Silicon fuzz with a clean and fuzz Balance, early 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 Maestro Super Fuzz FZ-1S.

An emitter follower splits the guitar two ways. One path stays clean, the other goes through two gain stages and a pair of transistors used as diodes. A dual Balance pot mixes the two into a last gain stage.

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 splits the guitar two ways from its emitter. The clean path goes straight to one half of the Balance pot. The fuzz path goes through Sustain into Q3, which runs near saturation with its collector about 0.75 V, then Q4, which idles nearly off. So the note has to be fairly large before anything gets through. Q5 and Q6 then hold it near 0.6 V either way. The Fuzz switch feeds that to the Balance pot through 470 kΩ, or through a filter that passes the treble. Q2 brings the mix back up about 6 times. As drawn the fuzz path is quiet: at full fuzz and full Volume a 200 mV pick comes out at about 40 mV. The drawing doesn’t show which way each half of the Balance pot turns.

Download.

Parts and what they do

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

C1, 100 nF
Input coupling cap.
R5, 100 kΩ
Top of Q1’s base bias.
R4, 47 kΩ
Bottom of Q1’s base bias. With R5 it holds the base near 2.9 V and sets the input impedance near 32 kΩ.
Q1, 2N5088
Emitter follower. No voltage gain. It drives both paths from a low impedance.
R6, 15 kΩ
Q1’s emitter load.
C2, 50 nF
Couples Q1 into the fuzz path.
Sustain, 100 kΩ pot, linear taper
Sustain, a 100 kΩ linear pot. It sets how much signal reaches Q3.
R13, 680 Ω
Under the Sustain pot, so a little signal gets through at 0.
C6, 10 nF
From the Sustain wiper to ground. Against the pot it trims the treble going into Q3, more with the knob down.
C7, 20 nF
Coupling cap into Q3.
R14, 1.5 MΩ
From Q3’s collector to its base. It sets Q3’s bias and feeds some of the signal back.
Q3, 2N5088
First gain stage of the fuzz path.
R15, 150 kΩ
Q3’s collector load.
R16, 470 Ω
Q3’s emitter resistor, unbypassed.
C8, 20 nF
Coupling cap into Q4.
R18, 1 MΩ
Top of Q4’s base bias.
R17, 47 kΩ
Bottom of Q4’s base bias. With R18 it holds the base near 0.4 V, so Q4 idles close to off.
Q4, 2N5088
Second gain stage, emitter grounded. Biased near off, it turns on only on the positive half of the wave.
R19, 100 kΩ
Q4’s collector load.
C9, 5.6 nF
Coupling cap toward the clipping transistors. Against R20 it cuts below about 420 Hz.
R20, 68 kΩ
Series resistor into the clipping transistors.
Q5, Q6, 2N5088 base-emitter, one each way
Q5 and Q6, each wired as a diode, back to back to ground. They hold the fuzz path near 0.5 V either way. Modeled as one base-emitter junction each.
R23, 470 kΩ
In one Fuzz switch position, the fuzz path reaches the Balance pot through this.
R21, 220 kΩ
First series leg of the filter.
C10, 680 pF
Across R21. Above about 1 kHz it passes more than R21 does.
R24, 100 kΩ
From the filter’s middle to ground.
R22, 220 kΩ
Second series leg of the filter.
C11, 680 pF
Across R22.
Balance, 30 kΩ pot, linear taper
Balance, the fuzz half of a dual 30 kΩ linear pot. Turned up, more fuzz reaches Q2.
R8, 33 kΩ
Mixing resistor from the fuzz half.
C3, 220 nF
Couples Q1 into the clean half of the Balance pot.
Balance (clean), 30 kΩ pot, linear taper
The clean half of the Balance pot, turned by the same knob. It turns the other way, so the clean signal fades as the fuzz comes up. The drawing doesn’t show which way each half turns.
R7, 33 kΩ
Mixing resistor from the clean half.
C4, 50 nF
Coupling cap into Q2.
R10, 1 MΩ
From Q2’s collector to its base, for bias.
R9, 470 kΩ
From Q2’s base to ground. With R10 it sets Q2’s collector voltage.
Q2, 2N5088
Output gain stage for both paths.
R12, 10 kΩ
Q2’s collector load.
R11, 1.5 kΩ
Q2’s emitter resistor, unbypassed. With R12 it holds Q2’s gain near 6.
C5, 1 µF electrolytic
Output coupling cap.
Volume, 100 kΩ pot, audio taper
Volume, a 100 kΩ audio pot.

About this version

2N5088
Negative ground, 9 V. 2N5088 transistors, one of the two types the drawing lists. Q5 and Q6, used as diodes, are modeled as base-emitter junctions.

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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