Push-pull 6V6 amp with bias tremolo, 1964 style

Pedal and tube amp circuits, solved part by part the way SPICE (the circuit simulator engineers use) does it, while you listen and watch. Turn the knobs, change any part, or tap a wire to put a probe on it.

How to use the bench

Every part is simulated. Nothing you hear is a recording.

  1. 1Pick a circuit

    Choose one from the menu, or type a name, maker or year in Find a circuit. The buttons under the menu switch between versions.

  2. 2Set the knobs and the input

    Turn the circuit’s knobs on the front panel. In the Input box, choose Guitar or Signal generator, and the note or chord.

  3. 3Press Play

    Press Play through the circuit to hear a picked note or chord. Play the string is the guitar by itself.

  4. 4Tap a part

    Tap any part on the schematic to change its value, or swap the transistor, tube or diode. The red numbers are DC voltages, and they update as you go.

  5. 5Tap a wire

    Tap a wire to put the probe there. The Probe graph and the spectrum show the signal there.

  6. 6Try a change

    Press Try it on a card under Try this to hear one change, before and after. Modify this circuit, below the schematic, adds or removes parts.

Good to know

Modeled after the Fender AA964 Princeton.

One 7025 gain stage and the blackface tone stack into the volume, then a second stage, a split-load inverter and two 6V6GTs in fixed bias. The tremolo swings the power tubes’ bias. The Princeton Reverb without the reverb.

Input

Add

Peak voltage from the pickup for each note. Strum hard and a single coil puts out a few hundred millivolts. When you play a chord, its strings share this level.

How it’s played
Tap a part to change its value, or a wire to probe it. To add or take out parts, open Modify this circuit, below. Red numbers are DC voltages to ground, not warnings. Scroll or drag sideways to see the whole circuit.
Modify this circuit Add parts, take them out, or put one in series with another, then hear what it does. Tap to open.

Pick what to do, then tap the schematic. Your changes show in purple. Any change can go on a switch beside the knobs, from the menu in its row under Your changes.

Add a part

Tap where each leg goes. Tap both ends of a part to put the new one across that part.

Change a part

With Remove a part on, tap any part of the pedal or amp to take it out, and tap it again to put it back. Tapping a purple part you added deletes it.

Turn on Add in series with a leg and tap a part. In that part’s box, below the schematic, pick which leg to lift and what goes in the gap, such as a resistor between a cap and ground or a stopper in front of a grid.

Change a wire

With Cut into a wire on, tap any wire. Pick a part from the menu and it goes in the gap, wired in. Volume pot puts a pot there with the input on one end, the output on the wiper and the other end to ground.

With Run a wire on, tap a leg, then where the wire goes. Drag any piece of a wire you ran, or of an added part’s lead, to square it up or move it.

Keep and share

Your changes stay in this browser, on this device, for your next visit. Nothing is sent to us. Anyone using this browser can see them. They’re lost if you clear your browsing data or use a private window, so save a file to keep a copy. A shared link carries the circuit after the #, a part browsers never send to a server.

Test tone for the graphs
The graphs use a steady tone. Play sounds a plucked string that dies away.

Input

pickup, unloadedat the pedal input

Output

at the amp input

Probe

Spectrum

Frequency response

What the curves mean

The ink curve is the response to a very quiet signal, from the generator to the probe. The blue curve is measured with pink noise at the level you set, like an audio analyzer. When stages clip, the blue curve pulls away from the ink one and the gray coherence line (right-hand scale) drops below 1. Then no single curve tells the whole story. Both use the signal generator, whichever Input tab is picked.

Output

Bias

every voltage and current at rest

What to listen for. The tone stack comes right after the first gain stage, and the second stage makes up the level the stack loses. With no reverb mixer in the way, the second stage drives the split-load inverter straight, and the feedback from the speaker goes to its cathode. The tremolo is a phase-shift oscillator whose swing moves both 6V6GT grids through the bias line, so the whole output stage rises and falls.

Download.

Parts and what they do

Stock values. Tap a part on the schematic to change it.

