Five-transistor fuzz with a split-phase pair and germanium diodes, late 1960s 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 Ace Tone FM-2 Fuzz Master.

A gain stage and a phase splitter drive two transistors in opposite phase into one shared collector load, so the two halves of the wave fold together. Germanium diodes clip that, a switch picks a filter or a plain divider, and a high-gain last stage clips again.

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. Q2 splits Q1’s output into two copies in opposite phase, one to Q3 and one to Q4. Their collectors share R16, so on each half of the wave one or the other pulls it down. Both halves come out the same way up, a wave that repeats at twice the note, so the octave above is the strongest part of the sound. The germanium diodes then hold it to about 0.2 V either way. The Tone switch picks a bridged-T filter, which cuts the mids hardest near 1 kHz (about 20 dB under the divider there for a small signal), or a plain divider. Fuzz and Volume both come before Q5, whose gain is well over 100. Single notes up the neck show the octave best. The model leaves out the mismatch between the two transistors of the pair, which unbalances the fold in real units.

Download.

Parts and what they do

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

R1, 100 kΩ
Input pulldown.
C1, 1 µF electrolytic
Input coupling cap.
R5, 22 kΩ
From the battery to the top of Q1’s collector load.
R4, 10 kΩ
Q1’s collector load, below R5.
R6, 100 kΩ
Q1’s base bias, from between R5 and R4. That point follows the collector, so it also feeds some of the signal back.
R2, 15 kΩ
Bottom of Q1’s base bias.
C2, 100 pF
Collector to base on Q1. It rolls off the very top.
Q1, 2N3904
Input gain stage.
R3, 1.8 kΩ
Q1’s emitter resistor, unbypassed. With R4 it keeps Q1’s gain near 8.
C3, 1 µF electrolytic
Coupling cap into Q2.
R8, 220 kΩ
Top of Q2’s base bias.
R7, 100 kΩ
Bottom of Q2’s base bias.
Q2, 2N3904
Phase splitter. Equal resistors on collector and emitter, so its two outputs are the same size and opposite in phase.
R10, 10 kΩ
Q2’s collector load.
R9, 10 kΩ
Q2’s emitter load.
R11, 1 kΩ
In series from Q2’s collector toward Q3.
C4, 1 µF electrolytic
Coupling cap into Q3.
R14, 100 kΩ
Top of Q3’s base bias.
R13, 15 kΩ
Bottom of Q3’s base bias. With R14 it holds the base near 1.2 V.
R12, 1 kΩ
In series from Q2’s emitter toward Q4.
C5, 1 µF electrolytic
Coupling cap into Q4.
R18, 100 kΩ
Top of Q4’s base bias.
R17, 15 kΩ
Bottom of Q4’s base bias.
Q3, 2N3904
Driven from Q2’s collector. Its collector and emitter are joined to Q4’s.
Q4, 2N3904
Driven from Q2’s emitter, in opposite phase to Q3. On each half of the wave one of the pair pulls the shared collector down, so both halves come out the same way up.
R16, 10 kΩ
The pair’s shared collector load.
R15, 1.8 kΩ
The pair’s shared emitter resistor.
C6, 10 µF electrolytic
Bypasses R15, so the pair runs at high gain.
C7, 1 µF electrolytic
Coupling cap to the diodes.
D1, D2, 1N60 germanium, one each way
Germanium diodes back to back to ground. They hold the wave to a few tenths of a volt either way.
C8, 1 µF electrolytic
Coupling cap to the tone section.
R19, 47 kΩ
Top of the divider for the switch’s plain side.
R20, 10 kΩ
Bottom of the divider. The plain side is about a sixth of the level.
C9, 1 nF
Across the filter’s two resistors. It passes the treble around them.
R21, 22 kΩ
First leg of the filter.
R22, 10 kΩ
Second leg of the filter.
C10, 100 nF
From the filter’s middle to ground. It shunts the mids between the bass that R21 and R22 pass and the treble that C9 passes.
C13, 100 pF
Across the Fuzz pot.
Fuzz, 500 kΩ pot, linear taper
Fuzz, a 500 kΩ linear pot in series ahead of the Volume pot. Turned up it drops the resistance, so more signal reaches Q5 and Q5 clips harder.
R28, 47 kΩ
From the Fuzz wiper to the pot’s far lug, so at full the series resistance is about 43 kΩ.
Volume, 100 kΩ pot, linear taper
Volume, a 100 kΩ linear pot. It comes before Q5, so it also sets how hard Q5 is driven.
C11, 1 µF electrolytic
Coupling cap into Q5.
R24, 100 kΩ
Top of Q5’s base bias.
R23, 15 kΩ
Bottom of Q5’s base bias.
Q5, 2N3904
Last gain stage. With R25 bypassed, its gain is near R26 over R29, well over 100, so it clips.
R26, 10 kΩ
Q5’s collector load.
R25, 1.8 kΩ
Q5’s emitter resistor for DC.
C14, 4.7 µF electrolytic
Bypasses R25 above about 20 Hz.
R29, 68 Ω
The unbypassed part of Q5’s emitter resistance. It sets Q5’s gain.
C12, 1 µF electrolytic
Output coupling cap.
R27, 100 kΩ
Output pulldown.

About this version

2N3904
Negative ground, 9 V. 2N3904 transistors as listed. 1N60 germanium diodes stand in for the drawing’s unnamed ones.

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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