Two-channel 6V6 reverb amp with a Tone Expander, early 1960s

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-30RV Invader.

Two channels on one 6EU7, mixed into a second 6EU7 stage, then a tone network, a third stage, and a 12AU7 cathodyne into two cathode-biased 6V6s. 120 kΩ from the speaker to the second stage’s cathode, through the Tone Expander switch, is the feedback.

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 on one 6EU7, each with its own volume, mixed through 270 kΩ each into a second 6EU7 stage. From there 220 kΩ and .001 µF roll the treble off above about 500 Hz, the Tone pot bleeds off more, and a third stage drives a 12AU7 cathodyne with no driver ahead of it, into two cathode-biased 6V6s. 120 kΩ from the speaker to the second stage’s cathode cuts the gain about 10 dB at 1 kHz, and the loop runs around the treble cut, so the speaker sees less of it. The Tone Expander switch puts .0033 µF in that path. On low, only the treble comes back, so bass and mids come up: a steady A2 at the speaker about 2 dB louder, with harmonics going from about 2 % to 12 %. The reverb send comes off channel 2 ahead of its volume. The springs are a measured tank, played from its impulse response. The 6V6s idle at about 38 mA, about 11.4 W each on 250 Ω, and deliver about 12 W before the harmonics pass 5 %. The tube models are typical of the types, and the transformers are typical figures, not measured.

Download.