R1A, 68 kΩ
The input 1 resistor into the grid. With nothing in input 2 it sits in parallel with input 2’s resistor, so a guitar in input 1 sees 34 kΩ to the grid: enough to stop radio pickup, not enough to cut treble.
R1B, 68 kΩ
The input 2 resistor into the grid. With nothing in input 1, input 1’s tip is grounded, so the two 68 kΩ make a divider: input 2 is about 6 dB quieter and loads the guitar with 136 kΩ.
RG1, 1 MΩ
Input 1’s 1 MΩ to ground. It sets the load a guitar in input 1 sees, and with a patch lead in input 1 it is the end of the lead’s path.
V1a, 12AX7 / ECC83 / 7025
First half of the 7025 (a low-noise 12AX7), the input stage. It feeds the tone stack straight from its plate.
1500, 1.5 kΩ
Cathode resistor for the input stage. The drawing shows 1.5 V across it.
25µ, 25 µF electrolytic
Cathode bypass for the input stage, full gain down to about 10 Hz.
100k, 100 kΩ
Plate resistor for the input stage, from box B. With the tube’s own plate resistance it’s the source the tone stack is driven from.
250p, 250 pF
Treble cap, 250 pF, from the plate to the top of the treble pot.
100k, 100 kΩ
Slope resistor, 100 kΩ. With the treble cap it sets where the treble control starts to work and how deep the mid scoop goes.
.1µ, 100 nF
Bass cap, 0.1 µF.
.047µ, 47 nF
Mid cap, 0.047 µF.
Treble 250k, 250 kΩ pot, audio taper
Treble, 250 kΩ audio. Its wiper is the stack’s output, and it reaches well down into the mids.
Bass 250k, 250 kΩ pot, audio taper
Bass, 250 kΩ audio, used as a variable resistor between the bass cap and the mid cap.
6800, 6.8 kΩ
A fixed 6.8 kΩ where a middle pot would go. It works like a 10 kΩ audio mid pot set at about 9.
Vol 1M, 1 MΩ pot, audio taper
Volume, 1 MΩ audio. The stack drives its top, and the wiper feeds the second stage’s grid, which has no resistor of its own. There’s no bright cap on this one.
V2a, 12AX7 / ECC83 / 7025
12AX7, the driver. A cathodyne has no gain of its own, so this stage raises the preamp’s signal about 30 times, enough to drive the output tubes. Its grid connects straight to the preamp’s output with no coupling cap, so its path to ground is through the preamp’s volume and tone network.
100k, 100 kΩ
Plate resistor for the driver.
1.5k, 1.5 kΩ
Cathode resistor for the driver. It sets the driver’s bias, a volt or so. It sits on the feedback leg, so the fed-back signal moves the driver’s cathode.
25µ, 25 µF electrolytic
Bypass cap across the driver’s cathode resistor, for full gain above a few hertz. It goes to the top of the feedback leg, not to ground, so the feedback still reaches the cathode.
.022µ, 22 nF
Coupling cap from the driver to the cathodyne.
V2b, 12AX7 / ECC83 / 7025
The cathodyne, or split-load inverter. The same current runs through its plate resistor and its cathode resistors, which are about equal, so the plate swings one way and the cathode the other by about the same amount. The cathode drives tube A, the plate tube B. Its gain is slightly under 1 on each side.
56k, 56 kΩ
The cathodyne’s plate resistor, the output to tube B.
1k, 1 kΩ
The cathodyne’s bias resistor. The grid returns below it, so the cathode sits this far above the grid.
56k, 56 kΩ
The cathodyne’s cathode load, the same value as its plate resistor. The cathode above the bias resistor is the output to tube A.
1M, 1 MΩ
Grid resistor for the cathodyne, returned to the junction of its cathode resistors so the grid sits near the cathode’s high DC voltage.
2.7k, 2.7 kΩ
The feedback resistor from the speaker. With the 47 Ω below it, it sends about 2 % of the speaker voltage back to the driver’s cathode, which lowers the gain, flattens the response and tightens the bass.
47, 47 Ω
The bottom of the feedback loop, 47 Ω to ground under the driver’s cathode resistor. It isn’t bypassed, so the fed-back speaker voltage moves the cathode. The driver’s current runs through it too, so it’s kept small.
