Three-transistor fuzz with a boost mode, 1972 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 Roland AF-100 Bee Baa.

Q1 is a treble-leaning gain stage. In fuzz mode two more stages follow it, then one of two tone networks, a Tone control and Volume. In boost mode Q1 goes straight to the output through its own path, with Boost Level in front of it.

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. In fuzz mode Q1, Q2 and Q3 are three gain stages in a row, about 80 dB together at 200 Hz, so a guitar note leaves Q3 as a near square wave even with Sustain at 0. Sustain changes the texture more than the amount of fuzz. Tone Select then picks a network: Bee is a bridged T that cuts about 15 dB near 350 Hz, Baa a plain divider with a small treble lift. In boost mode only Q1 is used, and its output reaches the amp through 6.8 nF, which cuts the bass: the boost peaks near 700 Hz. Volume works in fuzz mode only. The kit drawing was followed. It has no factory drawing behind it and uses 2N5088s for the 2SC644s.

Download.

Parts and what they do

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

Boost level, 100 kΩ pot, audio taper
Boost Level, a 100 kΩ audio pot across the input. In boost mode it sets how much guitar reaches Q1, and so the level.
R1, 10 kΩ
Under Boost Level, so a little signal gets through at 0.
Sustain, 100 kΩ pot, audio taper
Sustain, a 100 kΩ audio pot across the input. In fuzz mode it sets how much guitar reaches Q1.
R2, 1 kΩ
Under Sustain.
R3, 1 kΩ
Series resistor into Q1.
C1, 1 µF
Coupling cap into Q1.
R25, 1 kΩ
Feeds Q1 from the 9 V. With C17 it filters Q1’s supply.
C17, 47 µF electrolytic
Filter cap on Q1’s supply.
R4, 100 kΩ
Top of Q1’s base divider.
R5, 18 kΩ
Bottom of Q1’s base divider. With R4 it holds the base near 1.3 V.
Q1, 2N5088
First gain stage, used in both modes.
C2, 100 pF
From Q1’s collector to its base. It trims Q1’s gain at the top of the range.
R7, 10 kΩ
Q1’s collector load.
R6, 1.8 kΩ
Q1’s emitter resistor. It sets the current.
C3, 10 µF electrolytic
Bypasses R6, so Q1 has full gain above about 9 Hz.
C4, 1 µF
Output coupling cap of Q1, into the Boost switch.
R8, 220 kΩ
Holds the boost path at ground for DC.
C5, 6.8 nF
Boost path coupling cap. It cuts the bass: in boost mode the output is about 15 dB down at 100 Hz from its peak near 700 Hz.
R9, 10 kΩ
Boost path into the output.
C6, 1 µF
Coupling cap into Q2.
R10, 100 kΩ
Top of Q2’s base divider.
R11, 15 kΩ
Bottom of Q2’s base divider, about 1.2 V.
Q2, 2N5088
Second gain stage.
C7, 4.7 nF
From Q2’s collector to its base. It trims Q2’s treble.
R13, 10 kΩ
Q2’s collector load.
R12, 1.5 kΩ
Q2’s emitter resistor.
C8, 10 µF electrolytic
Bypasses R12.
C9, 1 µF
Coupling cap into Q3.
R14, 100 kΩ
Top of Q3’s base divider.
R15, 15 kΩ
Bottom of Q3’s base divider.
Q3, 2N5088
Third gain stage. By here a guitar note is clipped into a near square wave.
R17, 10 kΩ
Q3’s collector load.
R16, 1.5 kΩ
Q3’s emitter resistor.
C11, 10 µF electrolytic
Bypasses R16.
C12, 1 µF
Coupling cap into the tone networks.
R18, 33 kΩ
Baa network: the series resistor into R20.
C13, 2.2 nF
Baa network: with R19, a path around R18 that lifts the treble a little. Some units have 4.7 nF.
R19, 47 kΩ
Baa network: in series with C13.
R20, 10 kΩ
Baa network: to ground. With R18 it drops the level about 12 dB.
C15, 2.2 nF
Bee network: the treble path of the bridged T.
R21, 47 kΩ
Bee network: the bass path, first half.
R22, 22 kΩ
Bee network: the bass path, second half.
C14, 100 nF
Bee network: from the bass path’s middle to ground. It cuts the mids, deepest near 350 Hz.
Tone, 100 kΩ pot, audio taper
Tone, a 100 kΩ audio pot from Tone Select to ground. Turned down, more of C16 is left to ground and the treble drops.
C16, 22 nF
From the top of the Tone pot to its wiper. At full Tone it is shorted out.
R23, 68 kΩ
Series resistor into Volume.
Volume, 100 kΩ pot, audio taper
Volume, a 100 kΩ audio pot. Fuzz mode only.
R24, 47 kΩ
From the Volume wiper to the output. It keeps the two paths apart where they meet.

About this version

2N5088
Negative ground, 9 V. 2N5088 transistors, as the kit drawing has them in place of the original 2SC644, which the library doesn’t have.

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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