Bucket-brigade chorus and vibrato, 1976 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-1 Chorus Ensemble.

An MN3002 delay chip holds a copy of the guitar 1.3 to 4.1 ms behind. In Chorus a slow triangle sweeps the delay, and the copy mixed with the dry signal moves in pitch against the note. In Vibrato a faster rounded wave sweeps it, and the stereo jack carries the copy alone.

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. IC-1 raises the guitar 34 dB after Level. About a tenth of its output goes to the mono jack as the dry signal. The wet side lifts the treble from about 1 kHz, cuts above about 8 kHz and drives the MN3002, which holds the signal 1.3 to 4.1 ms. A second low-pass after the chip cuts the treble back down, so the chip’s hiss drops with it. The wet signal goes to the stereo jack. With nothing plugged into the stereo jack, the mono jack mixes the dry and wet signals in about equal parts. In Chorus a triangle of about ±10 V sweeps the clock, and Intensity sets how fast: 0.4 to 3 sweeps a second. The delay swings over the same range at any setting, so a faster sweep moves the pitch farther. In Vibrato IC-4b runs in a loop with Rate’s network at 3.7 to 13 sweeps a second, and Depth sets how far the delay swings. As drawn, the mono jack still mixes the vibrato with the dry signal. The real clock is a multivibrator of four transistors. In the model a ramp clock stands in for it, set to the drawing’s 5 to 16 µs. The input level switch and mic amp, the noise gate Q12, the peak LEDs, Effect / Normal, and the transformer and regulators are left out. The chip’s clock ripple isn’t modeled.

Download.