.1µ, 100 nF
Coupling cap to tube A’s grid.
.1µ, 100 nF
Coupling cap to tube B’s grid.
V3, 6V6GT
6V6GT, tube A of the push-pull pair, cathode grounded, biased from the fixed bias supply.
V4, 6V6GT
6V6GT, tube B of the pair.
220k, 220 kΩ
Grid leak for tube A, returned to the bias supply. Driven into grid current, the coupling cap charges through it and the bias goes colder for a moment. That’s blocking distortion.
220k, 220 kΩ
Grid leak for tube B.
1k, 1 kΩ
Dropping resistor from B+ to the screen supply. The screens sit slightly below the plates.
20µ, 20 µF electrolytic
Filter cap on the screen supply.
18k, 18 kΩ
Dropping resistor from the screen supply node to the node for the middle node, as on the Fender AA964 Princeton drawing. The current drawn past it drops the voltage, so each stage down the chain runs lower than the one before, and a hard note that pulls B+ down pulls these down too.
20µ, 20 µF electrolytic
Filter cap at the node for the middle node. With the dropping resistor it filters the supply and keeps the stages on this node from coupling to each other through it.
18k, 18 kΩ
Dropping resistor from the node for the middle node to the node for the preamp, driver and inverter, as on the Fender AA964 Princeton drawing. The current drawn past it drops the voltage, so each stage down the chain runs lower than the one before, and a hard note that pulls B+ down pulls these down too. The preamp runs off this node instead of its own HT supply.
20µ, 20 µF electrolytic
Filter cap at the node for the preamp, driver and inverter. With the dropping resistor it filters the supply and keeps the stages on this node from coupling to each other through it.
OT, 8.2 kΩ to 8 Ω, 10 H primary, 10 mH leakage
The output transformer, 8.2 kΩ plate to plate into 8 Ω, center tap on B+. Tap any piece of it to change its figures. The core saturates as the flux nears its knee. Every figure is typical of the part, not measured.
PT, 340 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 340 V ac from the grounded center tap, as a sine source at the mains frequency. The value is typical of the part, not measured.
PT, 340 V ac, 60 Hz
The other half of the winding, from the grounded center tap. It runs opposite the first half, so the rectifier charges the reservoir on every half cycle.
PT R, 70 Ω
Resistance of one half of the winding, plus the primary’s share. With the rectifier’s own drop it sets how far B+ sags on a hard note. Typical of the part, not measured.
PT R, 70 Ω
Resistance of the other half. Typical of the part, not measured.
V5, GZ34
One plate of the GZ34 rectifier. It conducts near the peak of its half of the winding, and its own drop rises with the current, which adds to the sag.
V5, GZ34
The rectifier’s other plate, on the other half cycle.
20µ, 20 µF electrolytic
The reservoir cap. It feeds the output transformer’s center tap. It charges in short pulses at each peak of the mains and drains between them, so B+ carries ripple, and on a hard note it sags.
PT, 340 V ac, 60 Hz
The end of the high-voltage winding the bias comes off, 340 V ac, the same end as the rectifier’s first plate. It’s drawn as its own source so the bias supply’s average can be found. Keep it equal to the winding beside the rectifier.
100k, 100 kΩ
Series resistor into the bias rectifier, 100k Ω. There’s no bias tap, so the bias comes straight off the high-voltage winding, and this resistor drops most of it.
Bias diode, 1N4001
The bias rectifier, a silicon diode turned so the supply comes out negative.
25µ, 25 µF electrolytic
Filter cap on the bias supply, its plus side to ground. It also holds the Intensity pot’s bias end still, so the LFO swings the wiper.
27k, 27 kΩ
The bias supply’s load, 27k Ω to ground. With the 100k Ω above it, it sets the bias. There’s no bias pot, so change this resistor to change the bias. Bigger runs the tubes colder.
