Four-transistor sustainer, 1970s 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 Electro-Harmonix Big Muff π.

A boost, two clipping stages with diodes in their feedback, a passive tone stack, then a recovery stage, all four transistors common-emitter. Values from a traced mid-’70s version. The circuit changed constantly over its life.

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. Each clipping stage clips a signal the stage before already clipped. The 470 pF caps around three of the four transistors roll off the top at every step, so the fizz never builds up. The tone stack fades a low-pass against a high-pass. At noon it carves out the mids, and that’s the scoop. Probe each collector in turn and watch the wave go from sine to square.

Download.

Parts and what they do

Stock values for the Silicon NPN version. Tap a part on the schematic to change it.

C14, 100 µF electrolytic
Supply filter cap.
C1, 100 nF
Input coupling cap.
R1, 39 kΩ
Input series resistor. It sets most of the input impedance and, with the stage’s shunt feedback, its gain.
Q1, 2N5088
Input boost. Shunt feedback from collector to base sets both its bias and its gain.
R2, 100 kΩ
Base to ground. With R3 it sets the collector voltage.
R3, 470 kΩ
Collector-to-base feedback.
C2, 470 pF
Feedback cap. Its effect is multiplied by the stage gain, so it rolls off the treble well below what its size suggests.
R4, 100 Ω
Emitter resistor.
R5, 15 kΩ
Collector load.
C3, 1 µF
Coupling cap to the sustain pot.
Sustain, 100 kΩ pot, audio taper
How much of the boosted signal reaches the clipping stages.
R7, 1 kΩ
Keeps a little signal flowing with the sustain all the way down.
C4, 1 µF
Coupling cap into the first clipping stage.
R8, 8.2 kΩ
Series input resistor for the first clipping stage. With R10 it sets the stage gain.
Q2, 2N5088
First clipping stage.
R9, 100 kΩ
Base to ground.
R10, 470 kΩ
Collector-to-base feedback.
C5, 470 pF
Feedback cap, rolling off the treble.
C6, 100 nF
Blocks DC from the diodes, so they only see the signal. Smaller values leave the bass unclipped.
D1, D2, 1N914 / 1N4148, one each way
First pair of clipping diodes, from collector to base. They limit how far the collector can swing from the base.
R12, 100 Ω
Emitter resistor.
R11, 15 kΩ
Collector load.
C7, 1 µF
Coupling cap into the second clipping stage.
R13, 8.2 kΩ
Series input resistor for the second clipping stage.
Q3, 2N5088
Second clipping stage.
R16, 100 kΩ
Base to ground.
R15, 470 kΩ
Collector-to-base feedback.
C8, 470 pF
Feedback cap, rolling off the treble.
C9, 100 nF
Blocks DC from the second diode pair.
D3, D4, 1N914 / 1N4148, one each way
Second pair of clipping diodes.
R14, 100 Ω
Emitter resistor.
R17, 15 kΩ
Collector load.
C10, 3.9 nF
High-pass cap in the tone stack. With R19 its corner is near 1 kHz.
R19, 39 kΩ
High-pass resistor.
R18, 100 kΩ
Low-pass resistor in the tone stack. With C11 its corner is near 160 Hz in this version. Later versions used 22 k to 39 k.
C11, 10 nF
Low-pass cap.
Tone, 100 kΩ pot, linear taper
Fades between the low-pass and high-pass outputs. The gap between their corners is the mid scoop.
C12, 100 nF
Coupling cap into the recovery stage.
Q4, 2N5088
Recovery stage. Makes up the level the tone stack throws away.
R21, 100 kΩ
Base to ground.
R22, 390 kΩ
Collector-to-base bias resistor.
R23, 2.2 kΩ
Emitter resistor, unbypassed, so the gain is about R24 / R23.
R24, 10 kΩ
Collector load.
C13, 100 nF
Output coupling cap.
Volume, 100 kΩ pot, audio taper
Output level.

Versions

Silicon NPN
Negative ground. Every stage is biased by feedback from its own collector, so it tolerates almost any high-gain silicon transistor.
Second drawing
The values of a second, unsigned drawing of the circuit: 1 µF couplings, 500 pF feedback caps, and a tone stack with 39 kΩ and 10 nF on the low-pass side and 4 nF and 100 kΩ on the high-pass side, both corners near 400 Hz. Its input has the 39 kΩ before the 1 µF, which this build leaves the other way round.
Russian, 2000s, modeled after the Electro-Harmonix Big Muff π (Russian, 2000s)
The black Russian box of the 2000s, from a kit maker’s drawing. 12 kΩ collectors, 390 Ω emitters on the first three stages, 47 nF ahead of each diode pair, 100 nF couplings, a tone stack of 39 kΩ and 10 nF against 3.9 nF and 22 kΩ, and Q4 biased by 470 kΩ from the battery over 100 kΩ.
2002 reissue, modeled after the Electro-Harmonix Big Muff π (2002 reissue)
New Sensor’s drawing EC 3003A of 2 December 2002. 10 kΩ collectors, 390 Ω emitters on the clipping stages, 1 µF couplings, 1N6263 Schottky diodes (the BAT41 preset stands in), both tone corners near 700 Hz and 1.9 kHz, linear pots, and Q4 biased by 470 kΩ from the battery over 100 kΩ.

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.

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.

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