Parts and what they do

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

R1A, 100 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 50 kΩ to the grid: enough to stop radio pickup, not enough to cut treble.
R3A, 100 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 100 kΩ make a divider: input 2 is about 6 dB quieter and loads the guitar with 200 kΩ.
R2A, 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.
R1B, 100 kΩ
The channel 2 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 50 kΩ to the grid: enough to stop radio pickup, not enough to cut treble.
R3B, 100 kΩ
The channel 2 channel’s input 2 resistor into the grid. With nothing in input 1, input 1’s tip is grounded, so the two 100 kΩ make a divider: input 2 is about 6 dB quieter and loads the guitar with 200 kΩ.
R2B, 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, 6EU7
First half of a 6EU7, channel 1’s input stage. It shares its cathode resistor and bypass cap with channel 2’s half.
V1b, 6EU7
Second half of the same 6EU7, channel 2’s input stage.
100k, 100 kΩ
Plate resistor for V1a. The chart reads +190 V on the plate.
100k, 100 kΩ
Plate resistor for V1b, +190 V on the chart.
1.5k, 1.5 kΩ
One cathode resistor for both halves of V1, so it carries both currents. The chart reads +1.75 V.
20µ, 20 µF electrolytic
Bypass for V1’s shared cathode resistor, full gain on both channels.
.02µ, 20 nF
Coupling cap from V1a to Volume 1.
.02µ, 20 nF
Coupling cap from V1b to Volume 2 and the Reverb pot, which both start here.
Vol 1, 1 MΩ pot, audio taper
Channel 1’s volume, 1 MΩ audio. Its wiper goes through 270 kΩ to the mix.
Vol 2, 1 MΩ pot, audio taper
Channel 2’s volume, 1 MΩ audio. It sets only the dry signal. The reverb send comes off ahead of it, so with Volume 2 at 0 only reverb is left.
270k, 270 kΩ
Mixing resistor from Volume 1’s wiper to V2a’s grid. With the other 270 kΩ it keeps the two volumes from loading each other much.
270k, 270 kΩ
Mixing resistor from Volume 2’s wiper to V2a’s grid.
V2a, 6EU7
First half of the second 6EU7, the gain stage after the mix. The feedback from the speaker goes to its cathode.
100k, 100 kΩ
Plate resistor for V2a. The chart reads +160 V on the plate.
1.5k, 1.5 kΩ
Cathode resistor for V2a, +1.2 V on the chart. It’s also the bottom of the feedback divider: 120 kΩ above it, so about 1.2 % of the speaker’s voltage comes back.
.005µ, 5 nF
Only .005 µF across V2a’s cathode resistor, so the stage has its full gain only from about 20 kHz up. Below that the cathode resistor lowers the gain and the feedback works on it.
120k, 120 kΩ
The feedback resistor, 120 kΩ from the speaker. With Preamp only or another power amp, its far end is grounded.
.0033µ, 3.3 nF
The Tone Expander’s cap, in the feedback path. On low the switch is open, so the feedback comes through this cap alone. It passes treble and blocks bass, so less bass is fed back and the bass comes up.
220k, 220 kΩ
From V2a’s plate to the treble cut cap. With the .001 µF it rolls the treble off above about 500 Hz, before the tone pot.
.001µ, 1 nF
The fixed treble cut, .001 µF to ground behind 220 kΩ. The feedback loop runs around it, so the speaker sees less of the cut than this node.
.02µ, 20 nF
Coupling cap to V3a’s grid.
Tone, 1 MΩ pot, audio taper
Tone, 1 MΩ audio, from V3a’s grid to ground, with .005 µF from the grid to the wiper. At full the wiper sits on the grid, so the cap does nothing. Turned down, the cap reaches further down the track and bleeds more treble to ground. Up is assumed to be brighter.
.005µ, 5 nF
The tone pot’s cap, from V3a’s grid to the wiper.
V3a, 6EU7
First half of the third 6EU7, the last gain stage. It drives the cathodyne, which has no gain of its own, so it brings the whole swing the 6V6s need.
100k, 100 kΩ
Plate resistor for V3a, from the 263 V node. The chart reads +180 V on the plate.
1.5k, 1.5 kΩ
Cathode resistor for V3a, not bypassed, +1.3 V on the chart.
.005µ, 5 nF
Coupling cap into the cathodyne’s grid. The cathodyne’s grid resistor goes to its cathode, so the grid looks like several megohms and the .005 µF still passes the low E.
Reverb, 1 MΩ pot, audio taper
Reverb, 1 MΩ audio, off channel 2’s coupling cap. Its wiper is the reverb send’s grid. Turn it up to send more to the springs.
V2b, 6EU7
Second half of the second 6EU7, the reverb send’s gain stage.
470k, 470 kΩ
Plate resistor for V2b, +122 V on the chart.
4.7k, 4.7 kΩ
Cathode resistor for V2b, not bypassed, +1.25 V on the chart.
.01µ, 10 nF
Coupling cap to the reverb driver.
470k, 470 kΩ
The reverb driver’s grid resistor.
V4b, 12AU7 / ECC82
The 12AU7A’s second half, the reverb driver. Its first half is the cathodyne. The chart reads +8.6 V on its cathode, about 8.6 mA.
1k, 1 kΩ
Cathode resistor for the reverb driver, not bypassed.
Reverb T, 10 kΩ to 8 Ω, 10 H primary, 50 mH leakage
The reverb transformer, from the driver’s plate to the reverb unit. It loads the driver and the supply. The primary’s 1.5 kΩ is set from the chart’s 13 V across it at 8.6 mA. The rest is typical of the part, not measured.
Unit in R, 800 mΩ
Resistance of the reverb unit’s input coil. The current through it drives the springs, which play from a measured tank’s impulse response. Typical of a long spring tank, not measured.
Unit in L, 1.3 mH
Inductance of the reverb unit’s input coil. Typical of a long spring tank, not measured.
Unit out L, 360 mH
The reverb unit’s output coil, from the recovery grid to ground. The springs come back in series with it. Typical of a long spring tank, not measured.
Unit out R, 200 Ω
Resistance of the unit’s output coil. Typical, not measured.
100k, 100 kΩ
Grid resistor for the recovery stage.
.001µ, 1 nF
From the recovery grid to ground. It trims the springs’ highest treble.
V3b, 6EU7
Second half of the third 6EU7, the reverb recovery stage. It brings the springs back up, into V3a’s grid.
470k, 470 kΩ
Plate resistor for the recovery stage, +122 V on the chart.
4.7k, 4.7 kΩ
Cathode resistor for the recovery stage, +1.2 V on the chart.
20µ, 20 µF electrolytic
Bypass for the recovery stage’s cathode.
47k, 47 kΩ
From the recovery plate toward V3a’s grid, where the reverb joins the dry signal. With the .005 µF it also loads V3a’s grid slightly.
.005µ, 5 nF
The reverb return’s coupling cap into V3a’s grid.
V4a, 12AU7 / ECC82
12AU7 (ECC82), 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.
56k, 56 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.
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. It’s the grid’s only path to ground, and the cathode follows the grid, so the preamp sees much more than 1M Ω.
.02µ, 20 nF
Coupling cap to tube A’s grid.
.02µ, 20 nF
Coupling cap to tube B’s grid.
V5, 6V6GT
6V6GT, tube A of the push-pull pair, its cathode on the shared cathode resistor, so it biases itself.
V6, 6V6GT
6V6GT, tube B of the pair.
220k, 220 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.
220k, 220 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.
20µ, 20 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.
3 H, 3 H
The choke from the reservoir cap to B+, as on the amp’s drawing. The output transformer’s center tap, the screens and everything after them hang past it, so all of their current runs through it. It passes DC with little drop. With the cap after it, it would filter ripple, but ripple isn’t modeled here. Typical of the part, not measured.
55, 55 Ω
The choke’s winding resistance, typical, not measured. With the output tubes’ whole current through it, it’s most of the DC drop between the reservoir and B+.
10µ, 10 µF electrolytic
Filter cap on B+ after the choke, where the output transformer’s center tap and the screens hang.
4.7k, 4.7 kΩ
Dropping resistor from the screen supply node to the node for the inverter, the third stage and the reverb driver, as on the Gibson GA-30RV Invader 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 inverter, the third stage and the reverb driver. With the dropping resistor it filters the supply and keeps the stages on this node from coupling to each other through it.
6.8k, 6.8 kΩ
Dropping resistor from the node for the inverter, the third stage and the reverb driver to the node for the preamp, as on the Gibson GA-30RV Invader 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 preamp. With the dropping resistor it filters the supply and keeps the stages on this node from coupling to each other through it.
OT, 8 kΩ to 8 Ω, 10 H primary, 10 mH leakage
The output transformer, 8 kΩ plate to plate into 8 Ω, center tap on B+ after the choke. 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, 320 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 320 V ac from the grounded center tap, as listed in the drawing’s voltage chart, as a sine source at the mains frequency.
PT, 320 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.
V7, 5Y3GT
One plate of the 5Y3GT 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.
V7, 5Y3GT
The rectifier’s other plate, on the other half cycle.
20µ, 20 µF electrolytic
The reservoir cap. It feeds the choke, and past it the output transformer’s center tap and everything else. 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.
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 6EU7 uses the 12AX7 model, and the 12AU7A the 12AU7 model. The transformers and the reverb unit are typical, not on the drawing.

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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