Tube tape echo with three playback heads on buttons, 1965 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 Mk III.

A tube tape echo with a tape loop and three playback heads, after the Watkins Copicat Mk III. Any mix of the three heads can be in at once, so the repeats come at 100, 200 and 300 ms together. A transistor follower drives 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 Volume 1 sets its level. From the wiper, 470 k goes to the record amp and 1 M to the output line. V2A, the pentode half of a 6BR8, drives the record head through 100 k. The three playback heads sit in series on V3A’s grid. Each button either bridges its head or puts it in the chain, and any of them can be in at once. A head’s output rises 6 dB an octave. The 2.2 k across the heads holds it flat above about 600 Hz with all three in, about 1.7 kHz with one. Each button’s third pole puts a 220 k on V3B’s grid, so adding heads doesn’t raise the echo much. Sustain feeds the echo back to the record amp through 1.8 M, Echo Volume sets the echo at the output, and the footswitch grounds the echo line. TR1, an OC200, follows the output line on a −18 V supply made from the bias oscillator.

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.
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.
470, 470 Ω
The two pre-amplifiers’ shared cathode resistor, 470 Ω.
50µ, 50 µF electrolytic
Across the shared cathode resistor, 50 µF: full gain on both halves.
100k, 100 kΩ
V1A’s plate resistor, 100 k.
100k, 100 kΩ
V1B’s plate resistor, 100 k.
.05µ, 50 nF
Coupling cap from V1A to Volume 1, .05 µF.
Volume 1, 500 kΩ pot, audio taper
Volume 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 Volume 2, .05 µF.
Volume 2, 500 kΩ pot, audio taper
Volume for input 2, 500 k, set at half. With nothing in input 2 it only loads the mix.
1M, 1 MΩ
From Volume 1’s wiper to the output line, 1 M: the dry guitar.
470k, 470 kΩ
From Volume 1’s wiper to the record amp’s grid, 470 k.
1M, 1 MΩ
From Volume 2’s wiper to the output line, 1 M.
470k, 470 kΩ
From Volume 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, and the echo comes onto its wiper through R26. Turned up, more of each repeat is recorded again. At 0 the wiper is grounded and the echo plays once.
V2A, 6AN8 pentode
The pentode half of the 6BR8, the record amp. The 6AN8 pentode model stands in for it.
680, 680 Ω
V2A’s cathode resistor, 680 Ω.
50µ, 50 µF electrolytic
V2A’s cathode bypass, 50 µF.
330k, 330 kΩ
V2A’s screen resistor, 330 k.
.1µ, 100 nF
V2A’s screen bypass, 0.1 µF.
100k, 100 kΩ
V2A’s plate resistor, 100 k.
.1µ, 100 nF
Coupling cap from V2A’s plate toward the record head, 0.1 µF.
470p, 470 pF
From the record feed to ground. The value is hard to read on the drawing: 470 pF here. It trims the record drive’s top end and keeps bias off V2A.
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 V2A’s plate voltage across the band.
200p, 200 pF
From the record head to the bias oscillator, 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 Mk III 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). The heads are wired in series. Every tape figure is typical and can be changed.
2.2k, 2.2 kΩ
Across the heads, 2.2 k. With the heads’ inductance it rolls off their rising output above about 600 Hz with all three in, about 1.7 kHz with one: the replay EQ.
V3A, 12AX7 / ECC83 / 7025
Half an ECC83, the replay amp’s first stage.
2.2k, 2.2 kΩ
V3A’s cathode resistor, 2.2 k.
50µ, 50 µF electrolytic
V3A’s cathode bypass, 50 µF.
100k, 100 kΩ
V3A’s plate resistor, 100 k.
.01µ, 10 nF
Coupling cap to V3B, .01 µF.
470k, 470 kΩ
V3B’s grid leak, 470 k.
220k, 220 kΩ
Button A’s third pole puts this 220 k from V3B’s grid to ground while head 1 is in. More heads in, more cut here.
220k, 220 kΩ
Button B’s third pole puts this 220 k from V3B’s grid to ground while head 2 is in. More heads in, more cut here.
220k, 220 kΩ
Button C’s third pole puts this 220 k from V3B’s grid to ground while head 3 is in. More heads in, more cut here.
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, not bypassed, so the stage runs at lower gain.
47k, 47 kΩ
V3B’s plate resistor, 47 k.
.01µ, 10 nF
Coupling cap out of the replay amp, .01 µF.
100k, 100 kΩ
From the replay amp to the echo line, 100 k. The footswitch grounds the echo line after it.
Echo Volume, 500 kΩ pot, audio taper
Echo Volume, 500 k, audio taper (assumed). It sets how much echo joins the guitar on the output line. Sustain takes the echo before it, so it doesn’t change the repeats.
2.2M, 2.2 MΩ
From Echo Volume’s wiper to the output line, 2.2 M.
1.8M, 1.8 MΩ
From the echo line to Sustain’s wiper, 1.8 M: the feedback path for the repeats.
TR1, OC200
An OC200, a silicon PNP, as an emitter follower on the output line. It drives the cable and amp from a low impedance. The drawing marks its emitter at −3 V.
470k, 470 kΩ
TR1’s base bias from the −18 V line, 470 k. The output line’s resistors to ground are the other half of the divider.
10k, 10 kΩ
TR1’s emitter resistor to ground, 10 k.
.01µ, 10 nF
Output coupling cap, .01 µF. Into an amp’s 1 M input it passes down to about 16 Hz.
Osc grid, -54 V
A stand-in, not a part of the unit: the bias oscillator’s grid, which sits well below ground from its own rectified swing. −54 V puts the −18 V line near −18 V with TR1 running. Typical.
82k, 82 kΩ
From the oscillator’s grid to the −18 V line, 82 k.
50µ, 50 µF electrolytic
Filter cap on the −18 V line, 50 µF.
T2, 250 V ac, 50 Hz
The high-voltage winding, 250 V a.c. on the drawing, as a sine at the mains frequency.
T2 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 oscillator’s line, 2.2 k.
20µ, 20 µF electrolytic
Filter cap on the oscillator’s line, 20 µF. The drawing marks this line 285 V.
Osc, 20 kΩ
A stand-in for the bias oscillator’s current, which isn’t modeled.
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. The drawing marks this line 260 V.

About this version

As drawn, 1965
WEM’s Mk III drawing of 30 March 1965, one guitar in input 1. The power transformer, the oscillator’s draw and grid voltage 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.

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.

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