Push-button tone amp with tremolo, 1959 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 Gibson GA-80 Vari-Tone.

Two channels, each a 12AX7 half into a 5879 pentode. The second has six push buttons: a regular tone control, or a 15 H choke with one of five caps that notches a band out. A 12AX7 oscillator swings that 5879’s screen for the tremolo.

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. Two channels meet at the cathodyne’s grid. Each one is a 12AX7 half into a 5879 pentode whose screen sits near 20 V on a 4.7 or 6.8 MΩ resistor, so each 5879 runs on about a third of a milliamp. Channel 1’s Tone is a 2 MΩ pot with its wiper looped to the input end and .01 µF to ground on a tap a quarter of the way from the volume end, with 330 pF across the pot. Turned down, the cap loads the channel. Turned all the way up, the bass at 100 Hz drops about 25 dB and 3 kHz about 12 dB, so the channel gets thinner and brighter. The Vari-Tone channel has six push buttons. Button I is a plain treble cut. Buttons II to VI each hang the 15 H choke and one cap from the signal to ground, which notches out one band: about 1.3 kHz, 750 Hz, 410 Hz, 240 Hz and 90 Hz. The notches are narrow and deep, 30 dB or more right at their centers with the choke’s resistance assumed to be 500 Ω. The tremolo is on the same channel: a 12AX7 phase-shift oscillator, about 5.5 to 11 times a second, swings V3’s screen through the Depth pot, 750 kΩ and .1 µF, so V3’s gain rises and falls. Three .0047 µF caps keep that slow swing on V3’s plate out of the power amp. With 0.2 V in, the Vari-Tone channel already clips V3 at 3 on its volume. The tube models are typical of the types, not measured.

Download.

