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How Guitar Tube Amplifiers Work

The non-electronic tech's guide to understanding electric guitars and amplifiers.

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 Fender 5F1 Champ guitar amplifier. Once you understand the simple 5F1 you'll be able to understand more complicated amps.

5F1 Champ Amplifier

We'll start with an uncluttered Triode Electronics 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.

Triode Electronics 5F1 Champ Guitar Amplifier Layout Diagram

Note the output transformer (OT) is not shown in this diagram.

5F1 Chassis

Controls on top, Circuit Board inside, tubes on bottom: V3 Rectifier Tube on left, V2 Power Tube in center, V1 Preamp Tube on right. The Power Transformer and Output Transformer are attached to the other side of the chassis.

Annotated Layout With Signal Flow and Component Numbers

I added component numbers to this layout that match the schematic diagram below. Tracing the signal flow on this layout diagram and the schematic below will help you understand how this amp works.

Signal Flow Overview

Signal flow is shown above using orange arrows. Signal from guitar enters at upper right guitar jack J1 or J2 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 and on to the speaker. I have added component numbers to this diagram that match the schematic below.

Weber 5F1 Schematic

The signal flow seems much simpler on the amp's schematic. Component numbers match the Annotated Layout diagram above. V1A is one half of tube V1, V1B is the other half.


How Amps Work

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 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.

Guitar Pickup

The black and white wires leaving the pickup are the two ends of the single wrapped coil wire.

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.

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 signal's voltage and current alternate between positive and negative (up and down). The tight wave spacing is an indication of high frequency and pitch. The wave height (amplitude) 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.

WARNING

Amplifiers have large capacitors that store enough electricity to kill even when 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's alternating current audio signal enters the amplifier at guitar jack J1 or J2. J1 is the Hi input and J2 is the Lo, -6dB quieter input. Resistor R1 on Jack2 is the 'input resistor.' It 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 volts.] The '1M' written on R1 is its rating of 1 megaohm. See more on impedance here and see this web page for more information 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. [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.]

First Preamp Stage

Signal from resistor R2 travels down blue wire to tube V1A's grid then out the plate to capacitor C1. Tube V1 is split into two identical halves, A & B.

After going through R2 or R3 the audio signal flows down the blue wire to the preamp tube's pin 2 (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 red wire to capacitor C1, which is a 'coupling capacitor' or 'cap.' Coupling caps are sometimes also called 'blocking caps.' The 0.022F written on the cap is it's rating of 0.022 micro Farads (0.000,000,022 Farads).

High voltage DC (direct current) power used by the tube is brought in through resistor R5, which is a 'load resistor.' The red wire between tube pin 1 (plate) and R5 carries up to 250 volts DC. The red wire carries both the AC audio signal and the high voltage DC power the tube needs. Coupling capacitor C1 allows the AC audio signal to pass through but blocks the DC on the red wire.

Volume and Second Preamp/Output Stage Driver

Signal flows from C1 to the Volume pot then down the blue 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 light blue 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 is a variable resistor. Volume knob left = more resistance = less signal and lower volume. Volume knob right = less resistance = higher volume. [Bonus info: The volume pot also functions as V1B's grid stopper and grid leak resistors]

The signal then flows down the blue wire all the way to tube V1B's pin 7 (grid). V1B is the second half of the preamp tube. This stage has a dual function. It acts as the second amplification stage which helps recover some signal lost by the volume and tone controls. V1B is also the output stage driver which prepares the signal for the power tube. The audio signal leaves tube V1B via pin 6 (plate) and flows up the red 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 to a split: to resistor R9, and down via the light blue 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.

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 also referred to as the output tube. V2 is the final stage of voltage amplification. The signal enters at pin 5 (grid) and leaves via pin 3 (plate). It then goes to the output transformer (OT) which 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 its secondary (output) winding.

