Octave fuzz with a dry blend and tone bypass, 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 Fender Blender.

A gain pair and Sustain pot drive a phase splitter, two diodes fold the wave into an octave, a transistor amplifies it into germanium clipping diodes, then a tone network, Level and a make-up stage. Blend mixes in the clean signal.

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 gains about 30 and drives the follower Q2, whose emitter feeds back to Q1’s base through R6 and R3 for the bias. A light pick already clips the pair. Sustain sends part of it to Q3, which splits it into two copies in opposite phase. R17 holds both octave diodes conducting, so Q4’s base follows whichever copy is lower: the wave folds and repeats at twice the note. Q4 amplifies that into two germanium diodes to ground. The tone network blends a highs path and a lows path, Level and Q5 make up the loss, and Blend mixes the effect with the preamp’s signal from under the Sustain pot. Probe K3 and E3, then QB, to see the fold.

Download.

Parts and what they do

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

R1, 15 kΩ
Series resistor at the input.
C1, 100 nF
Input coupling cap.
R2, 150 kΩ
Q1’s base to ground.
R3, 560 kΩ
Q1’s base bias, fed back from Q2’s emitter through R6.
Q1, 2N5088
First gain stage.
R5, 1.5 kΩ
Q1’s emitter resistor, unbypassed (1.5 kΩ in Fender’s list, 15 kΩ in the redraw).
R4, 47 kΩ
Q1’s collector load.
Q2, 2N5088
Emitter follower, direct coupled to Q1’s collector.
R6, 120 kΩ
Q2’s emitter load and the DC feedback path to Q1’s bias.
C2, 50 nF
Filters the feedback node, so the feedback works on the bias only.
C4, 10 µF electrolytic
Couples Q2’s emitter to the Sustain pot.
R7, 10 kΩ
Across the Sustain pot and R9, the follower’s load.
Sustain, 10 kΩ pot, audio taper
A 10 kΩ audio pot over 680 Ω. Its wiper feeds the octave section. Its bottom end is the dry signal for Blend, about a fifteenth of the preamp’s output.
R9, 680 Ω
Under the Sustain pot. The dry signal for Blend is taken across it (Fender’s list reads 680K, see the file header).
C5, 100 nF
Coupling cap into the phase splitter.
R10, 220 kΩ
Top of Q3’s base bias.
R11, 150 kΩ
Bottom of Q3’s base bias.
Q3, 2N5088
Phase splitter. Equal resistors on collector and emitter, so the two copies are the same size and opposite in phase.
R12, 8.2 kΩ
Q3’s collector load (5 %).
R13, 8.2 kΩ
Q3’s emitter load (5 %).
C6, 10 µF electrolytic
Couples the collector’s copy to its diode.
R15, 27 kΩ
The collector copy’s diode load to ground.
C7, 10 µF electrolytic
Couples the emitter’s copy to its diode.
R16, 27 kΩ
The emitter copy’s diode load to ground.
D1, 1N60 germanium
From Q4’s base to the collector copy. R17 holds both diodes conducting, so Q4’s base follows whichever copy is lower.
D2, 1N60 germanium
From Q4’s base to the emitter copy. Between them the two diodes pick the lower of two opposite waves, which repeats at twice the note.
R17, 100 kΩ
Feeds the octave diodes and Q4’s base from the supply.
Q4, 2N5088
Gain stage after the octave diodes.
R18, 10 kΩ
Q4’s collector load.
R19, 820 Ω
Q4’s emitter resistor.
C8, 10 µF electrolytic
Bypasses R19, so Q4 has its full gain.
C9, 10 µF electrolytic
Coupling cap into the clipping diodes.
D3, D4, 1N60 germanium, one each way
Two germanium diodes to ground, one each way. They clip at about ±0.3 V.
C10, 10 µF electrolytic
Coupling cap into the tone network.
C12, 2.7 nF
To the Tone pot’s bright end: the highs path.
Tone, 25 kΩ pot, linear taper
A 25 kΩ linear pot between the highs path and the lows path. Clockwise is brighter.
R22, 3.9 kΩ
From the Tone pot’s dark end to the lows path.
R20, 27 kΩ
The lows path: with C11 to ground, a low-pass at about 59 Hz.
C11, 100 nF
The lows path’s cap to ground.
R23, 47 kΩ
The Tone bypass path to Level.
R24, 100 kΩ
From the Tone wiper to Level. The switch shorts it with Tone in.
Level, 50 kΩ pot, linear taper
A 50 kΩ linear pot, the effect’s level into the make-up stage.
C13, 10 µF electrolytic
Coupling cap into the make-up stage.
R26, 10 kΩ
Series resistor into Q5’s base.
R27, 150 kΩ
Top of Q5’s base bias.
R28, 18 kΩ
Bottom of Q5’s base bias.
Q5, 2N5088
Make-up gain stage for the effect.
R31, 10 kΩ
Q5’s collector load.
R29, 3.9 kΩ
With C14, treble feedback from Q5’s collector to its base. It trims the highs.
C14, 1 nF
The treble feedback’s cap.
R30, 820 Ω
Q5’s emitter resistor.
C15, 10 µF electrolytic
Bypasses R30.
C16, 10 µF electrolytic
Couples the effect to the Blend pot.
Blend, 100 kΩ pot, audio taper
A 100 kΩ audio pot between the effect and the dry signal under Sustain. Clockwise is more effect.
C3, 100 nF
Output coupling cap.

About this version

2N3391 stand-ins
Negative ground, 9 V. 2N5088 values stand in for the 2N3391 and 2N3391A, silicon NPN of similar gain.

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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