Octave fuzz with a split-phase transistor pair, 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 Univox Super-Fuzz.

A two-transistor preamp, then a phase splitter driving two transistors in opposite phase into one collector load. Their currents add, so the note cancels and its octave stays. Germanium diodes clip the result, and the Tone switch picks a mid scoop or a flat divider.

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 is a gain stage of about 18, direct coupled to the follower Q2. DC feedback from Q2’s emitter through R6 and R3, filtered by C4, holds both stages’ bias. Expander takes part of Q2’s output to Q3, which splits it into two equal copies in opposite phase. Q4 and Q5 share a collector load and a bypassed emitter, one copy each. Their currents rise and fall against each other, so the note cancels in the sum, while the bend in each transistor’s turn-on is the same in both halves, so the second harmonic adds. The octave trimmer evens up the two. Past a few tens of millivolts at the pickup the pair saturates, and the germanium diodes clip what’s left to about 0.2 V. The Tone switch picks the scoop network or the flat divider, and Q6 makes up about 14 dB. The model leaves out the gain spread between real transistors, which sets how well the note cancels.

Download.

Parts and what they do

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

R1, 22 kΩ
Series resistor at the input (100 kΩ in the earlier FY-6).
C1, 10 µF electrolytic
Input coupling cap.
R2, 100 kΩ
Q1’s base to ground.
R3, 100 kΩ
Q1’s base bias, fed from Q2’s emitter through R6.
Q1, 2SC539
First gain stage.
R4, 1.8 kΩ
Q1’s emitter resistor, unbypassed, so Q1’s gain is about 18.
R5, 47 kΩ
Q1’s collector load.
C2, 2 nF
From Q1’s collector to the supply. With R5 it rolls off the top above about 1.7 kHz (1 nF in the FY-6).
Q2, 2SC828
Emitter follower, direct coupled to Q1’s collector.
R7, 10 kΩ
Q2’s emitter load.
R6, 470 kΩ
DC feedback from Q2’s emitter back to Q1’s base. It holds both stages’ bias.
C3, 10 µF electrolytic
Couples Q2’s emitter into R8.
R8, 47 kΩ
With C4 to ground, part of the feedback network on Q1’s bias.
C4, 100 nF
Filters the feedback node, so the feedback works on bias and the lowest notes only.
C5, 10 µF electrolytic
Coupling cap into the Expander pot.
Expander, 50 kΩ pot, linear taper
A 50 kΩ linear pot: how much of the preamp’s signal goes on to the phase splitter.
R9, 3.3 kΩ
Under the Expander pot, so it never turns all the way off (a jumper in the FY-6).
C6, 10 µF electrolytic
Coupling cap into the phase splitter.
R10, 220 kΩ
Q3’s base bias from the supply.
R11, 150 kΩ
Q3’s base bias to ground.
Q3, 2SC828
Phase splitter. Equal resistors on collector and emitter, so the two outputs are the same size and opposite in phase.
R12, 10 kΩ
Q3’s collector load.
R13, 10 kΩ
Q3’s emitter load.
C7, 10 µF electrolytic
Couples Q3’s collector to Q4.
R15, 470 Ω
Base stopper into Q4.
C8, 10 µF electrolytic
Couples Q3’s emitter to Q5.
R14, 470 Ω
Base stopper into Q5.
R16, 100 kΩ
Q4’s base bias from the supply.
R17, 22 kΩ
Q4’s base bias, to ground through one side of the octave trimmer.
R21, 100 kΩ
Q5’s base bias from the supply.
R20, 22 kΩ
Q5’s base bias, to ground through the other side of the octave trimmer.
Octave trim, 10 kΩ pot, linear taper
A 10 kΩ trimmer between the bottoms of R17 and R20, wiper to ground. Turning it raises one transistor’s bias and lowers the other’s, to even up the pair so the note itself cancels.
Q4, 2SC539
One half of the octave pair, fed from Q3’s collector.
Q5, 2SC539
The other half, fed the opposite phase.
R18, 10 kΩ
The pair’s shared collector load. Both halves pull on it, so the wave here repeats at twice the note.
R19, 1.8 kΩ
The pair’s shared emitter resistor (2 kΩ in the FY-6).
C9, 10 µF electrolytic
Bypasses the shared emitter resistor.
C10, 10 µF electrolytic
Coupling cap into the diodes.
D3, D4, 1N60 germanium, one each way
Two DA-90 germanium diodes to ground, one each way. Modeled as 1N60s.
C11, 10 µF electrolytic
Coupling cap into the tone networks.
R22, 47 kΩ
Top of the Flat divider.
R23, 10 kΩ
Bottom of the Flat divider.
R24, 22 kΩ
Lows path of the Scoop network.
C12, 1 nF
Highs path of the Scoop network.
R25, 10 kΩ
Joins the Scoop output to the lows path.
C13, 100 nF
Shunts the middle of the lows path to ground.
Balance, 50 kΩ pot, linear taper
Output level, a 50 kΩ linear pot.
C14, 10 µF electrolytic
Coupling cap into the output stage.
R26, 100 kΩ
Q6’s base bias from the supply.
R27, 15 kΩ
Q6’s base bias to ground.
Q6, 2SC539
Output stage, to make up what the tone networks drop.
R28, 10 kΩ
Q6’s collector load.
R29, 1 kΩ
Q6’s emitter resistor, unbypassed, so its gain is about 5.
C15, 10 µF electrolytic
Output coupling cap.
R30, 100 kΩ
Output pulldown.
C16, 10 µF electrolytic
Across the supply, so the stages don’t talk to each other through the battery’s resistance.

About this version

2SC539 and 2SC828
As the 1968 factory drawing marks them: 2SC539 in the first stage, the octave pair and the output stage, 2SC828 in the follower and the phase splitter.

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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