Tube tape echo with three playback heads in series, 1961 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 Watkins Copicat.

A tube tape echo with a tape loop and three playback heads, after the Watkins Copicat. One head plays at a time, picked by the head switches. Sustain sends the replay back to the record amp, and Echo Volume sets how much echo joins the guitar at the output.

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. The guitar goes into V1A, half an ECC83, and Gain sets its level. From Gain’s wiper, 470 k goes to the record amp and 1 M to the output, so the dry guitar reaches the output with no stage after it. The record amp, V2, is the pentode half of a 6BR8. It drives the record head through 100 k, so the head current follows the signal across the band. The three playback heads sit in series. A switch either puts a head in the chain or bridges it. On the 1961 Copicat only one head is in at a time, so the echo comes 100, 200 or 300 ms behind the note. V3A and V3B are the replay amp. A head’s output rises 6 dB an octave, and the 47 k and .005 µF on V3B’s grid even it out from about 60 Hz to about 680 Hz. Above that the replay keeps rising until the tape’s own losses cut in, so the repeats are bright. Each head switch has a third contact in the output rows, which sets the replay level. Sustain sends the replay back to the record amp’s grid, and Echo Volume sets the echo at the output.

Download.

Parts and what they do

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

1M, 1 MΩ
Input 1’s grid leak, 1 M. The jack goes straight to the grid.
V1A, 12AX7 / ECC83 / 7025
Half an ECC83, input 1’s pre-amplifier.
1M, 1 MΩ
Input 2’s grid leak, 1 M. Nothing is plugged into input 2 here.
V1B, 12AX7 / ECC83 / 7025
The other half of the ECC83, input 2’s pre-amplifier. It shares V1A’s cathode resistor.
1.5k, 1.5 kΩ
The two pre-amplifiers’ shared cathode resistor, 1.5 k.
25µ, 25 µF electrolytic
Across the shared cathode resistor, 25 µF: full gain on both halves.
100k, 100 kΩ
V1A’s plate resistor, 100 k high stability.
100k, 100 kΩ
V1B’s plate resistor, 100 k high stability.
.05µ, 50 nF
Coupling cap from V1A to Gain, .05 µF.
Gain, 500 kΩ pot, audio taper
Gain for input 1, 500 k, audio taper (assumed). It sets the guitar’s level into the record amp and to the output alike.
.05µ, 50 nF
Coupling cap from V1B to Gain 2, .05 µF.
Gain 2, 500 kΩ pot, audio taper
Gain for input 2, 500 k, set at half. With nothing in input 2 it only loads the mix.
1M, 1 MΩ
From Gain’s wiper to the output, 1 M: the dry guitar.
470k, 470 kΩ
From Gain’s wiper to the record amp’s grid, 470 k.
1M, 1 MΩ
From Gain 2’s wiper to the output, 1 M.
470k, 470 kΩ
From Gain 2’s wiper to the record amp’s grid, 470 k.
Sustain, 2 MΩ pot, linear taper
Sustain, 2 M, taper assumed linear. It hangs from the record amp’s grid to ground with the replay on its wiper. Turned up, more of each repeat is recorded again. At 0 the wiper grounds the replay line and the echo stops.
V2, 6AN8 pentode
The pentode half of the 6BR8, the record amp. The 6AN8 pentode model stands in for it.
680, 680 Ω
V2’s cathode resistor, 680 Ω.
25µ, 25 µF electrolytic
V2’s cathode bypass, 25 µF.
330k, 330 kΩ
V2’s screen resistor, 330 k.
.1µ, 100 nF
V2’s screen bypass, 0.1 µF.
100k, 100 kΩ
V2’s plate resistor, 100 k.
.1µ, 100 nF
Coupling cap from V2’s plate toward the record head, 0.1 µF.
100k, 100 kΩ
In series with the record head, 100 k. It is far bigger than the head’s impedance, so the head current follows V2’s plate voltage across the band.
200p, 200 pF
