Three-transistor overdrive with diodes in the feedback, 1982 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 Ibanez OD-850 Overdrive.

A transistor stage drives Distortion, then two gain stages with two diodes and a cap from collector to base. A fourth transistor with a twin-T network around it does the tone.

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 brings the guitar up about 27 dB into Distortion. Q2 and Q3 each have a gain near 47 from their 470 kΩ feedback over the 10 kΩ in front. Two diodes and 100 nF sit across that feedback too. Past about 0.6 V either way the diodes conduct, and the gain drops to what the 100 nF allows, which is little in the treble and more in the bass. So turning Distortion up makes the note louder and fuller more than more broken up. Q4 with the twin-T network is the tone stage. It rolls off the treble from about 2 kHz, and Tone brings some of it back.

Download.

Parts and what they do

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

R23, 1 MΩ
From the input to ground.
R1, 10 kΩ
Series resistor into Q1.
C1, 1 µF electrolytic
Input coupling cap.
R2, 100 kΩ
From Q1’s base to ground. With R3 it sets Q1’s collector voltage.
R3, 470 kΩ
Collector to base. It biases Q1 and feeds part of the signal back.
C2, 330 pF
Collector to base. It trims Q1’s treble.
Q1, 2N5088
First gain stage.
R4, 10 kΩ
Q1’s collector load.
R5, 100 Ω
Q1’s emitter resistor, unbypassed.
C3, 1 µF electrolytic
Couples Q1 to the Distortion pot.
Distortion, 100 kΩ pot, linear taper
Distortion, a 100 kΩ linear pot. It sets how much of Q1’s output reaches Q2.
R6, 1 kΩ
Under Distortion.
C4, 1 µF electrolytic
Coupling cap toward Q2.
R7, 10 kΩ
Into Q2’s base. Q2’s feedback works against it, so it sets the stage’s gain.
R8, 100 kΩ
From Q2’s base to ground.
R9, 470 kΩ
Collector to base. With R7 it holds Q2’s gain near 47 before the diodes conduct.
C6, 330 pF
Collector to base. It trims Q2’s treble.
D3, D4, 1N914 / 1N4148, one each way
Two 1N914s back to back. Once the swing is over about 0.6 V either way they conduct, the feedback rises and the gain drops.
C5, 100 nF
In series with the diodes, so they don’t upset Q2’s bias.
Q2, 2N5088
Second gain stage, with soft clipping in its feedback.
R10, 10 kΩ
Q2’s collector load.
R11, 180 Ω
Q2’s emitter resistor.
C7, 1 µF electrolytic
Coupling cap toward Q3.
R12, 10 kΩ
Into Q3’s base.
R13, 100 kΩ
From Q3’s base to ground.
R14, 470 kΩ
Collector to base.
C9, 330 pF
Collector to base. It trims Q3’s treble.
D1, D2, 1N914 / 1N4148, one each way
Two 1N914s back to back, as on Q2.
C8, 100 nF
In series with the diodes.
Q3, 2N5088
Third gain stage, the same as Q2.
R15, 10 kΩ
Q3’s collector load.
R16, 180 Ω
Q3’s emitter resistor.
C10, 1 µF electrolytic
Couples Q3 into the tone section.
R27, 10 kΩ
From the Tone pot’s top into the twin-T network.
R25, 47 kΩ
Twin-T network, first leg.
C16, 47 nF
Across R25.
R26, 47 kΩ
Twin-T network, second leg.
C17, 47 nF
Across R26.
R24, 10 kΩ
From the network’s middle into Q4.
R28, 10 kΩ
From the network’s far end to Q4’s output, so the network is also Q4’s feedback.
Tone, 100 kΩ pot, linear taper
Tone, a 100 kΩ linear pot from Q3’s side to Q4’s output. Its wiper feeds Q4 through 2.2 nF, so it fades the treble between the two.
C15, 2.2 nF
Passes only the treble from the Tone wiper.
R18, 22 kΩ
In series with C15 into Q4.
C12, 1 µF electrolytic
Coupling cap into Q4.
R20, 220 kΩ
Top of Q4’s base divider.
R19, 33 kΩ
Bottom of Q4’s base divider, about 1.2 V.
Q4, 2N5088
Tone stage gain, inverting.
R21, 5.6 kΩ
Q4’s collector load.
R22, 1 kΩ
Q4’s emitter resistor.
C11, 10 µF electrolytic
Bypasses R22, so Q4 has full gain.
C13, 1 µF electrolytic
Output coupling cap of Q4.
R17, 47 kΩ
Series resistor into Level.
Level, 100 kΩ pot, linear taper
Level, a 100 kΩ linear pot.

About this version

2N5088
Negative ground, 9 V. 2N5088 transistors as the kit drawing has them.

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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