The output transformer's windings are really just two coils. The input, or primary coil uses electric current flowing through the coil to generate a magnetic field or flux. This magnetic field fluctuates with the AC signal voltage. The output, or secondary coil uses the primary's magnetic field to generate a current in its coil. You can alter the voltage and current from primary to secondary by changing the ratio of coil wraps from primary coil to secondary.

Transformers

   

Current flowing into the primary winding (above left) induces magnetic flux flow around the transformer core which in turn induces an electric 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 'shell' 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 90 out of phase with one another like this reduces interference and noise.

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 move the speaker cone.

The output transformer sends the higher current signal through the blue 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 force. This magnetic force 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.

Speaker Voice Coil is an Electromagnet

Electric current flowing through the speaker's voice coil generates a magnetic force. When the electric current 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 experience as sound--the sweet sound of electric guitar. For every movement of a guitar string the amplifier generates a corresponding movement of the speaker cone. 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.

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.

Bonus Info: How Microphones Work

Dynamic microphones work exactly in reverse of how a speaker works. It has 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.

Microphone

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 movement. 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 how an electric guitar generates an AC voltage signal.

Negative Feedback

The 5F1 amplifier uses negative feedback (NFB) to reduce distortion and improve stability but a drawback is it also reduces overall amplifier gain. A blue wire running from the speaker jack carries the amplified audio signal through R13 and injects the feedback at V1B's pin 8 (cathode). R13 controls the level of feedback passed to the cathode.

So the main purpose of a guitar amplifier is to take the tiny electrical signal generated by the guitar's pickup 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.


Power Supply

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 (or 100, 220 or 240 volts AC in other countries) runs through the power switch S1, which is located on the volume pot. From the power switch the 120v AC runs to fuse F1. The layout and schematic have the power switch and fuse reversed--having the fuse first is preferred. 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.

Bonus Info: Alternating Current (AC) voltage is normally given in volts RMS (root-mean-square), which is a form of voltage averaging equal to the DC equivalent voltage. Always consider AC voltage as RMS unless it is specified as peak Vp or peak-to-peak Vpp. Our standard 120 volts AC wall power is RMS. You can convert RMS voltage to peak-to-peak voltage by multiplying 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). Wall power goes from zero to +169.7 volts, then down through zero to -169.7 volts (339.4 volts peak-to-peak), then back up to zero 60 times per second. This is why AC electrical noise picked up by guitar amps is described as a 60 Hertz hum (poorly filtered DC from a full wave rectifier will give you 120 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).

120 AC volts RMS (average) wall power equals 339.4 volts peak-to-peak.

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 power supply 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 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 runs to the power transformer (PT), through its primary winding, then back to the wall plug via the white neutral wire. The power transformer must be designed for your wall power (100, 120, 220 or 240 volts). The power transformer has three secondary windings. The first winding steps the 120v AC up to 650 volts AC. Two other small secondary windings step the 120v AC down to 6.3 volts AC and 5 volts AC. The 6.3 volts is used to power the pilot light and heat the preamp and power tubes' heater filaments which heat the tubes' cathodes. The 5 volts is used to directly heat the rectifier tube's cathode.

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), which the amplifier's electronics actually need to function. 340 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). Europeans like to call B+ HT (HT stands for High Tension).

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. The '16F 475V' written on the cap is its rating of 16 micro Farads and 475 volts. These capacitors 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.

You'll notice resistor R10 between C3 and C4 and R11 between C4 and C5. The '10K 2W' written on R10 is its rating of 10,000 ohms and 2 watts. These step down resistors step the 340 volts DC (B+) down to 295 volts DC (B+2) then to 250 volts DC (B+3). The 340 volts DC direct from the rectifier is tapped off to feed directly to the output transformer's primary input which is also connected to the power tubes' plates. The 295 volts DC B+2 is connected to the power tubes' pins 4--the screen grids. The 250 volts DC B+3 is used to power the preamp tube but it's stepped down by the load resistors, R5 and R7. With no guitar signal present the 'idle' voltage at the preamp tube's plate pins 1 and 6 will be around 150 volts DC.

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.