Parts and what they do

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

R1, 56 kΩ
The channel 1 channel’s 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 28 kΩ to the grid: enough to stop radio pickup, not enough to cut treble.
R3, 56 kΩ
The channel 1 channel’s input 2 resistor into the grid. With nothing in input 1, input 1’s tip is grounded, so the two 56 kΩ make a divider: input 2 is about 6 dB quieter and loads the guitar with 112 kΩ.
R2, 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.
R4, 56 kΩ
The vari-tone channel’s 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 28 kΩ to the grid: enough to stop radio pickup, not enough to cut treble.
R6, 56 kΩ
The vari-tone channel’s input 2 resistor into the grid. With nothing in input 1, input 1’s tip is grounded, so the two 56 kΩ make a divider: input 2 is about 6 dB quieter and loads the guitar with 112 kΩ.
R5, 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
Half of the 12AX7, channel 1’s first stage.
220k, 220 kΩ
Plate resistor, 220 kΩ on the drawing. The voltage table reads +127 V on this plate and on V1b’s, which has 100 kΩ, so one of the two values or readings is off.
1.5k, 1.5 kΩ
Cathode resistor, +0.9 V on the drawing.
20µ, 20 µF electrolytic
Cathode bypass cap, full gain down to about 5 Hz.
.022µ, 22 nF
Coupling cap to the tone network.
330p, 330 pF
Across the Tone pot. With the pot’s resistance up, the treble passes through this cap and the bass has to go through the pot.
.01µ, 10 nF
From the Tone pot’s tap to ground. With the pot turned down the wiper shorts the tap to the input and this cap sits straight on the channel. Turned up, more of the track stands between them, the cap cuts less, and the bass has to come through the pot while the treble goes through the 330 pF.
Tone 2M, 2 MΩ pot, audio taper
Channel 1’s Tone, 2 MΩ, its wiper looped to the input end and the .01 µF on a tap a quarter of the way from the volume end. Brighter clockwise: turned down the whole track is shorted and the .01 µF loads the channel. Turned up the bass has to come through more of the track while the treble passes the 330 pF, so the channel gets thinner and brighter.
Vol 1M, 1 MΩ pot, audio taper
Channel 1 volume, 1 MΩ. The wiper is V2’s grid. No taper is marked, so audio is assumed.
V2, 5879
Channel 1’s 5879 pentode.
470k, 470 kΩ
Plate resistor. The drawing reads +160 V on the plate.
4.7M, 4.7 MΩ
Screen resistor. It holds the screen down near +20 V on the drawing, so the tube runs on a fraction of a milliamp.
.047µ, 47 nF
Screen bypass cap.
3.3k, 3.3 kΩ
Cathode resistor, +1.10 V on the drawing.
20µ, 20 µF electrolytic
Cathode bypass cap.
.01µ, 10 nF
Coupling cap toward the mixing node.
100k, 100 kΩ
Mixing resistor from channel 1.
V1b, 12AX7 / ECC83 / 7025
The other half of the 12AX7, the Vari-Tone channel’s first stage.
100k, 100 kΩ
Plate resistor, 100 kΩ on the drawing.
1.5k, 1.5 kΩ
Cathode resistor, +0.9 V on the drawing.
20µ, 20 µF electrolytic
Cathode bypass cap.
.022µ, 22 nF
Coupling cap to the Vari-Tone bus.
Tone 2M, 2 MΩ pot, audio taper
Button I’s Tone, 2 MΩ as a variable resistor in series with .01 µF from the bus to ground. Turned up, its resistance rises, the cap shunts less treble away and the channel gets brighter. It works only with button I in. No taper is marked, so audio is assumed.
.01µ, 10 nF
Button I’s tone cap. Its far end goes to ground through a closed contact on each of buttons II to VI.
15 H, 15 H
The Vari-Tone choke, 15 H. In series with whichever preset cap is switched in, it makes a path to ground that’s lowest at one frequency, so the channel loses that band.
500, 500 Ω
The choke’s winding resistance, typical of a small 15 H choke, not on the drawing. It sets how deep each notch goes.
.001µ, 1 nF
Button II’s cap. The first preset cap. Its label can’t be read on this copy of the drawing, so .001 µF is assumed, the next step down in the .003, .01, .03 run. With the 15 H choke it notches about 1.3 kHz.
.003µ, 3 nF
Button III’s cap. With the choke it notches about 750 Hz.
.01µ, 10 nF
Button IV’s cap. With the choke it notches about 410 Hz.
.03µ, 30 nF
Button V’s cap. With the choke it notches about 240 Hz.
.22µ, 220 nF
Button VI’s cap. With the choke it notches about 90 Hz, so it thins out the low end. The label reads .22 on this copy. It could be .022, which would notch about 280 Hz.
Vol 1M, 1 MΩ pot, audio taper
The Vari-Tone channel’s volume, 1 MΩ, its top on the bus. The wiper is V3’s grid. No taper is marked, so audio is assumed.
V3, 5879
The Vari-Tone channel’s 5879. Its screen sits at about 20 V, and the tremolo swings it, so the stage’s gain rises and falls.
220k, 220 kΩ
Plate resistor. The drawing reads +240 V on the plate.
6.8M, 6.8 MΩ
Screen resistor, 6.8 MΩ, down to about +20 V on the drawing.
.047µ, 47 nF
Screen bypass cap. It holds the screen still at guitar frequencies but lets the slow tremolo swing through.
4.7k, 4.7 kΩ
Cathode resistor, +1.35 V on the drawing.
20µ, 20 µF electrolytic
Cathode bypass cap.
.1µ, 100 nF
Blocking cap from the tremolo feed to V3’s screen.
750k, 750 kΩ
From the Depth wiper to the screen. With the screen’s 6.8 MΩ and .047 µF it sets how much of the oscillator’s swing reaches the screen.
.0047µ, 4.7 nF
First of three .0047 µF caps from V3’s plate to the mixing node, with 1 MΩ to ground after the first two. They keep the tremolo’s slow swing on the plate out of the power amp, and cut some bass with it.
1M, 1 MΩ
Ladder resistor to ground.
.0047µ, 4.7 nF
Ladder cap.
1M, 1 MΩ
Ladder resistor to ground.
.0047µ, 4.7 nF
Last ladder cap.
100k, 100 kΩ
Mixing resistor from the Vari-Tone channel.
.002µ, 2 nF
Coupling cap from the mixing node to the cathodyne’s grid. It’s small, so it trims the deep bass, below about 80 Hz into the cathodyne’s grid resistor.
330k, 330 kΩ
Feed resistor for the oscillator.
10µ, 10 µF electrolytic
Filter cap at the foot of the feed resistor, so the Depth pot’s top end holds still.
Depth, 250 kΩ pot, audio taper
Depth, 250 kΩ. It’s the oscillator’s plate resistor, and its wiper picks part of the plate’s swing for V3’s screen. Turned up, the wiper moves toward the plate and the swing grows. No taper is marked, so audio is assumed.
V4b, 12AX7 / ECC83 / 7025
