Two-6V6 reverb amp with grid tremolo, 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-19RVT Falcon.

One 6EU7 gain stage, then Volume, Tone and a reverb send off the same node, a second 6EU7 stage, and a cathodyne with no driver into two cathode-biased 6V6s. The tremolo swings both output grids through their grid leaks.

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. One 6EU7 stage, then three pots off one node: Volume to the second stage’s grid, Reverb to the reverb send, and Tone, whose wiper sends the signal through 500 pF around the volume or through .005 µF to ground. The second stage has only .02 µF across its cathode resistor, so it has its full gain only from about 3 kHz up. Dry and reverb meet on the inverter’s grid through 500 pF each. The inverter is a cathodyne with no driver ahead of it, into two cathode-biased 6V6s with no feedback. The tremolo oscillator is the other half of the cathodyne’s tube. Its swing goes through the Depth pot to where the 6V6s’ grid leaks meet, so both grids move together and the output stage’s gain rises and falls, about 4 to 8 times a second. The springs are a measured tank, played from its impulse response. The 6V6s idle on 270 Ω at about 19 V and deliver about 10 W before they clip. 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 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.
R1B, 100 kΩ
The 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.
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, 6EU7
First half of a 6EU7, the input stage, cathode bypassed for full gain.
220k, 220 kΩ
Plate resistor for V1a. The drawing reads +135 V on the plate.
2.7k, 2.7 kΩ
Cathode resistor for V1a, +1 V on the drawing.
20µ, 20 µF electrolytic
Bypass for V1a’s cathode resistor.
.02µ, 20 nF
Coupling cap to the node where Volume, Reverb and Tone all start.
Vol 2M, 2 MΩ pot, audio taper
Volume, 2 MΩ audio. Its wiper is V3’s grid. There’s no master, so it sets how hard everything after it is pushed.
Reverb 2M, 2 MΩ pot, audio taper
Reverb, 2 MΩ audio. Its wiper feeds the reverb driver. Turn it up to send more to the springs.
Tone 1M, 1 MΩ pot, audio taper
Tone, 1 MΩ audio. The signal comes in on the wiper. One end goes through 500 pF to V3’s grid, around the volume pot. The other goes through .005 µF to ground. Turned up, the wiper moves toward the 500 pF, so more treble gets around the volume and less is bled away. Up is assumed to be brighter.
.005µ, 5 nF
The tone pot’s treble cut, to ground.
500p, 500 pF
The tone pot’s bright cap, to V3’s grid.
V3, 6EU7
One half of a 6EU7, the second gain stage.
220k, 220 kΩ
Plate resistor for V3. The drawing reads +135 V on the plate.
2.7k, 2.7 kΩ
Cathode resistor for V3, +1 V on the drawing.
.02µ, 20 nF
Only .02 µF across V3’s cathode resistor, so the stage has full gain only in the treble, from about 3 kHz up. Below that the cathode resistor lowers the gain.
500p, 500 pF
Only 500 pF into the inverter’s grid. With the cathodyne’s high input resistance it still passes the guitar’s range, and it meets the reverb’s 500 pF there.
100k, 100 kΩ
From the Reverb pot’s wiper to V1b’s grid.
V1b, 6EU7
Second half of the first 6EU7, the reverb send’s gain stage.
100k, 100 kΩ
Plate resistor for V1b, +180 V on the drawing.
3.9k, 3.9 kΩ
Cathode resistor for V1b, not bypassed, +1.3 V on the drawing.
.005µ, 5 nF
Coupling cap to the reverb driver. Into 470 kΩ it passes everything above about 70 Hz.
470k, 470 kΩ
The reverb driver’s grid resistor.
V2 triode, 7199 (triode section)
The 7199’s triode section, the reverb driver. Its cathode resistor isn’t bypassed. The drawing reads +9 V on it, about 9 mA.
1k, 1 kΩ
Cathode resistor for the reverb driver.
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 2.2 kΩ is set from the drawing’s 20 V across it at 9 mA. The rest is typical of the part, not measured.