Here's a More Complex Amplifier Layout from Weber

The 5E3P Tweed Proluxe features a dual 6L6GT tube push-pull output stage and fixed bias. The signal flow is very similar to the 5F1 Champ and is shown using orange arrows. Red arrows show the power flow. Right input jacks are the 'Normal/Mic' channel, left are 'Bright/Instrument' channel. Upper jacks are 'Hi' and the lower are -6dB 'Lo.' The tone control's 500pF capacitor is a high frequency volume bypass 'bright cap' which keeps low volume from becoming too dark. The 0.0047F capacitor is a standard tone low pass filter that bleeds high freqs to ground as the tone control is turned down. Volume and tone controls interact with one another.

Bonus Info: How Fender multiple guitar input jacks and channel jumpering works


How Tubes Work

In the diagram below the tube's glass shell holds a vacuum. A vacuum is needed to keep the electrodes from burning--the same reason it's needed in a light bulb. The DC power source on the right applies high voltage DC (250 volts for the 12AX7 preamp tube) to the Load Resistor which lowers the voltage at the tube's 'plate.' The tube's cathode is grounded so there is up to 250 volts between the plate and cathode. The positive charge on the plate would love to pull electrons from the grounded cathode but they 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.

Vacuum Tube Triode Amplifier

Audio signal voltage enters the tube on the left at the Grid and exits at the Plate. The 12AX7 tube used in our amplifier is a 'dual triode' design. 'Triode' means it has three electrodes, the grid, plate and cathode.  'Dual' means it has two of the above triode circuits built into one glass tube shell. The two circuits are referred to as V1A and V1B (Valve #1 A and B).

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. He also discovered he could put a positively charged 'plate' inside the tube to collect the boiling electrons and create a current from the hot filament, or 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.'

Later someone discovered you could put an electrically charged metal screen, or 'grid,' between the cathode and plate to block the electron flow. If you fluctuate the electric charge of the 'control grid' you fluctuate the current flow between the cathode and plate. This effect can be used as a signal amplifier by applying a low level signal to the grid which modulates the much larger flow of electrons between the cathode and plate (the plate is also called the anode).

The 12AX7 tube used in our amplifier is a 'dual triode' design. 'Triode' means it has three electrodes, the grid, plate and cathode.  'Dual' means it has two separate triode circuits built into one tube. The two circuits are referred to as V1A and V1B (Valve #1 A and B). V1A is the first preamp and V1B functions as the second preamp and output stage driver. Some simple tubes like half-wave rectifiers have only a cathode and plate and are called diodes (two electrodes). V3 has four electrodes so it's called a tetrode. A pentode has five electrodes. I guess you could say a light bulb is a uniode with just one electrode ;)

Notice in the diagram above we have both a filament and a cathode. In most tubes the cathode is indirectly heated by a filament heater. In the layout diagram at the top of this webpage the power transformer on the left sends 6.3 volts AC through the twisted green wires to power the pilot light and V1 and V2's filament heaters. The rectifier tube's cathode is directly heated by 5 volts AC through 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 tube diagram again, the guitar audio signal enters on the left at 'input voltage.' The signal flows to the tubes '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 too. The voltage fluctuations on the plate are the amplified 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 from the cathode to the plate (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. We will discuss later how the tube's load resistor transforms the circuit from a current amplifier to a voltage amplifier.

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 audio signal rides on top of the tube's high voltage DC power.

How the Load Resistor Works

In the diagram 'R' is a load resistor which allows a small flow of current from the tube's plate to make a large change in voltage thus transforming the tube from a current amplifier to 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 between the plate and load resistor. Slow the flow of electrons from cathode to plate and the voltage rises from positive current flowing through the load resistor. Stop the flow completely and the voltage on both sides of the load resistor will equalize at 250 volts.

One way to describe how a tube is used as a voltage amplifier is with a water hose analogy. Think of the high voltage supply 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 rate. 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. Now put your thumb over the end of the hose--this 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. This simulates the control grid stopping the electron flow from the cathode to the plate--now flow = no voltage drop across the load resistor.

