Four-transistor fuzz with a Gain pot and diodes before the tone stack, late 1960s 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 Vox Supa Tone Bender.

Two feedback gain stages with a Gain pot between them, a third stage with germanium diodes in its feedback, a second diode pair to ground, a low-pass and high-pass tone stack and a recovery stage.

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 feedback gain stages, with the Gain pot between them. Q2 has no resistor from base to ground, so it idles low, its collector near 2.3 V, and clips by itself on big notes. Q3’s germanium diodes, in its feedback through C8, limit how far its collector swings. D3 and D4 then hold the tone stack’s input to about 0.16 V either way. The tone stack fades a low-pass near 480 Hz against a high-pass near 1 kHz, and Q4 brings the level back up about 3.7 times. C1, 10 nF, cuts the low strings: a small signal at 100 Hz comes out about 13 dB under one at 1 kHz. The model leaves out the spread between real germanium diodes.

Download.

Parts and what they do

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

C1, 10 nF
Input coupling cap. Against the stage’s input it trims the deep bass.
R1, 33 kΩ
Input series resistor. With R3’s feedback it sets Q1’s gain.
Q1, 2N3904
Input gain stage. Feedback from collector to base sets its bias and its gain.
R2, 100 kΩ
Base to ground. With R3 it sets the collector voltage.
R3, 470 kΩ
Collector-to-base feedback.
C2, 560 pF
Feedback cap. It rolls off the top end.
R5, 100 Ω
Emitter resistor.
R4, 15 kΩ
Collector load.
C3, 100 nF
Coupling cap to the Gain pot.
Gain, 10 kΩ pot, audio taper
Gain, R6 on the drawing, a 10 kΩ audio pot. It sets how much of Q1’s output reaches Q2.
R7, 1 kΩ
Under the Gain pot, so a little signal gets through at 0.
C4, 100 nF
Coupling cap into Q2.
R8, 8.2 kΩ
Series input resistor for Q2. With R9 it sets the stage’s gain.
Q2, 2N3904
Second gain stage. It has no diodes, so when it clips, the transistor itself clips.
R9, 470 kΩ
Collector-to-base feedback, Q2’s only bias. With no resistor from base to ground, the collector sits low, near 2.3 V.
C5, 560 pF
Feedback cap. It rolls off the top end.
R11, 100 Ω
Emitter resistor.
R10, 10 kΩ
Collector load.
C6, 100 nF
Coupling cap into Q3.
R12, 8.2 kΩ
Series input resistor for Q3.
Q3, 2N3904
Clipping stage, with diodes in its feedback.
R13, 100 kΩ
Base to ground.
R14, 470 kΩ
Collector-to-base feedback.
C7, 560 pF
Feedback cap. It rolls off the top end.
C8, 100 nF
Blocks DC from the diodes, so they only see the signal.
D1, D2, 1N60 germanium, one each way
Germanium diodes back to back from base to collector. They limit how far the collector swings from the base.
R16, 100 Ω
Emitter resistor.
R15, 15 kΩ
Collector load.
C9, 100 nF
Coupling cap into the tone stack.
D3, D4, 1N60 germanium, one each way
Germanium diodes back to back to ground at the tone stack’s input. They hold it to a few tenths of a volt either way.
C10, 4.7 nF
High-pass cap in the tone stack. With R17 its corner is near 1 kHz.
R17, 33 kΩ
High-pass resistor.
R18, 33 kΩ
Low-pass resistor in the tone stack. With C11 its corner is near 480 Hz.
C11, 10 nF
Low-pass cap.
Tone, 100 kΩ pot, linear taper
Tone, R19 on the drawing, 100 kΩ linear. It fades between the low-pass and high-pass sides.
C12, 100 nF
Coupling cap into the recovery stage.
R20, 470 kΩ
Top of Q4’s base bias, from the battery.
R21, 100 kΩ
Bottom of Q4’s base bias.
Q4, 2N3904
Recovery stage. It makes up some of the level the tone stack loses.
R23, 2.7 kΩ
Emitter resistor, unbypassed, so the gain is about R22 over R23, near 3.7.
R22, 10 kΩ
Collector load.
C13, 100 nF
Output coupling cap.
C14, 10 µF electrolytic
A 10 µF cap beside C13, as drawn. With it there, C13 makes no audible difference.
Volume, 10 kΩ pot, audio taper
Volume, R24 on the drawing, a 10 kΩ audio pot.

About this version

2N3904
Negative ground, 9 V. 2N3904 transistors, one of the two types the drawing lists. 1N60 germanium diodes stand in for the drawing’s unnamed ones.

Sources

  • L.A. Holt, drawing of the Vox Suppa Tone Bender (2006)

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.

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