Oil can echo with transistor amps, 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 Morley EDL Electrostatic Delay Line.

An oil can echo after the Morley EDL. A spinning disc in oil holds the guitar as electric charge: one brush writes it, a second brush half a turn on reads it back. The disc never quite forgets, so every note comes round again each turn, darker each time.

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. Input Level feeds two amps. Q5 carries the dry guitar to the output. Q4 starts the echo path. Q3 and Q2 are the record amp, and they drive the writing brush. The 27 k, 68 k, 1 M trims and the two .001 µF caps from the brush back to the input set its gain and treble lift. The disc is anodized aluminum, so its oxide skin is a capacitor, and the brush leaves the guitar on it as charge. Half a turn later the reading brush picks it up. Q1 holds that brush near ground through 2.2 M, so the brush’s current becomes the echo voltage. A brush takes only part of what passes under it, and the writing brush overwrites only part, so every note comes round again a turn later. The charge leaks and the oil smears it on the way, so each turn is weaker and darker. Repeats feeds the echo back to Q4’s emitter through a 2.3 kHz filter, for more repeats. Echo Level sets the echo at the output.

Download.

Parts and what they do

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

Input Level, 100 kΩ pot, audio taper
Input Level, 100 k, audio taper (assumed). Its wiper feeds both the echo path and the dry path, so it sets how hard the disc is written and the dry level together.
.047µ, 47 nF
Coupling cap into Q4, .047 µF.
68k, 68 kΩ
Q4’s collector resistor, 68 k.
4.7M, 4.7 MΩ
From Q4’s collector to its base, 4.7 M. With R3 it sets the bias, and it holds the collector steady.
2.2M, 2.2 MΩ
Q4’s base to ground, 2.2 M.
Q4a, MPSA12 (half)
First half of Q4, an MPSA12 Darlington.
Q4b, MPSA12 (half)
Second half of Q4, the echo path’s input stage. The repeats come into its emitter.
18k, 18 kΩ
Q4’s emitter resistor, 18 k. The repeats come in across it, so the emitter is where the guitar and the repeats mix.
.047µ, 47 nF
From Q4’s emitter toward Repeats, .047 µF.
68k, 68 kΩ
From Repeats’ wiper into Q4’s emitter, 68 k.
.047µ, 47 nF
Coupling cap out of Q4, .047 µF.
1M, 1 MΩ
Into the record amp’s summing point, 1 M. The feedback network’s impedance against this sets the record amp’s gain.
1µ, 1 µF electrolytic
Coupling cap into Q3, 1 µF.
15M, 15 MΩ
Q3’s base from the 25 V line, 15 M.
2.2M, 2.2 MΩ
Q3’s base to ground, 2.2 M.
Q3a, MPSA12 (half)
First half of Q3, an MPSA12 Darlington.
Q3b, MPSA12 (half)
Second half of Q3, an emitter follower. Its very high input impedance keeps the summing point’s 1 M network free.
10k, 10 kΩ
Q3’s emitter resistor, 10 k.
1µ, 1 µF electrolytic
From Q3’s emitter to Q2’s base, 1 µF.
220k, 220 kΩ
From Q2’s collector to its base, 220 k.
25k, 25 kΩ pot, linear taper
Q2’s bias trim, 25 k, from its base to ground. With the 220 k it sets Q2’s current. Set so its collector sits near half the supply (assumed).
Q2, 2N3904
The record amp’s gain stage. The drawing calls it an MPS254, a part with no sheet found, so a 2N3904 stands in.
10k, 10 kΩ
Q2’s collector resistor, 10 k.
22, 22 Ω
Q2’s emitter resistor, 22 Ω.
10µ, 10 µF electrolytic
Across Q2’s emitter resistor, 10 µF. It bypasses the 22 Ω above about 720 Hz.
.1µ, 100 nF electrolytic
From Q2’s collector to the writing brush, 0.1 µF.
22k, 22 kΩ
From the writing brush to ground, 22 k.
27k, 27 kΩ
The first part of the record amp’s feedback, from the writing brush, 27 k.
Midrange, 1 MΩ pot, linear taper
The Midrange trim, 1 M, in the feedback. Between the two .001 µF caps it sets how much the middle of the band is lifted. Set at half (assumed).
.001µ, 1 nF
From the feedback’s first point to the Highs trim, .001 µF.
.001µ, 1 nF
From the feedback’s second point to the Highs trim, .001 µF.
Highs, 25 kΩ pot, linear taper
The Highs trim, 25 k, from the two .001 µF caps to ground. The caps shunt the feedback at high frequencies, so the record amp lifts the treble, and this sets how far. Set at half (assumed).
