Three-transistor germanium fuzz, 1965 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 Sola Sound Tone Bender Mk I.

An emitter follower, then two germanium gain stages coupled by caps. The Attack pot sets the second transistor’s bias and shunts its input, and Q3 runs on its own leakage.

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. Three stages, none of them with feedback. Q1 follows the input and drives Q2’s base through a 25 µF cap. Q2’s bias comes from R3 against R4 and the Attack pot, so Attack sets how hard Q2 is turned on and how much of the signal reaches it. Q3 has only an 8.2 kΩ from base to ground and idles nearly off, about 20 mV from the supply on its collector. It conducts only on the half of each swing that turns it on, so small notes die away suddenly and loud ones come out lopsided and fuzzy. The 2.2 MΩ in series with the Level pot keeps the output low: about 0.46 V rms with Level full. The model leaves out the leakage spread between real transistors, which moves Q3’s idle and the gating a lot.

Download.

Parts and what they do

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

C1, 10 nF
Input coupling cap. Q1’s input is only about 85 kΩ, since Q2’s base loads its emitter, so the lows are about 5 dB down at 100 Hz at Q1’s base.
R1, 1 MΩ
Q1’s base resistor to ground. There’s no bias from the supply, so Q1 runs on its leakage.
Q1, OC75, leaky (first-stage pick)
Emitter follower, voltage gain slightly under 1. It drives Q2’s base, which pulls a lot of signal current.
R2, 8.2 kΩ
Q1’s emitter load.
C2, 25 µF electrolytic
Coupling cap into Q2.
R3, 470 kΩ
Q2’s base bias from the supply.
R4, 1.8 kΩ
In series with the Attack pot from Q2’s base to ground. It sets the least resistance there.
Attack, 50 kΩ pot, linear taper
A 50 kΩ linear pot wired as a variable resistor from R4 to ground. Turned down it pulls Q2’s base bias toward zero and shunts the signal coming in.
Q2, 2G381
First gain stage, emitter to ground.
R5, 2.2 kΩ
Q2’s collector load.
C3, 100 nF
Coupling cap into Q3.
R6, 8.2 kΩ
Q3’s only bias, a resistor from base to ground. Q3 conducts on its leakage, so a leakier transistor idles lower on its collector.
Q3, 2G381
Second gain stage, emitter to ground, where most of the clipping happens.
R7, 15 kΩ
Q3’s collector load.
C4, 100 nF
Output coupling cap.
R8, 47 kΩ
Load to ground after the output cap.
R9, 2.2 MΩ
A 2.2 MΩ in series with the Level pot. With the 500 kΩ pot it divides the output by about 5.
Level, 500 kΩ pot, audio taper
Output level.

About this version

OC75 and 2G381s
Positive ground, so every voltage reads negative. An OC75 in the input stage and 2G381s in the two gain stages, as the kit drawing marks them.

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.

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.

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