Unit in R, 800 mΩ
Resistance of the reverb unit’s input coil, 0.8 Ω. The current through it drives the springs, which play from a measured tank’s impulse response. The drawing shows no figures, so these are a measured long tank’s, typical, not measured on this unit.
Unit in L, 1.1 mH
Inductance of the reverb unit’s input coil, 1.1 mH, a measured long tank’s figure. Typical, not measured on this unit.
Unit out L, 350 mH
The reverb unit’s output coil, 0.35 H, from the recovery grid to ground. The springs come back in series with it. Its figures are a measured long tank’s, typical, not measured on this unit.
Unit out R, 181 Ω
Resistance of the unit’s output coil, 181 Ω, a measured long tank’s figure.
100k, 100 kΩ
Grid resistor for the recovery stage.
.001µ, 1 nF
From the recovery grid to ground. It trims the springs’ highest treble.
V2 pentode, 7199 pentode (RCA sheet)
The 7199’s pentode section, the reverb recovery stage. Its screen sits at only about 15 V, so it runs on half a milliamp. It brings the springs back up into the mix.
470k, 470 kΩ
Plate resistor for the recovery stage, +64 V on the drawing.
1.5M, 1.5 MΩ
Screen resistor for the recovery stage, +15 V on the drawing.
.047µ, 47 nF
Screen bypass cap for the recovery stage.
2.2k, 2.2 kΩ
Cathode resistor for the recovery stage, +1 V on the drawing.
20µ, 20 µF electrolytic
Bypass for the recovery stage’s cathode.
500p, 500 pF
From the recovery plate to the inverter’s grid, 500 pF, the same as the dry path’s.
270k, 270 kΩ
Plate resistor for the oscillator, from the 240 V node. The drawing reads +130 V on the plate.
V4b, 6EU7
The other half of V4’s 6EU7, the tremolo oscillator.
2.7k, 2.7 kΩ
Cathode resistor for the oscillator, +1.1 V on the drawing.
20µ, 20 µF electrolytic
Bypass for the oscillator’s cathode.
.047µ, 47 nF
First cap of the oscillator’s phase-shift ladder.
47k, 47 kΩ
In series with the Speed pot, the ladder’s first leg. It sets the top speed.
Speed, 500 kΩ pot, reverse audio taper
Speed, 500 kΩ reverse audio, as a variable resistor in the ladder’s first leg. Turned up, its resistance drops and the oscillator runs faster. Its far end goes through the foot switch to ground.
.047µ, 47 nF
Second ladder cap.
240k, 240 kΩ
Second ladder leg to ground.
.047µ, 47 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.
510k, 510 kΩ
Grid leak for the oscillator, and the ladder’s last leg.
.22µ, 220 nF
Couples the oscillator’s swing from its plate to the Depth pot and blocks the plate’s DC.
Depth, 250 kΩ pot, reverse audio taper
Depth, 250 kΩ reverse audio, from the cap to ground. Its wiper is where the output tubes’ two grid leaks meet, so it sets how much of the swing reaches both grids together. Turned up, more of it does. On Preamp only, the wiper drives nothing.
V4a, 6EU7
6EU7, 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.
.02µ, 20 nF
Coupling cap to tube A’s grid.
.02µ, 20 nF
Coupling cap to tube B’s grid.
10k, 10 kΩ
Grid stopper for tube A. It keeps the tube from oscillating and softens grid current.
10k, 10 kΩ
Grid stopper for tube B.
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, returned with tube B’s to the tremolo’s Depth pot, so the oscillator swings both grids together. 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.
270, 270 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.
470, 470 Ω
Dropping resistor from B+ to the screen supply. The screens sit slightly below the plates.
10µ, 10 µF electrolytic
Filter cap on the screen supply.
4.7k, 4.7 kΩ
Dropping resistor from the screen supply node to the node for the inverter, the tremolo and the reverb driver, as on the Gibson GA-19RVT Falcon 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 tremolo 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.
10k, 10 kΩ
Dropping resistor from the node for the inverter, the tremolo and the reverb driver to the node for the preamp, as on the Gibson GA-19RVT Falcon 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+. 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, 290 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 290 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, 290 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 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.
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. The 7199 is typical of the type, set at one published point and the readings on this drawing. 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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