From the record head to the bias oscillator’s tank, 200 pF: it carries the bias. The oscillator isn’t drawn, so it returns to the oscillator’s supply line here.
Tape, Tape loop (Copicat transport, typical)
The tape loop, the record head and three playback heads. The tape runs at one speed, so the repeats come 100, 200 and 300 ms behind (typical figures, assumed). The heads are wired in series. Every tape figure is typical and can be changed.
V3A, 12AX7 / ECC83 / 7025
Half an ECC83, the replay amp’s first stage. The heads in series are its only grid return.
3.3k, 3.3 kΩ
V3A’s cathode resistor, 3.3 k.
25µ, 25 µF electrolytic
V3A’s cathode bypass, 25 µF.
100k, 100 kΩ
V3A’s plate resistor, 100 k high stability.
.05µ, 50 nF
Coupling cap from V3A, .05 µF.
1M, 1 MΩ
From the coupling cap to ground, 1 M.
470k, 470 kΩ
To V3B’s grid, 470 k. With R17 and C9 it forms the playback EQ.
47k, 47 kΩ
With C9, from V3B’s grid to ground, 47 k. The pair cuts 6 dB an octave from about 60 Hz to about 680 Hz, then holds. A playback head’s output rises 6 dB an octave, so this evens it out.
.005µ, 5 nF
The playback EQ’s cap, .005 µF.
V3B, 12AX7 / ECC83 / 7025
The other half of the ECC83, the replay amp’s second stage.
2.2k, 2.2 kΩ
V3B’s cathode resistor, 2.2 k.
25µ, 25 µF electrolytic
V3B’s cathode bypass, 25 µF.
100k, 100 kΩ
V3B’s plate resistor, 100 k high stability.
.01µ, 10 nF
Coupling cap out of the replay amp, .01 µF.
100k, 100 kΩ
From the replay amp to the replay output line, 100 k.
100k, 100 kΩ
Head 1’s level row, 100 k. With head 1 off, its switch puts this across R20, so the replay level rises.
220k, 220 kΩ
Head 1’s level row, 220 k to ground. With head 1 on, its switch puts this on the replay line, so the level drops.
100k, 100 kΩ
Head 2’s level row, 100 k. With head 2 off, its switch puts this across R20, so the replay level rises.
220k, 220 kΩ
Head 2’s level row, 220 k to ground. With head 2 on, its switch puts this on the replay line, so the level drops.
100k, 100 kΩ
Head 3’s level row, 100 k. With head 3 off, its switch puts this across R20, so the replay level rises.
220k, 220 kΩ
Head 3’s level row, 220 k to ground. With head 3 on, its switch puts this on the replay line, so the level drops.
1M, 1 MΩ
From the replay output line to ground, 1 M.
Echo Volume, 500 kΩ pot, audio taper
Echo Volume, 500 k, audio taper (assumed). It sets how much echo joins the guitar at the output. Sustain takes the replay before it, so it doesn’t change the repeats.
470k, 470 kΩ
From Echo Volume’s wiper to the output, 470 k. The output is high impedance, meant for an amp’s input.
T1, 230 V ac, 50 Hz
The high-voltage winding, as a sine at the mains frequency. Its voltage isn’t on the drawing: 230 V is set so the main line sits near 250 V.
T1 R, 50 Ω
The winding’s resistance. A typical value.
D1, 1N4001
The rectifier, half wave. A silicon part stands in for the drawing’s rectifier.
20µ, 20 µF electrolytic
First filter cap, 20 µF.
2.2k, 2.2 kΩ
Dropping resistor to the bias oscillator’s line, 2.2 k.
20µ, 20 µF electrolytic
Filter cap on the bias oscillator’s line, 20 µF.
Osc, 30 kΩ
A stand-in for the bias oscillator’s current, which isn’t modeled: 30 k, about 9 mA.
4.7k, 4.7 kΩ
Dropping resistor to the main line, 4.7 k.
20µ, 20 µF electrolytic
Filter cap on the main line, 20 µF.

About this version

As drawn, 1961
The Watkins drawing of 20 July 1961, one guitar in input 1. The power transformer, the oscillator’s draw and the tape figures are typical.

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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© 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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