68k, 68 kΩ
In the feedback, 68 k.
1M, 1 MΩ pot, linear taper
A 1 M trim in the feedback, with 30 pF across it. Set at half (assumed).
30p, 30 pF
Across the second 1 M trim. The drawing says “30MF”, read as 30 pF.
1M, 1 MΩ
From the feedback’s middle to ground, 1 M. With the trims either side it makes a T, so a small feedback current sets a large gain.
1M, 1 MΩ pot, linear taper
A 1 M trim, the feedback’s last part into the summing point. It sets the record level. Set at half (assumed).
Disc, Oil can (EDL disc, typical)
The oil can: an anodized disc turning in oil, a writing brush and a reading brush half a turn on. The disc turns once in about 300 ms, so the first echo comes at about 150 ms and the disc’s own repeats every 300 ms after that (typical figures, assumed). Every disc figure is typical and can be changed.
.047µ, 47 nF
From the reading brush to Q1’s base, .047 µF.
2.2M, 2.2 MΩ
From the read amp’s output back to the reading brush, 2.2 M. Q1’s gain holds the brush near ground, so the brush’s current flows through this, and the echo voltage is that current times 2.2 M.
4.7M, 4.7 MΩ
Q1’s collector to base, 4.7 M.
1.8M, 1.8 MΩ
Q1’s base to ground, 1.8 M.
Q1a, MPSA12 (half)
First half of Q1, an MPSA12 Darlington.
Q1b, MPSA12 (half)
Second half of Q1, the read amp.
68k, 68 kΩ
Q1’s collector resistor, 68 k.
10k, 10 kΩ
Q1’s emitter resistor, 10 k.
1µ, 1 µF electrolytic
Across Q1’s emitter resistor, 1 µF, for full gain above about 16 Hz.
1µ, 1 µF electrolytic
Coupling cap out of the read amp, 1.0 µF.
22k, 22 kΩ
From the read amp to Echo Level, 22 k.
Echo Level, 100 kΩ pot, audio taper
Echo Level, 100 k, audio taper (assumed). It sets the echo at the output. The repeats are taken before it, so it doesn’t change how long they last.
68k, 68 kΩ
From Echo Level’s wiper to the output, 68 k.
27k, 27 kΩ
The echo line’s 27 k to ground.
.1µ, 100 nF
Into the repeat filter, 0.1 µF.
1.8M, 1.8 MΩ
The repeat filter’s bias from the 25 V line, 1.8 M.
2.2M, 2.2 MΩ
The repeat filter’s bias to ground, 2.2 M.
47k, 47 kΩ
The repeat filter’s first 47 k.
47k, 47 kΩ
The repeat filter’s second 47 k.
.0022µ, 2.2 nF
From between the 47 k resistors to the follower’s emitter, .0022 µF. With the .001 µF it makes a two-pole low-pass near 2.3 kHz.
.001µ, 1 nF
The repeat filter’s cap to ground, .001 µF.
Q6a, MPSA12 (half)
First half of the repeat filter’s follower, an MPSA12 Darlington (not numbered on the drawing).
Q6b, MPSA12 (half)
Second half of the repeat filter’s follower.
47, 47 Ω
The follower’s collector resistor, 47 Ω.
22k, 22 kΩ
The follower’s emitter resistor, 22 k.
.1µ, 100 nF
From the follower to Repeats, 0.1 µF.
250k, 250 kΩ pot, linear taper
A 250 k trim above Repeats. It sets the most feedback Repeats can reach. Set at about 96 k, so the repeats first run away at Repeats 7.2 (assumed, as on the other delay units).
Repeats, 100 kΩ pot, linear taper
Repeats, 100 k, linear (assumed). It feeds the echo back into Q4’s emitter, so it is recorded again. Turned up, the repeats last longer, then run away.
.047µ, 47 nF
Coupling cap into the direct signal amp, .047 µF.
68k, 68 kΩ
Q5’s collector resistor, 68 k.
4.7M, 4.7 MΩ
Q5’s collector to base, 4.7 M.
2.2M, 2.2 MΩ
Q5’s base to ground, 2.2 M.
Q5a, MPSA12 (half)
First half of Q5, an MPSA12 Darlington.
Q5b, MPSA12 (half)
Second half of Q5, the direct signal amp.
8.2k, 8.2 kΩ
Q5’s emitter resistor, 8.2 k, left unbypassed: its gain is about 8.
1µ, 1 µF electrolytic
Coupling cap out of Q5, 1 µF.
68k, 68 kΩ
From Q5 to the output, 68 k.
27k, 27 kΩ
The direct line’s 27 k to ground. With nothing in Delay Out, its jack joins the echo line and the direct line here.
47µ, 47 µF electrolytic
Filter cap on the 25 V line, 47 µF. The wall transformer, the doubler and the first filter stages are drawn as the supply.
22k, 22 kΩ
Bleeder on the 25 V line, 22 k.

About this version

As drawn, 1981
Morley’s drawing of 3 June 1981, on its 25 V line. The disc 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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