. Common dielectrics are mica, polypropylene, ceramic and even paper and oil.
How capacitors block DC but let AC pass: Caps are made with sandwiched but separated conductive plates. The separated plates cannot flow DC current but AC fluctuates between positive and negative voltage. When the AC guitar signal negative voltage (excess electrons) is applied to the input plate the electrons repel electrons on the output plate so they move off the plate and flow out of the capacitor. When a positive voltage is applied (scarcity of electrons) to the input plate the output plate attracts electrons so electrons flow into the capacitor.
That's how capacitors really work 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.
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 R7. 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 out the blue wire 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 guitar 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. For you mechanical types you can think of the output transformer as a gearbox that alters speed (voltage) and torque (current). The power tubes send large voltage swing into the output transformer and you can think of this as high speed from an "engine". The transformer gears this high speed (high speed, low torque=high voltage, low current) down for much less speed (less voltage swing) but much more torque (current).
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, decrease damping factor 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", size 3AG, 1/4 inch (6mm) wide by 1 1/4 inch (30mm) long. The fuse is a 250 volt, 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 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 325 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 325 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. 325V AC on each high voltage secondary wire 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" current 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+ 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. Here's an online RC Ripple Filter Calculator.
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 stages 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.
Although the Champ does not use a choke many amps do use them to filter the power supply. Typically the choke is placed between the power tube plate and power tube screen power nodes. This is done as a cost savings measure. A choke would have to be very large and expensive to filter the entire power supply for a 50 or 100 watt amplifier.
When electrical current flows through a wire it creates a magnetic field around the wire. Chokes are inductors that use this magnetic field to reduce changes in voltage and current. When no current flows through a wire (you can have voltage but no current) there is no magnetic field generated around the wire. When current increases some of the current will be used to grow a magnetic field around the wire. When current decreases the magnetic field shrinks and the magnetic energy is converted into electrical current. These inductor properties "fight" current changes.
A choke is simply one long wire wound in many loops. A choke will have two leads which are simply the two ends of the one long wire. The wire is usually looped around an iron core which makes it work better. Looping the wire increases the effect of the induced magnetic field ("inductor" comes from the word "induced").
When power supply voltage ripple flows through a choke, the choke "fights" the ripple. As ripple voltage increases, ripple current increases through the choke. The choke will convert some of the current increase into a magnetic field. The choke's magnetic field is stored energy. As the power supply ripple voltage decreases the choke's magnetic field collapses and converts into current. So increasing voltage and current are cut, and decreasing voltage and current are reinforced which reduces the amplitude of voltage ripple.
This simple looped wire is a small air core inductor.
Inductors in AC circuits work the same way. Inductance "fights" changes in current and AC audio signals are made up of voltage and current changes. A small inductor can remove high frequencies. A larger value inductor can remove medium and high frequencies. A very high value inductor, like the choke in power supplies, can remove all audio frequencies.
How is it that the 5F1 Champ uses 0.75 amps of 6.3 volt current, 2 amps of 5 volt current and 42ma of high voltage current and yet it uses a 2 amp fuse? It's due to power conversion.
This is the power conversion for the 5F1 Champ
First we will convert the current from amps to watts:
The Champ's 5Y3 rectifier tube datasheet shows it uses 2 amps of 5v heater current: 2a x 5v = 10 watts (this is also be called 10 VA)
The 6V6GT power tube datasheet shows it 0.45 amps of 6.3v heater current: 0.45a x 6.3v = 2.835 watts
The 12AX7 datasheet shows it uses 0.3 amps of 6.3v heater current: 0.3a x 6.3v = 1.89 watts
The amp uses 42ma of high voltage DC.
We have a typical 19v drop across the 470 ohm power tube cathode resistor: 19v / 470 = 40ma
We have a 1.5v drop across both of the 12AX7 cathode resistors: 1.5v / 1.5k = 1ma each, 2ma total
42ma x 370v = 15.5 watts
Total the watts: 10w + 2.835w + 1.89w + 15.5w = 30.225 watts of power used by the power transformer secondaries.
Now we convert the secondary watts to primary watts:
If our mains voltage is 125 volts then 30.225w / 125v = 0.24 amps of mains current. We can estimate that the transformer looses about 10% for a total of 0.26 amps, so a 2 amp fuse has a lot of headroom for turn-on power surge.
If our mains voltage is 240 volts then 30.225w / 240v = 0.13 amps + 0.01 amp efficiency loss for a total of 0.14 amps, so a 1 amp fuse could be used.
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
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