Two-stage blend fuzz, 1968 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 Mosrite Fuzzrite (silicon version).

Two transistor gain stages in a row, and a Depth control that blends between the output of the first and the second instead of setting their gain.

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 first stage is a mild, fairly clean boost, and the second clips hard. Depth crossfades between them through two tiny 2.2 nF caps, so both paths are thin on bass. Each stage inverts, so the two signals are out of phase. Mid-knob they partly cancel, and that’s where the hollow, octave-like sounds are. Sweep Depth slowly with the probe on its wiper. The Axis build biases both transistors with dividers and bypasses their emitters, for much more gain per stage and a less extreme blend.

Download.

Parts and what they do

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

C1, 47 nF
Input coupling cap.
R3, 470 kΩ
Collector-to-base feedback. It biases Q1 and keeps its gain modest (Fuzzrite).
Q1, 2N3904
First gain stage. Its output feeds both the second stage and one end of the Depth blend.
R4, 470 kΩ
Q1 collector load: 470 kΩ on the Fuzzrite, so Q1 runs at only a few microamps.
C3, 47 nF
Coupling cap into the second stage.
R8, 470 kΩ
Collector-to-base feedback for Q2 (Fuzzrite).
Q2, 2N3904
Second gain stage, where most of the clipping happens.
R9, 470 kΩ
Q2 collector load.
C6, 2.2 nF
First-stage side of the blend. At 2.2 nF it’s a steep bass cut.
R12, 22 kΩ
With C6, a high-pass filter and a level cut on the first-stage side of the blend.
C8, 2.2 nF
Second-stage side of the blend.
Depth, 350 kΩ pot, linear taper
Blends from the first stage (down) to the second (up). Its outer lugs are wired so turning it up moves the wiper toward the second stage.
Volume, 33 kΩ pot, audio taper
Output level. At 33 kΩ it also loads the blend and trims the bass further.

Versions

Fuzzrite
Negative ground, 9 V. Collector-feedback bias on both stages.
Axis / Foxey Lady, modeled after the Electro-Harmonix Axis Fuzz and two-knob Guild Foxey Lady
The same topology with divider bias and bypassed emitter resistors on both stages, sold under both names.

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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