Two-6BM8 tremolo amp, 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-8T.

A 12AX7 into one volume and one tone, a second 12AX7 stage, and a 6BM8 triode as a cathodyne into the pentodes of two 6BM8s. The tremolo, another 6BM8 triode, 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. Two 12AX7 stages, with one volume and one tone between them. The tone pot’s wiper sits on the volume’s top: one end bleeds treble to ground through .005 µF, the other carries it past the volume through 500 pF. The second stage’s cathode has only .02 µF across it, so its lows get less gain. A 6BM8’s triode is the cathodyne, and the two 6BM8 pentodes are the output, in cathode bias with no feedback. The tremolo oscillator is the other 6BM8’s triode. Its swing goes through .25 µF to the Depth pot, whose wiper is where the output grids’ leaks meet, so both grids move together. With the power amp on, Depth at 3 swings a steady note about 18 dB, full about 31 dB, and Frequency runs it from about 3.8 to 8.3 times a second. There’s no 6BM8 model on the bench, so its pentode runs on the 6V6GT’s and its triode on the 12AX7’s.

Download.

Parts and what they do

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

R1A, 56 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 28 kΩ to the grid: enough to stop radio pickup, not enough to cut treble.
R1B, 56 kΩ
The 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.
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 12AX7, the input stage.
100k, 100 kΩ
Plate resistor for V1a. The chart reads +130 V on the plate.
1.5k, 1.5 kΩ
Cathode resistor for V1a, +1 V on the chart.
20µ, 20 µF electrolytic
Bypass for V1a’s cathode resistor, full gain down to a few hertz.
.02µ, 20 nF
Coupling cap from V1a to the top of the volume pot.
Vol 1M, 1 MΩ pot, audio taper
Volume, 1 MΩ audio. Its wiper is V1b’s grid. There’s no master.
Tone, 1 MΩ pot, audio taper
Tone, 1 MΩ audio, its wiper on the volume’s top. Turned up, the wiper moves toward the 500 pF that carries treble past the volume to V1b’s grid. Turned down, it moves toward the .005 µF to ground, which bleeds treble off. Up is assumed to be brighter.
500p, 500 pF
The tone pot’s bright cap, to the volume’s wiper.
.005µ, 5 nF
The tone pot’s cut cap, to ground.
V1b, 12AX7 / ECC83 / 7025
Second half of the 12AX7. It drives the cathodyne, which has no gain of its own.
100k, 100 kΩ
Plate resistor for V1b. The chart reads +130 V on the plate.
2.2k, 2.2 kΩ
Cathode resistor for V1b, +1 V on the chart.
.02µ, 20 nF
Only .02 µF across V1b’s cathode resistor, so the stage has full gain only on the treble, above about 3.6 kHz, and less on the lows.
500p, 500 pF
Only 500 pF into the cathodyne’s grid. The cathodyne’s grid resistor goes to its cathode, so the grid looks like a few megohms and the 500 pF still passes most of the guitar’s range.
270k, 270 kΩ
Plate resistor for the oscillator, from the screen node. The chart reads +85 V on the plate.
V3a, 6BM8 / ECL82 (triode section)
The triode of V3’s 6BM8, the tremolo oscillator.
2.7k, 2.7 kΩ
Cathode resistor for the oscillator, +1.5 V on the chart.
20µ, 20 µF electrolytic
Bypass for the oscillator’s cathode resistor.
.047µ, 47 nF
First cap of the oscillator’s phase-shift ladder.
47k, 47 kΩ
In series with the Frequency pot, the ladder’s first leg. It sets the top speed.
Freq, 500 kΩ pot, reverse audio taper
Frequency, 500 kΩ reverse audio, as a variable resistor, its far end grounded through the foot switch. Turned up, its resistance drops and the oscillator runs faster.
.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.
.25µ, 250 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, 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.
V2a, 6BM8 / ECL82 (triode section)
6BM8 triode, 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.
V2b, 6BM8 / ECL82 (pentode section)
6BM8, tube A of the push-pull pair, its cathode on the shared cathode resistor, so it biases itself.
V3b, 6BM8 / ECL82 (pentode section)
6BM8, 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.
22k, 22 kΩ
Dropping resistor from the screen supply node to the node for the cathodyne and preamp, as on the Gibson GA-8T 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 cathodyne and 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.
PT, 280 V ac, 60 Hz
One half of the power transformer’s high-voltage winding, 280 V ac from the grounded center tap, as a sine source at the mains frequency. The value is typical of the part.
PT, 280 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, 300 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.
PT R, 300 Ω
Resistance of the other half. Typical of the part.
V4, 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.
V4, 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 12AX7 model is fitted to plate charts, and the 6BM8 triode model to the Philips ECL82 chart.

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. Caps are ideal, and the battery or HT is a voltage behind a resistance.

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