Parts and what they do

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

−14 V, -14 V
The −14 V rail. The pedal’s Q17 regulator makes it from the mains, modeled as a steady −14 V source (the transformer and regulators are not drawn). It feeds the followers’ emitter resistors, the delay chip’s bias divider and IC-1’s and IC-4’s negative supply.
R20, 10 kΩ
Top of the delay chip’s divider, from +14 V. Its bottom, about +4.4 V, is a supply pin of the chip (the drawing marks +5 V).
C13, 10 µF electrolytic
Filters the +4.4 V line.
R19, 8.2 kΩ
Middle of the divider. Its bottom, about −3.5 V, is the delay chip’s input bias (the drawing marks −4 V).
C15, 10 µF electrolytic
Filters the input bias line.
R22, 10 kΩ
Lower part of the divider. Its bottom, about −13 V, is the chip’s gate supply.
C16, 10 µF electrolytic
Filters the −13 V line.
R23, 1 kΩ
Bottom of the divider, to −14 V.
Level, 50 kΩ pot, audio taper
Level, 50 kΩ audio taper, on the input. It sets how hard the guitar drives IC-1 and the delay chip. Its 50 kΩ is the input impedance too.
R8, 10 kΩ
In series with IC-1’s + input.
IC-1, LM741 / µA741
Input amplifier, non-inverting, a gain of 48 (34 dB) above about 34 Hz. Its output feeds both the dry and the wet sides. TA7136P on the drawing, modeled as a 741 on ±13 V.
R11, 470 kΩ
Feedback resistor. Against R12 it sets IC-1’s gain at 48.
R12, 10 kΩ
Bottom of the feedback divider.
C07, 470 nF
Keeps DC out of the feedback divider, so IC-1’s DC gain is 1. With R12 it sets the low cut near 34 Hz.
R13, 22 kΩ
Top of a divider into the wet side. With R15 it passes about 0.3 of IC-1’s output for low notes.
C08, 6.8 nF
Across R13. It lets more treble through, rising from about 1 kHz to 3.4 kHz, so the delay chip’s hiss can be cut back after it.
R15, 10 kΩ
Bottom of the divider. It also holds Q2’s base near 0 V.
R14, 10 kΩ
Low-pass filter resistor.
C12, 4.7 nF
Feedback cap from Q2’s emitter. It sharpens the low-pass’s knee.
R16, 10 kΩ
Low-pass filter resistor.
C11, 1 nF
Last cap of the low-pass ahead of the chip. With R14, R16 and C12 it rolls off above about 8 kHz, so the clock doesn’t fold high notes back down.
Q2, 2SC1815
Emitter follower of the low-pass. It drives the delay chip. 2SC900 on the drawing, modeled as a 2SC1815, typical.
R18, 10 kΩ
Q2’s emitter load, to −14 V.
C14, 100 nF
Couples Q2 into the delay chip.
R21, 100 kΩ
Holds the delay chip’s input at the −3.5 V bias.
IC-2, MN3002
MN3002 delay line, 512 stages. It holds 256 samples and passes each on after 256 clock periods, 1.3 to 4.1 ms here. The two outputs are drawn as one pin. Its level and DC are set from the drawing: the wet comes out at half the input (the drawing shows 4 V p-p in and 2 V p-p out), and its outputs sit near −6 V.
R24, 100 kΩ
Load on one of the chip’s two outputs.
R25, 100 kΩ
Load on the other output.
VR01, 5 kΩ pot, linear taper
Balance trim, 5 kΩ, between the chip’s two outputs. The real one evens out the clock left on the two outputs. The model has one output, so both ends are on it.
Q7, 2SC1815
Emitter follower after the chip. It drives the second low-pass. 2SC900, modeled as a 2SC1815.
R40, 10 kΩ
Q7’s emitter load, to −14 V.
R41, 22 kΩ
First resistor of the low-pass after the chip.
C26, 4.7 nF
First cap of the second low-pass. With R41 it cuts the treble from about 1.5 kHz, taking back what C08 lifted.
R42, 6.8 kΩ
Low-pass filter resistor.
C28, 1 nF
Feedback cap from Q10’s emitter.
R43, 15 kΩ
Low-pass filter resistor.
C29, 1 nF
Second feedback cap from Q10’s emitter.
R44, 15 kΩ
Low-pass filter resistor.
C27, 470 pF
Last cap of the second low-pass. The wet side is down 3 dB near 4.5 kHz, and the chip’s steps are smoothed out.
Q10, 2SC1815
Emitter follower of the second low-pass. 2SC900, modeled as a 2SC1815.
R46, 10 kΩ
Q10’s emitter load, to −14 V.
C30, 470 nF electrolytic
Couples the wet signal toward the stereo output.
R48, 2.2 kΩ
In series with the wet signal. Q12, a JFET that shorts the wet side to ground when no note is playing, connects here. It is left out, so the model is the pedal with a note playing.
R90, 2.2 kΩ
In series with the wet signal, ahead of Q13.
R50, 100 kΩ
Holds Q13’s drain at 0 V.
Q13, 2SK30A-GR
Switching JFET, on with the effect on: its gate sits at 0 V and it passes the wet signal with a few hundred ohms. Effect / Normal (S3) pulls its gate to about −5.6 V to turn it off, not drawn. Not named on the drawing, modeled as a 2SK30A.
C33, 22 nF