Now spray some water with your thumb and the pressure will rapidly drop in the hose between your finger and clamped hose. This simulates the control grid allowing electrons to flow to the plate. Add more thumb pressure to decrease the water spray and the hoes pressure increases, allow more to flow by your thumb and the pressure drops. The partially clamped hose causes more pressure fluctuations when small amounts of water is 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 fluctuation when small amounts of current is 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 fluctuations.

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, 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 electron flow). Pins 4 & 9 connect to the heater filaments. The '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.

6V6GT Power Tube

The 6V6GT is a tetrode because it has 4 electrodes: cathode, plate, control grid and screen grid. The screen grid 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 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.

So to review, the filament heats the cathode, the cathode gives off electrons, the plate attracts the free electrons but the grid controls the flow--the control grid is the valve that controls the flow of electrons through the tube.

Back to "First Preamp Stage"

Tube Bias Circuits

V1 and V2 use 'common cathode' biasing, also referred to as 'self biasing.' 'Common Cathode' means the cathode is tied to (common) ground through resistors. V1A's bias voltage is set by resistor R4 which is connected to V1's pin 3 and cathode. V1B's bias is set by R6. V2's bias is set by R8. Capacitor C6 is an AC bypass cap that bleeds unwanted AC around R8 to ground which helps decrease feedback and increase V2's gain. In most guitar amplifiers there is also a bypass cap around V1's bias resistors R4 and R6.

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. This voltage difference between the cathode and control grid is called tube 'bias.' Common cathode tubes use bias resistors placed between the cathode and ground to set the bias voltage. The much more powerful 5e3p amplifier shown below uses an 'adjustable fixed bias' system that applies a negative voltage (usually between -30 to -45 volts DC) to the power tubes' control grids.

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 large 50uF Filter (Smoothing) Capacitor smoothes out the pulses and the Bias Adjustment Pot adjusts the amount of negative DC that flows to the Power Tubes' Control Grids.


The Rectifier Tube V3

The rectifier tube, V3 is used to convert 650 volts AC from the Power Transformer into 340 volts DC. Note that when you measure between ground and either of V3's AC input pins you'll see 325 AC volts, but if you measure between the two AC input pins you'll see 650 AC volts. V3 is different from the preamp and power tubes in that It has two plates and 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.

Rectifier Tubes

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.

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.

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 relatively large capacitor.

5Y3GT Rectifier

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 Tension (HT) 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.


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 center tap provides the path for returning current. If you use a transformer without a center tap then a 4 diode bridge rectifier is needed to provide a current return path from the amp circuit back to the transformer.

Bridge Rectifier Current Flow During Top Half of AC Wave

All four diodes act as one-way valves that allow current to flow in only one direction. The two diodes on the left side are the 'bridge' from the amp circuit back to the 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 at zero volts and does not pull.

Bridge Rectifier Current Flow During Bottom Half of AC Wave

 

With a Center Tap Two Diodes are Enough

Conventional two diode rectifier showing current flow during the top 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 at zero volts so only the 'push' half of the AC wave (orange arrows) is converted into DC power.

 

A tube rectifier works the same way as this 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.

 

Conventional Rectifier and No Center Tap

With no center tap there's no return path to the transformer so this will not work.

Fixed bias amplifiers often use a single diode to rectify 50V AC supplied by a single wire from the power transformer. The single diode functions as a half wave rectifier and uses the power transformer's center tap as the current return path to the transformer.


Big Picture Amp Power

The electric company's generators (spinning at 60Hz or 60 revolutions per second or 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 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 GZ34S rectifier's pins 4 and 6 to charge its plates. Every AC half cycle the pin 4 plate is charged positive (scarcity of electrons) while the pin 6 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+ wire attached to it (removing electrons from the wire creates a positive voltage).

During the next AC half cycle the pin 6 plate is charged positive and pulls electrons from the cathode while the pin 4 plate does nothing. Because the GZ34S 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 them 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 are half wave rectifiers.

