Octave fuzz with a phase splitter and diode rectifier, 1971 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 fOXX Tone Machine.

A two-transistor phase splitter feeds two germanium diodes that pass each output’s negative half to one node, which doubles the note. A gain stage clipped by two more diodes, a Big Muff-like tone network and a recovery stage follow.

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 biased together by DC feedback from Q2’s emitter. Q2’s collector and emitter carry equal and opposite copies of the note. Each goes through a 10 µF cap to a germanium diode, and the two diodes’ anodes meet at a node pulled up by 100 kΩ. That node follows whichever side is lower, so it dips twice per cycle, and that is the octave. Q3 amplifies it, and two more germanium diodes clip it to about 0.2 to 0.3 V. The tone network takes a 1 nF treble path and a 22 kΩ and 47 nF bass path to either end of the Tone pot, and Q4 makes up the loss. The model leaves out mismatch between the two diodes and between the phase splitter’s two outputs in a real unit, which leaves some of the note in the octave.

Download.

Parts and what they do

Stock values for the 1 nF treble cap version. Tap a part on the schematic to change it.

C1, 100 nF
Input coupling cap.
Q1, 2N5133
Input gain stage.
R2, 47 kΩ
Q1’s collector load.
R3, 1 kΩ
Q1’s emitter resistor, unbypassed.
R4, 47 kΩ
Q1’s base bias, from the feedback middle.
R5, 100 kΩ
From the feedback middle to ground.
C3, 100 nF
Filters the feedback middle, so the DC feedback carries little signal.
R6, 100 kΩ
DC feedback from Q2’s emitter. It holds both stages’ bias.
C2, 1 nF
From Q2’s emitter back to Q1’s base. It cuts the gain at high frequencies.
Q2, 2N5133
Phase splitter. With equal resistors on collector and emitter, its two outputs are the same size and opposite in phase.
R7, 4.7 kΩ
Q2’s collector load.
R8, 4.7 kΩ
Q2’s emitter load.
C4, 10 µF electrolytic
Couples the collector side to its diode.
R10, 100 kΩ
Holds the collector side’s diode at ground at DC.
D2, 1N34A germanium
Germanium diode, cathode toward the collector side. It conducts while that side swings low.
C5, 10 µF electrolytic
Couples the emitter side to its diode.
R9, 100 kΩ
Holds the emitter side’s diode at ground at DC.
D1, 1N34A germanium
Germanium diode, cathode toward the emitter side, through the Octave switch.
R13, 100 kΩ
Pulls the rectifier node up, so it follows whichever side swings lower, minus a diode drop. The two sides take turns, so the node swings twice per cycle.
C6, 10 µF electrolytic
Coupling cap after the rectifier.
R26, 12 kΩ
In series before the Sustain pot, on one kit maker’s drawing only.
Sustain, 50 kΩ pot, linear taper
A 50 kΩ linear pot: how much of the rectified signal reaches Q3.
R12, 220 Ω
Under the Sustain pot.
C7, 10 µF electrolytic
Coupling cap into Q3.
R14, 150 kΩ
Q3’s base bias from the supply.
R15, 15 kΩ
Q3’s base bias to ground.
Q3, 2N5133
Gain stage into the clipping diodes.
R16, 10 kΩ
Q3’s collector load.
R17, 1 kΩ
Q3’s emitter resistor.
C9, 10 µF electrolytic
Bypasses R17, so Q3’s gain is high above about 16 Hz (22 µF in some units).
C8, 10 µF electrolytic
Coupling cap into the diodes.
D3, D4, 1N34A germanium, one each way
Two germanium diodes to ground, one each way.
C10, 1 nF
The treble path to the Tone pot’s top (3 nF on another drawing).
R18, 22 kΩ
The bass path’s series resistor.
C11, 47 nF
With R18, the bass path’s low-pass to ground.
R19, 4.7 kΩ
From the bass network to the Tone pot’s bottom.
Tone, 50 kΩ pot, linear taper
A 50 kΩ linear pot between the treble and bass paths. Clockwise is brighter.
C12, 10 µF electrolytic
Coupling cap into Q4.
R21, 470 kΩ
Q4’s base bias from the supply.
R22, 47 kΩ
Q4’s base bias to ground.
Q4, 2N5133
Recovery stage, to make up what the tone network drops.
R23, 10 kΩ
Q4’s collector load.
R24, 1.5 kΩ
Q4’s emitter resistor, unbypassed.
C13, 10 µF electrolytic
Output coupling cap.
Volume, 50 kΩ pot, linear taper
Output level, a 50 kΩ linear pot.

Versions

1 nF treble cap
The tone network’s treble cap at 1 nF, which the kit maker calls the original value.
Red case, 1.8 nF
The treble cap at 1.8 nF, which the kit maker lists for the later red units. Not checked against a unit.

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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From Swell Amplification, swellamps.com.