Bucket-brigade chorus, 1979 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 Boss CE-2 Chorus.

An MN3007 delay chip holds a copy of the guitar about 4 ms behind, and a slow triangle swings that delay through the chip’s MN3101 clock. Mixed equal with the dry signal, the copy’s pitch rises and falls a little against the note.

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. IC1.2 lifts the treble 15 dB, and IC1.1 takes it back down after the mix, so the delay chip’s hiss comes down with it. The wet side has a three-pole low-pass near 6.6 kHz before the chip and another after it, so the clock’s steps stay out of the output. The clock runs at 116 to 147 kHz at half Depth, a delay of 3.5 to 4.4 ms. At full Depth it runs 108 to 164 kHz, 3.1 to 4.8 ms, and at 0 the delay holds near 3.9 ms. Rate runs the sweep from about 0.3 to 3.6 times a second. As the delay grows the copy’s pitch drops a little, and as it shrinks the pitch rises. C14 into R22 cuts the wet side below about 100 Hz, so low notes come through mostly dry. The clock is a ramp oscillator, and the model stands it in with a clock run by current: RCV is a stand-in sized to the pedal’s clock range. The footswitch, its flip-flop, the switching JFET and the CHECK LED 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.

C29, 100 µF electrolytic
Bulk cap across the supply.
R52, 33 Ω
Feeds the clock side from the supply. With C30 it keeps the clock’s current spikes off the audio.
C30, 220 µF electrolytic
Filters the clock supply.
R27, 4.7 kΩ
Top of the bias divider.
VR3, 10 kΩ pot, linear taper
Bias trimmer, 10 kΩ. Its wiper is the 4.5 V line the audio works around, and through Q2 it sets the DC on the delay chip’s input. The factory set it for the cleanest delay. At half turn its wiper sits at 4.5 V.
R28, 4.7 kΩ
Bottom of the bias divider.
C18, 47 µF electrolytic
Filters the 4.5 V bias line.
C1, 47 nF
Input coupling cap.
R1, 1 kΩ
In series with Q1’s base.
R2, 470 kΩ
Biases Q1’s base at 4.5 V and sets the input impedance near 400 kΩ.
Q1, 2SC1815
Input emitter follower. 2SC1815 typical, the drawing shows only an NPN.
R3, 10 kΩ
Q1’s emitter load.
C2, 470 nF electrolytic
Couples the follower into the pre-emphasis stage.
R4, 47 kΩ
Input resistor of the pre-emphasis stage. Against R6 it sets a gain of 1 for low notes.
C3, 6.8 nF
With R5 beside R4, it lifts the treble: the gain starts rising near 410 Hz and levels off near 2.3 kHz at about 5.7 (15 dB).
R5, 10 kΩ
In series with C3. It sets where the treble lift levels off.
R6, 47 kΩ
Feedback resistor of the pre-emphasis stage.
C4, 100 pF
Rolls the top end off above about 34 kHz.
R7, 10 kΩ
Biases the + input at 4.5 V.
IC1.2, RC4558
Pre-emphasis stage, inverting. Treble goes up 15 dB before the delay so the chip’s hiss can come down the same amount after it. Both the dry and the wet paths start here. A 4558-class op-amp, typical, not named on the drawing.
C5, 33 nF
Couples the signal into the delay side. With R8 it cuts below about 48 Hz.
R8, 100 kΩ
Holds the filter’s input at 4.5 V, so Q2’s base and the delay chip’s input sit at a set DC.
R9, 10 kΩ
First resistor of the low-pass ahead of the delay chip.
C6, 3.3 nF
First pole of the low-pass.
R10, 10 kΩ
Low-pass filter resistor.
C7, 8.2 nF
Feedback cap from Q2’s emitter. It squares up the low-pass’s knee.
R11, 10 kΩ
Low-pass filter resistor.
C8, 470 pF
Last cap of the low-pass. With C6, C7 and the three 10 kΩ it makes a three-pole low-pass near 6.6 kHz, so the clock doesn’t fold high notes back down.
Q2, 2SC1815
Emitter follower of the low-pass. It drives the delay chip. 2SC1815 typical, the drawing shows only an NPN.
R12, 10 kΩ
Q2’s emitter load.
R13, 4.7 kΩ
Into the delay chip’s input.
IC3, MN3007
MN3007 delay line, 1024 stages. It holds 512 samples and passes each on after 512 clock periods, a few milliseconds here. The two outputs are drawn as one pin.
R14, 56 kΩ
Load on the delay chip’s outputs, to the supply.
C10, 33 nF
Couples the delayed signal into the second low-pass.