V6, 12AX7 / ECC83 / 7025
The tremolo’s oscillator, half a 12AX7. The three caps after its plate turn the signal around at one frequency, and the oscillator runs at that frequency, a few times a second. In the amp it’s the other half of the phase inverter’s tube.
220k, 220 kΩ
Oscillator plate resistor, from the screen supply.
3.3k, 3.3 kΩ
Oscillator cathode resistor.
25µ, 25 µF electrolytic
Oscillator bypass cap. At a few hertz it isn’t a full short, and it adds some of the phase shift that keeps the loop oscillating.
.02µ, 20 nF
First ladder cap.
Speed, 3 MΩ pot, reverse audio taper
Speed, 3M Ω reverse audio, used as a variable resistor. Less resistance runs the oscillator faster.
100k, 100 kΩ
Sets the top speed. It’s all that’s left with Speed on 10.
.01µ, 10 nF
Second ladder cap.
1M, 1 MΩ
Middle resistor of the ladder, returned to the cathode. The footswitch grounds this point.
.01µ, 10 nF
Third ladder cap.
1M, 1 MΩ
Oscillator grid leak.
1M, 1 MΩ
Couples the LFO from the oscillator’s plate. It keeps the bias network from loading the oscillator much. With the cap below it, it makes a low-pass that rounds the LFO and cuts a fast speed more.
.02µ, 20 nF
To ground below the resistor off the plate. Together they round the LFO’s wave.
.1µ, 100 nF
Blocks the oscillator’s DC, so only its swing reaches the Intensity pot.
Intensity, 250 kΩ pot, linear taper
Intensity, 250k Ω linear. One end gets the LFO, the other sits on the bias supply, and the wiper is the bias line for both output tubes. At 0 the wiper is on the bias supply and the bias holds still. Turned up, more of the LFO rides on the bias, so both grids move together and the output stage’s gain rises and falls. Which way it turns isn’t marked on the drawing. Up is assumed to be more.
Coil R, 8.69 Ω
Voice coil resistance, the floor of the speaker’s impedance. The shape of a 1x12 open-back cab with an alnico 12 in, measured (a 93 Ω peak near 74 Hz on 8 Ω), scaled to the 8 Ω tap. One real cab, not every speaker. The Speaker menu by the play buttons is separate. It plays the note through a speaker and cab impulse response, with either load.
Coil L, 256 µH
Voice coil inductance. It makes the impedance rise through the treble. The feedback holds the output steadier against that than it would be without feedback.
Loss L, 464 µH
With Loss R across it, the treble rises and then levels off, like the losses in a real coil.
Loss R, 14.3 Ω
Limits what Loss L adds to the treble rise.
Loss L2, 2.13 mH
A second step in the treble rise, with Loss R2 across it: a real voice coil’s losses keep climbing from a few hundred hertz.
Loss R2, 2.19 Ω
Limits what Loss L2 adds.
Tank R, 83.7 Ω
Height of the bass resonance, about 92 Ω in all at its peak. The feedback holds the output steadier against that than it would be without feedback.
Tank L, 21.1 mH
With Tank C, sets the resonance: 74 Hz.
Tank C, 220 µF
With Tank L, sets the resonance. With Tank R, it sets how narrow the peak is.

About this version

As drawn
The 7025 is modeled as a 12AX7 (a low-noise version of it), fitted to its data sheets. Typical of the type, not measured.

How this tool works. Every part is an equation, and the whole circuit is solved several times per audio sample: 192,000 samples a second while you drag a knob, 384,000 once you let go. Transistors use Ebers-Moll with gain, leakage, Early effect, base resistance, junction capacitance and temperature. Their charge storage is set at the bias point, close for small signals and rough while they switch hard. Tubes use Koren’s equations fitted to each datasheet, plus grid current. Diodes are exponential junctions with series resistance. Presets are typical of each type, not measured parts. Caps are ideal, and the battery or HT is a voltage behind a resistance.

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