Bucket-brigade chorus with a Depth switch, 1981 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 Electro-Harmonix Small Clone.

An MN3007 delay chip holds a copy of the guitar about 5 to 8 ms behind, on a CD4047 clock. A slow triangle swings the clock, and the copy mixed with the dry signal rises and falls in pitch against the note. The Depth switch sets how far it swings.

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. IC1A lifts the treble about 15 dB, and IC1B cuts it back after the mix, so the delay chip’s hiss comes down with it. Q1 and Q2 each sit in a three-pole low-pass that keeps the clock’s steps out of the output. The triangle reaches the clock through R37 and C16, and R31 sets the clock’s slow half from it: the higher the triangle, the slower the clock and the longer the delay. With Depth at Low the delay swings from about 5.0 to 5.6 ms, and at High from 6.2 to 8.3 ms. Rate runs the sweep from about 0.4 to 9.4 times a second. The CD4047 is modeled as a clock run by the current through R31, fitted to its charging law within 2 %. No measured Small Clone clock was found, so the delay figures are worked out. The footswitch and the bypass are left out, so the model is the pedal with the effect on. The chip’s clock ripple at its outputs isn’t modeled.

Download.

Parts and what they do

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

R32, 47 Ω
Feeds the audio side from the supply.
C19, 220 µF electrolytic
Filters the audio supply.
R33, 56 kΩ
Top of the 4.5 V bias divider.
R34, 56 kΩ
Bottom of the 4.5 V bias divider.
C20, 47 µF electrolytic
Filters the 4.5 V bias line.
R35, 47 Ω
Feeds the LFO and the clock side from the supply, so the clock’s current spikes stay off the audio.
C21, 220 µF electrolytic
Filters the LFO supply.
R36, 10 kΩ
With C22 it smooths the voltage on Q3’s base.
C22, 10 µF electrolytic
Smoothing cap on Q3’s base.
Q3, 2N5088
Emitter follower, a 2N5088. Its emitter is the clean supply for the clock and the delay chip, about 0.6 V under its base.
C23, 10 µF electrolytic
Filters the clock and delay supply.
C1, 33 nF
Input coupling cap. With R1 it cuts below about 22 Hz.
R1, 220 kΩ
Biases IC1A’s + input at 4.5 V and sets the input impedance near 220 kΩ.
R2, 1 kΩ
In series with IC1A’s + input.
IC1A, RC4558
Pre-emphasis stage, a half of an RC4558. Its gain is 1 for low notes and rises to about 5.9 (15 dB) for the treble, so the delay chip’s hiss can come down the same amount after it. Both the dry and the wet paths start here.
R3, 33 kΩ
Feedback resistor of the pre-emphasis stage.
R4, 6.8 kΩ
With C2 it sets the treble lift: it levels off near 2.3 kHz at a gain of 1 + 33 k / 6.8 k.
C2, 10 nF
With R3 and R4 it starts the treble lift near 400 Hz.
C5, 1 µF
Couples the signal into the delay side, onto the bias trim’s wiper.
Bias, 100 kΩ pot, linear taper
Bias trim, 100 kΩ. Its wiper sets the DC on Q1’s base, so on the delay chip’s input. Set at half here, assumed: 4.5 V on the wiper and about 5.6 V on the chip.
R11, 3.3 kΩ
First resistor of the low-pass ahead of the delay chip.
C6, 3.3 nF
First pole of the low-pass.
R12, 10 kΩ
Low-pass filter resistor.
C8, 15 nF
Feedback cap from Q1’s emitter. It squares up the low-pass’s knee.
R13, 10 kΩ
Low-pass filter resistor.
C7, 470 pF
Last cap of the low-pass. With C6, C8 and the three resistors it makes a three-pole low-pass, so the clock doesn’t fold high notes back down.
Q1, 2N5087
Emitter follower of the low-pass, a 2N5087 (PNP). Its emitter sits about 0.6 V over its base.
D2, 1N914 / 1N4148
Lifts the delay chip’s input one more diode drop over Q1’s emitter. 1N914.
R14, 12 kΩ
