Germanium Darlington fuzz with a bias Balance, 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 Baldwin-Burns Buzzaround.

A germanium Darlington pair as a booster, a Sustain pot into a third transistor, and a Timbre pot that pans between a bass and a treble path. Balance sets the third transistor’s collector supply, which changes its tone as well as its level.

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. Q1 and Q2 are a Darlington pair, about 38 dB of gain at 1 kHz into the Sustain pot. Q3 has no bias of its own. Its base returns to ground through the Sustain pot, so it runs on its leakage, and its collector supply is only what the Balance pot passes, at most about 2.4 V. So Q3 clips early both ways, and D2 clips the swing that turns it off. The Timbre pot takes the output from between a bass path, 100 nF, and a treble path, 1 nF with 10 kΩ to ground. The model leaves out the leakage spread between real germanium transistors, which moves Q3’s idle a lot.

Download.

Parts and what they do

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

C1, 100 nF
Input coupling cap.
R1, 100 kΩ
Lower leg of Q1’s base divider.
R2, 470 kΩ
Upper leg of Q1’s base divider, from the supply.
C2, 100 pF
From the collectors back to Q1’s base. It cuts the pair’s gain at high frequencies.
Q1, OC81D
First half of the Darlington pair. Its emitter drives Q2’s base.
R4, 10 kΩ
Q1’s emitter to ground, so Q1 passes some current of its own.
Q2, OC81D
Second half of the Darlington pair, collector shared with Q1.
R5, 3.3 kΩ
Q2’s emitter resistor. It sets the pair’s current.
C4, 4.7 µF electrolytic
Bypasses R5, so the pair’s gain is high above about 10 Hz.
R3, 10 kΩ
The pair’s collector load. The kit has 1 kΩ and a 50 kΩ trimmer, set to about 10 kΩ in all. The model uses one fixed 10 kΩ.
C3, 4.7 µF electrolytic
Coupling cap into the Sustain pot.
Sustain, 100 kΩ pot, linear taper
A 100 kΩ linear pot wired as a volume control into Q3’s base. At zero it shuts the signal off.
D2, 1N270 germanium
Germanium diode from Q3’s base to ground. It conducts on the swing that turns Q3 off, so it clips that side before Q3.
Q3, AC128, high gain
The fuzz stage, emitter to ground. Its base has no bias but the Sustain pot to ground, so it runs on its leakage.
R6, 15 kΩ
Q3’s collector load, fed from the Balance wiper.
R7, 27 kΩ
Feeds the Balance pot from the supply. With the 10 kΩ pot, the most Q3’s supply reaches is about 2.4 V.
Balance, 10 kΩ pot, linear taper
A 10 kΩ linear pot from R7 to ground. Its wiper is Q3’s supply, so it sets Q3’s collector voltage, its level and how hard it clips.
C5, 100 nF
To the Timbre pot’s bass end.
C6, 1 nF
To the Timbre pot’s treble end.
R8, 10 kΩ
From the treble end to ground. With C6 it passes only the highs.
Timbre, 100 kΩ pot, linear taper
A 100 kΩ linear pot between the two paths, the output on its wiper. Clockwise is the bass end, counterclockwise the treble end.

About this version

Germanium PNP set
Positive ground, so every voltage reads negative. Transistors within the kit maker’s selection advice: low leakage for the pair, a leakier, higher-gain Q3.

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.