Back to the Output Stage.


How the Long Tail Pair Phase Inverter Works

Signal flow shown with red arrows. 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.

The Long Tail Pair (LTP) Phase Inverter (also called the cathode-coupled phase inverter) is the most popular phase inverter in guitar amplifiers. Unlike the non-amplifying cathodyne phase inverter it 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 helps it drive big bottle amps using 6L6 or larger power tubes. The LTP is a true differential amplifier and uses both halves of a triode (usually a 12AX7).

V3A has a dual function. It acts as a normal gain stage (outputs inverted signal at the plate) but also acts as a cathode follower (outputs non-inverted signal at its cathode).

The input signal flows onto V3A's grid, V3B's grid is held at a constant voltage and capacitor C20 removes all AC signal. Since the grid is held constant, voltage fluctuations on the cathode create an amplified signal on V3B's plate.

V3A and V3B's cathodes are tied together. All of V3B's input signal flows from V3A's cathode.

R36 is the tail resistor that creates the relatively high voltage (34v DC in the Bassman) needed for the cathode follower function of V3A. It also supplies a near constant current flow shared between the two cathodes.

R34 is the bias resistor and creates the voltage difference between both tubes' grid and cathode.

R37 and R38 are simply grid leak resistors which leak off 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 the two halves of the LTP.

The Presence control (R35 and C21) removes a variable amount of high frequency from Negative Feedback therefore boosting high frequency output (C21 shunts the AC NFB signal voltage to ground).

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 V3A's 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 V3A's cathode a positive voltage is created on the cathode (scarcity of electrons = positive voltage). This positive signal voltage is also present in V3B's cathode because the cathodes are directly connected. Since V3B's 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 V3B's 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.


How Spring Reverb Works

Old school spring reverb literally uses springs to delay and replicate a signal. In the schematic below the audio signal enters on the upper left and gets boosted by the Reverb Driver amplifier. The Reverb Driver is needed to generate the power to physically move the reverb spring.

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 causes the transducer's magnet to move. The springs are attached to the moving magnet. The movement travels down the spring and causes movement of the Output Transducer magnet at the other end. The moving magnet generates the 'wet' reverb signal voltage in the transducer output coil. 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) 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 a signal voltage in the output coil.

Reverb Tank

A reverb tank is simply made up of two transducers connected by two or more springs.

The time it takes for the spring movement to travel from input transducer to output is the reverb delay. Multiple springs add multiple delays. The original spring movement doesn't actually stop at the output transducer. A diminished 'wave' is reflected back toward the input transducer, bounces off it and returns in weakened form to the output transducer generating multiple diminishing reverb 'reflections.'


Input and Output Impedance Bridging

It took me a long time to understand why input and output impedance matters. Here's my layman's explanation:

Impedance is only a factor in alternating current (AC) circuits. Impedance is made up of three things that impede or restrict AC current flow: resistance, inductance and capacitance. 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).

power = volts x current

 But we're not concerned with maximum power transfer, we want maximum voltage transfer because an electric guitar generates an AC signal voltage--the audio signal IS the voltage changes. We really don't care about the current generated by the guitar's coil, it's the coil's voltage changes that the guitar amplifier will use to move the speaker cone. Since we favor voltage over current we can use an impedance mismatch to trade voltage for current. The intentional mismatch reduces the current but boosts the signal voltage the amplifier receives.

Guitar Output Impedance & Amplifier Input Impedance

Guitar circuit on left, amplifier input on the right.

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. The guitar is connected to the amplifier input jack. 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. Resistor R1 on the 5F1 amplifier's guitar Jack 2 adds 1,000,000 ohms of input impedance to boost the signal voltage from the guitar.

The same principal applies between amplifier circuits. Substitute the guitar for the output stage driver tube V1B, sending a 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 does, it adds input impedance to the driver tube V2.  Back to Signal Input.

Want to learn more? Check out the valve wizard's website. He has lots of great tube amplifier information.

Rob Robinette

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