With R60, slows Q13’s turn on and off, so the switch doesn’t click.
R60, 1 MΩ
Feeds Q13’s gate from the S3 line.
R61, 10 kΩ
Holds the S3 line at 0 V with the effect on.
R54, 4.7 kΩ
Load on the stereo output. Against R48 and R90 it halves the wet signal.
R92, 470 Ω
In series with the stereo output. With nothing plugged into it, the stereo jack’s switch joins it to the mono jack, so the two outputs mix there.
R47, 12 kΩ
The dry signal, from IC-1. Against R49 it passes about a tenth of IC-1’s output.
C31, 470 nF
Couples the dry signal (a non-polar cap).
R49, 1.5 kΩ
Bottom of the dry divider. It sets the dry level in the mix.
R52, 3.3 kΩ
In series with the dry signal.
R51, 4.7 kΩ
Load on the mono output.
R91, 470 Ω
In series with the mono output. R91 and R92 mix the dry and wet signals equally at the jack.
IC-4a, MC1458
Comparator with hysteresis, half a CA1458. Its output flips to the other rail each time the triangle reaches a peak, so it puts out a square wave of about ±12 V. In Vibrato it isn’t in the loop.
R81, 220 kΩ
Positive feedback that holds the comparator where it last flipped. Against R80 it sets the triangle’s height.
R80, 180 kΩ
From the triangle to the comparator. The triangle turns where it reaches 0.82 of the square, about ±10 V.
R82, 100 kΩ
From the square to Intensity.
Intensity, 10 kΩ pot, linear taper
Chorus Intensity, 10 kΩ linear. It sets how much of the square reaches the integrator, so how fast the triangle ramps: about 0.4 to 3 sweeps a second. The delay swings over the same range at any setting, so faster also means the pitch moves farther. It does nothing in Vibrato.
R65, 1.5 kΩ
Under Intensity. It sets the slowest sweep.
R57, 100 kΩ
From Intensity to the integrator, in Chorus.
R76, 18 kΩ
Integrator input resistor, in both modes. With C43 it sets the ramp’s speed.
C43, 82 nF
Integrator cap.
IC-4b, MC1458
Integrator, half a CA1458. In Chorus the square in makes a triangle out. In Vibrato it runs in a loop with Rate and the network after it, and puts out a rounded wave.
R77, 2.2 kΩ
From the integrator to the vibrato network.
Rate, 100 kΩ pot, reverse audio taper
Vibrato Rate, 100 kΩ reverse audio, used as a variable resistor. Its resistance falls as it turns up, so the vibrato runs faster: about 3.7 to 13 sweeps a second. It does nothing in Chorus.
R89, 220 kΩ
Across Rate. It sets the slowest vibrato.
D07, 1N914 / 1N4148
With D08, holds the vibrato loop’s swing to about ±0.5 V at this point, so the oscillation stays steady. 1S1555, modeled as a 1N914.
D08, 1N914 / 1N4148
The other half of the limiter, for the other direction.
C34, 10 µF electrolytic
First cap of the vibrato loop’s two lags, to −14 V.
R64, 3.9 kΩ
Between the two lags.
C35, 4.7 µF electrolytic
Second cap of the vibrato loop, to −14 V. Its top goes back to the integrator through S4a in Vibrato.
R79, 4.7 kΩ
From the integrator to the clock’s control line.
R78, 8.2 kΩ
From +14 V to the control line. With R79 it scales the LFO by about 0.63 and centers it near 4.8 V.
Depth, 50 kΩ pot, linear taper
Vibrato Depth, 50 kΩ linear. In Vibrato its wiper is the clock’s control, so it sets how far the delay swings. At 0 the delay holds near 1.5 ms. It does nothing in Chorus, where the clock gets the whole swing.
R83, 4.7 kΩ
Under Depth.
D05, 1N914 / 1N4148
Holds the control line from going under about −0.5 V, so the shortest delay is about 1.3 ms. In Chorus it flattens the bottom of each sweep. 1S1555, modeled as a 1N914.
R35, 1 kΩ
Into the clock’s control node.
C22, 1 µF electrolytic
Filters the clock’s control node.
R28, 47 kΩ
One of the two 47 kΩ timing resistors from the clock’s timing nodes to the −14 V side. In the model it sets the stand-in clock’s current.
Q3 to Q6, LM311 ramp (stand-in)
Stand-in for the clock, a multivibrator of four transistors (Q3, Q4 2SA493 and Q5, Q6 2SC536F) with its 250 pF timing caps C18 and C19. The control voltage on D03 and D04 sets how far each timing node is lifted each cycle, so a higher voltage makes a longer period. In the model it is a ramp whose length follows the control in a straight line: 5 µs at −0.5 V and 16 µs at about 10.8 V, the drawing’s 5 to 16 µs.

About this version

Board ET-10C
From the mains, on regulated ±14 V rails: the battery part at +14 V and a −14 V source. The audio works around 0 V. The delay chip’s input sits at −3.5 V from R19 and R22.

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.

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