Half of the second 12AX7, the tremolo oscillator. The other half is the cathodyne in the power amp.
1k, 1 kΩ
Cathode resistor. Its value isn’t on the drawing, so a typical 1 kΩ is assumed, which puts it near the drawing’s +0.4 V.
20µ, 20 µF electrolytic
Cathode bypass cap.
.033µ, 33 nF
First cap of the oscillator’s phase-shift ladder.
68k, 68 kΩ
In series with the Rate pot, the ladder’s first leg to ground. It sets the fastest rate.
Rate, 500 kΩ pot, linear taper
Rate, 500 kΩ as a variable resistor. Turned up, its resistance drops and the oscillator runs faster. Its far end goes through the footswitch to ground. No taper is marked, so linear is assumed.
.01µ, 10 nF
Second ladder cap.
680k, 680 kΩ
Second ladder leg to ground.
.01µ, 10 nF
Third ladder cap, into the grid. The three caps and legs turn the plate’s swing round by half a cycle at one frequency, so it oscillates there.
2.2M, 2.2 MΩ
Grid leak for the oscillator, and the ladder’s last leg.
V5, 12AX7 / ECC83 / 7025
12AX7, a cathodyne, or split-load inverter, with no driver ahead of it. 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, so the preamp has to bring the whole swing the output tubes need.
47k, 47 kΩ
The cathodyne’s plate resistor, the output to tube B.
1.5k, 1.5 kΩ
The cathodyne’s bias resistor. The grid returns below it, so the cathode sits this far above the grid.
47k, 47 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.
470k, 470 kΩ
Grid resistor for the cathodyne, returned to the junction of its cathode resistors so the grid sits near the cathode’s high DC voltage. It’s the grid’s only path to ground, and the cathode follows the grid, so the preamp sees much more than 470k Ω.
.022µ, 22 nF
Coupling cap to tube A’s grid.
.022µ, 22 nF
Coupling cap to tube B’s grid.
V6, 6L6GC
6L6GC, tube A of the push-pull pair, its cathode on the shared cathode resistor, so it biases itself.
V7, 6L6GC
6L6GC, tube B of the pair.
470k, 470 kΩ
Grid leak for tube A, to ground. The bias comes from the cathode resistor. Driven into grid current, the coupling cap charges through it and the bias goes colder for a moment. That’s blocking distortion.
470k, 470 kΩ
Grid leak for tube B.
250, 250 Ω
The shared cathode resistor. Every output tube’s current runs through it and lifts the cathodes above the grids, and that’s the bias. Driven hard, the pair draws more current on average, so the cathodes climb and the tubes run colder, which softens the clipping.
50µ, 50 µF electrolytic
Bypass cap across the cathode resistor, so the cathodes hold still at the signal’s frequencies. It charges and discharges slowly as the average current changes on a hard note.
25k, 25 kΩ
From the screen supply to the output tubes’ shared cathode, 25k Ω, as on the drawing. It sends a steady current down through the cathode resistor on top of the tubes’ own, so the cathodes sit higher and the tubes idle cooler than the resistor alone would set them.
4k, 4 kΩ
Dropping resistor from B+ to the screens. It’s large, so the screens sit about 30 V below the plates and drop further when they draw more current on a hard note.
10µ, 10 µF electrolytic
Filter cap on the screen supply.
33k, 33 kΩ
Dropping resistor from B+ to the node for the inverter, the 5879s and the tremolo, as on the Gibson GA-80 Vari-Tone 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.
10µ, 10 µF electrolytic
Filter cap at the node for the inverter, the 5879s and the tremolo. With the dropping resistor it filters the supply and keeps the stages on this node from coupling to each other through it.
47k, 47 kΩ
Dropping resistor from the node for the inverter, the 5879s and the tremolo to the node for the input stages, as on the Gibson GA-80 Vari-Tone 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.
10µ, 10 µF electrolytic
Filter cap at the node for the input stages. With the dropping resistor it filters the supply and keeps the stages on this node from coupling to each other through it.
OT, 6.6 kΩ to 8 Ω, 40 H primary, 10 mH leakage
The output transformer, 6.6 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, 350 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 350 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, 350 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, 90 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, 90 Ω
Resistance of the other half. Typical of the part, not measured.
V8, 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.
V8, GZ34
The rectifier’s other plate, on the other half cycle.
20µ, 20 µF electrolytic
The reservoir caps’ first. 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.
20µ, 20 µF electrolytic
Another reservoir cap, in parallel with the first.
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. With no feedback, the voltage at the speaker rises and falls with its impedance.
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. With no feedback, the voltage at the speaker rises and falls with its impedance.
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 12AX7 models are fitted to datasheet curves. The 5879 is typical of the type, fitted to one published point. None is 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.

Spot an error, or want a circuit added? Email info@swellamps.com. A schematic or a clear photo of one helps. Anything you send may be used to improve these models and our desktop app.

© 2026 Swell Amplification LLC. All rights reserved. The simulator, its circuit models, drawings, presets and text are the property of Swell Amplification LLC and may not be copied, reproduced, redistributed or used to build another product without written permission. The original circuits are historical designs documented in public sources, and no claim is made to them. Everything here is provided as is, without warranty of any kind, for study and personal use. Results are simulations of typical parts, not measurements of any real unit. Don’t rely on them for the safety of any build or repair. Tube amplifiers carry lethal voltages.

Product and company names are trademarks of their owners. They are used here only to identify the circuits these models are based on; this site is not affiliated with or endorsed by any of them.

From Swell Amplification, swellamps.com.