Q1’s emitter load, through D2, from the clock supply.
R15, 33 kΩ
Into the delay chip’s input.
IC2, MN3007
MN3007 delay line, 1024 stages. It holds 512 samples and passes each on after 512 clock periods, about 5 to 9 ms here. The two outputs are drawn as one pin.
R18, 39 kΩ
Load on the delay chip’s outputs, to the clock supply.
R19, 10 kΩ
First resistor of the low-pass after the delay chip. The chip’s output reaches Q2’s base with no coupling cap.
C10, 4.7 nF
First pole of the second low-pass.
R20, 39 kΩ
Low-pass filter resistor.
C11, 2.7 nF
Feedback cap from Q2’s emitter.
R21, 39 kΩ
Low-pass filter resistor.
C12, 180 pF
Last cap of the second low-pass. It smooths the chip’s steps.
Q2, 2N5088
Emitter follower of the second low-pass, a 2N5088.
R22, 10 kΩ
Q2’s emitter load.
C13, 1 µF
Couples the wet signal toward the mix.
R23, 220 kΩ
Holds the wet side at 4.5 V.
R24, 27 kΩ
Mixing resistor for the wet signal, 27 kΩ on the early units (the reissue used 20 kΩ, a louder wet side).
R5, 22 kΩ
Mixing resistor for the dry signal.
R6, 10 kΩ
From the mix point into IC1B.
IC1B, RC4558
De-emphasis and output stage, the other half of the RC4558. It cuts the treble back down by the 15 dB the first stage added.
R7, 33 kΩ
Feedback resistor of the output stage.
C3, 10 nF
With R8 beside R7 it cuts the treble by the same curve the first stage raised it.
R8, 6.8 kΩ
In series with C3. It sets where the treble cut levels off.
R9, 1 kΩ
In series with the output.
C4, 1 µF
Output coupling cap.
R10, 100 kΩ
Holds the output at 0 V so the jack doesn’t pop.
R25, 180 kΩ
Top of the LFO’s divider. IC3A’s + input sits at about 4.1 V.
R29, 68 kΩ
Middle of the LFO’s divider. IC3B’s − input sits at about 2.2 V.
R30, 82 kΩ
Bottom of the LFO’s divider.
IC3A, LM358
Integrator, a half of an LM358. The square wave in makes a triangle out, about 0.9 to 2.8 V.
C14, 2.2 µF electrolytic
Integrator cap, a 2.2 µF tantalum. A larger cap makes every Rate setting slower.
Rate, 1 MΩ pot, reverse audio taper
Rate, 1 MΩ reverse audio, used as a variable resistor. Its resistance falls as it turns up, so the triangle ramps faster.
C15, 47 nF
With R26 it rounds the square’s edges.
R26, 47 kΩ
From the square to Rate. It sets the fastest sweep with Rate full up.
IC3B, LM358
Comparator with hysteresis, the other half of the LM358. Each time the triangle reaches a turn its output flips to the other rail, so it puts out a square wave.
R27, 120 kΩ
From the triangle to the comparator. Against R28 it sets how far the triangle swings.
R28, 470 kΩ
Positive feedback that holds the comparator where it last flipped.
R37, 4.7 kΩ
From the triangle to the depth point. With C16 it rounds the triangle off. Not numbered on the drawing used.
R38, 2.7 kΩ
To the Depth switch. At Low it goes to ground, so the depth point gets about a third of the triangle. Not numbered on the drawing used.
C16, 10 µF electrolytic
Smoothing cap on the depth point.
R31, 39 kΩ
Timing resistor. The clock runs on the current through it, from the CD4047’s timing pin to the depth point. The higher the triangle, the less current, the slower the clock and the longer the delay.
IC4, CD4047 astable
CD4047 clock, its timing cap C17 100 pF. D3 charges C17 quickly each cycle and R31 sets the slow half, so the clock follows the depth point. Its two outputs are the delay chip’s two clock phases. D3 and C17 are part of the clock and not drawn.

About this version

MN3007, 1981
Negative ground, 9 V. The audio works around the 4.5 V from R33 and R34. The clock and the delay chip run on about 8.4 V from Q3.

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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