R15, 330 kΩ
Holds the second filter’s input at 4.5 V.
R16, 10 kΩ
First resistor of the low-pass after the delay chip.
C11, 3.3 nF
First pole of the second low-pass.
R17, 10 kΩ
Low-pass filter resistor.
C12, 8.2 nF
Feedback cap from Q3’s emitter.
R18, 10 kΩ
Low-pass filter resistor.
C13, 470 pF
Last cap of the second low-pass, the same as the first. It smooths the chip’s steps.
Q3, 2SC1815
Emitter follower of the second low-pass. 2SC1815 typical, the drawing shows only an NPN.
R19, 10 kΩ
Q3’s emitter load.
C14, 33 nF
Couples the wet signal toward the mixer.
R20, 1 MΩ
Holds the wet side at 4.5 V. It keeps the switch Q9 at the mixer’s DC.
R22, 47 kΩ
Mixing resistor for the wet signal. Q9, the switching JFET after it, is drawn as a wire: it is on with the effect on.
R21, 47 kΩ
Mixing resistor for the dry signal, equal to R22, so the two come in at the same level.
IC1.1, RC4558
Mixer and de-emphasis. It adds the dry and wet signals and takes the treble back down by the 15 dB the first stage added. A 4558-class op-amp, typical.
C15, 6.8 nF
With R23 beside R24 it cuts the treble by the same curve the pre-emphasis raised it, from about 410 Hz to 2.3 kHz.
R23, 10 kΩ
In series with C15.
C16, 100 pF
Rolls the top end off above about 34 kHz.
R24, 47 kΩ
Feedback resistor of the mixer. Against R21 and R22 each input comes through at a gain of 1 for low notes.
R25, 470 Ω
In series with the output.
C17, 1 µF electrolytic
Output coupling cap.
R26, 100 kΩ
Holds the output at 0 V so the jack doesn’t pop.
R33, 10 kΩ
Top of the LFO’s 4.5 V divider.
R34, 10 kΩ
Bottom of the LFO’s 4.5 V divider.
C20, 47 µF electrolytic
Filters the LFO’s 4.5 V line.
IC2.2, RC4558
Comparator with hysteresis. Its output flips to the other rail each time the triangle reaches a peak, so it puts out a square wave. A 4558-class op-amp, typical, not named on the drawing.
R30, 47 kΩ
Positive feedback that holds the comparator where it last flipped.
R29, 33 kΩ
From the triangle to the comparator. Against R30 it sets the triangle’s height at about 0.7 of the square’s.
Rate, 100 kΩ pot, linear taper
Rate, 100 kΩ linear. It sets how much of the square reaches the integrator, so how fast the triangle ramps: about 0.3 to 3.5 sweeps a second.
R31, 10 kΩ
Under Rate. It sets the slowest sweep.
R32, 1 MΩ
Integrator input resistor. With C19 it sets the ramp’s speed.
C19, 100 nF
Integrator cap. A smaller cap makes every Rate setting faster.
IC2.1, RC4558
Integrator. The square wave in makes a triangle out.
Depth, 100 kΩ pot, linear taper
Depth, 100 kΩ linear. It sets how much of the triangle reaches the clock, so how far the delay swings. At 0 the delay holds still.
R35, 220 kΩ
With C21 it rounds the triangle’s corners (about 72 Hz).
C21, 10 nF
Smoothing cap on Q4’s base.
Q4, 2SC1815
Emitter follower for the LFO. 2SC1815 typical, the drawing shows only an NPN.
R36, 4.7 kΩ
Top of a divider that halves Q4’s emitter voltage.
R37, 4.7 kΩ
Bottom of that divider.
D1, 1N914 / 1N4148
Feeds the LFO into the clock’s timing node. The higher the LFO, the more it adds, the faster the clock and the shorter the delay. 1S1588, modeled as a 1N914.
RCV, 56 kΩ
Stand-in, not on the pedal. It sizes the LFO’s pull on the model clock to match the pedal’s clock range.
R38, 150 kΩ
Timing resistor. It charges C22 and sets the slowest clock, with the LFO at the bottom of its swing.
IC4, MN3101
MN3101 clock, its timing cap C22 47 pF. More current into the timing pin runs the clock faster. Q5, D2, R39 and R40, which empty C22 each cycle, are part of the clock and not drawn.

About this version

As drawn
Negative ground, 9 V. The audio works around the 4.5 V the trimmer VR3 sets. The LFO has its own 4.5 V from R33 and R34.

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.

Product and company names are trademarks of their owners. They are used here only to identify the circuits these models are based on; this site is not affiliated with or endorsed by any of them.

From Swell Amplification, swellamps.com.