Three-transistor germanium fuzz, 1966 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 II.

The two-transistor fuzz with one more germanium stage in front of it. The first transistor has no bias network at all: it runs on its own leakage current.

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. The front stage makes the fuzz much hotter and less fussy about what drives it, but it only works because Q1 leaks. Its base has only a resistor to ground, so its only current is leakage times its gain. With a less leaky transistor it barely conducts, and the pedal gets thin and noisy. Try the OC81D build, which Sola Sound paired with a 100 kΩ base resistor for that reason, or open Q1 and turn its leakage down. Q2 and Q3 are the 1966 two-transistor fuzz’s feedback pair, and the Fuzz control is the same emitter bypass.

Download.

Parts and what they do

Stock values for the OC75, 1966 version. Tap a part on the schematic to change it.

C1, 10 nF
Cap to ground at the input. With the guitar’s pickup it trims the top end. Units had 5 to 15 nF, or none.
C2, 4.7 µF electrolytic
Input coupling cap.
R1, 10 kΩ
Q1’s only bias, a resistor from base to ground. Q1 conducts only its leakage current times its gain, and a bigger resistor multiplies it more.
Q1, OC75, leaky (first-stage pick)
Booster stage, biased by its own leakage. It needs a leaky transistor (100 to 300 µA) to work right.
R2, 10 kΩ
Q1 collector load. Q1 idles with its collector close to the supply.
C3, 100 nF
Coupling cap from the booster into the fuzz pair.
Q2, OC75, leaky (second-stage pick)
First transistor of the feedback pair, run nearly saturated: its collector sits a few tenths of a volt from ground.
R3, 47 kΩ
Q2 collector load, and through the direct connection, Q3’s base bias.
Q3, OC75, high gain
Second transistor of the pair, where most of the clipping happens. Use the highest-gain transistor of the three here.
R4, 8.2 kΩ
Most of Q3’s collector load.
R5, 470 Ω
The small end of Q3’s collector load. The output is across it.
R6, 100 kΩ
Feedback from Q3’s emitter to Q2’s base. It biases the pair (builds often use a trimmer here).
Fuzz, 1 kΩ pot, linear taper
Q3’s emitter resistor, partly bypassed by C5. Turning it up raises Q3’s gain and shunts away the feedback signal.
C5, 4.7 µF electrolytic
Emitter bypass cap on the fuzz pot.
C6, 10 nF
Output coupling cap (15 nF in some units, for slightly more bass).
Level, 100 kΩ pot, audio taper
Output level.

Versions

OC75, 1966
Positive ground, so every voltage reads negative. Leaky OC75s with a 10 kΩ base resistor on Q1, as in the first units.
Mk 1.5, modeled after the Sola Sound Tone Bender Mk 1.5
The version before, without the first transistor: the input goes straight to the feedback pair, so it’s the 1966 two-transistor fuzz with these values. One cap also differed in that version, and it isn’t changed in this build.
OC81D in Q1
The later arrangement for Q1, a less leaky OC81D with a 100 kΩ base resistor, said to sound the same. Q2 and Q3 are left as in the first build.

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.

From Swell Amplification, swellamps.com.