Transistor tape echo with a sliding record head, 1970 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 Maestro Echoplex EP-3.

A transistor tape echo with one playback head and a record head on a slider, after the Maestro Echoplex EP-3. Delay moves the record head along the tape; Echo Sustain sends the playback back to the record amp; Echo Volume pans between the dry signal and the echo.

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 Q5, a JFET with a gain of about 5. Its drain feeds the dry path to Echo Volume and the top of Record Level. Q3’s 470 k from collector to base holds its input near ground, so the record amp works on current. The .002 µF around Record Level carries most of that current above about 500 Hz, so Record Level sets mainly how much bass and low midrange is recorded. Q3 and Q4 lift the treble on the way to the record head (C10, then C12 and L1 in Q4’s emitter), since the tape loses the treble first. The tape runs at one speed, 7.5 inches a second here. Delay slides the record head along it, from 12.4 mm in front of the echo head to 143 mm, so the repeat comes 65 to 740 ms behind the note (both figures assumed). Q1 and Q2 bring the echo head up, with C19 and R25 in their feedback to cut back the head’s rising treble. Echo Sustain sends part of the echo back through C5 and R7 to the record amp, so each repeat is recorded again. C5 and R7 cut the bass below about 230 Hz on each pass, so the repeats thin out and turn brighter as they die away. At the starting settings a 1 kHz repeat comes back about 7.5 dB under the last, and a 3 kHz one about 4 dB under. The loop first runs away near Echo Sustain 7.2, at about 3.3 kHz. Echo Volume has its wiper grounded and pans between the dry end and the echo end. The bias oscillator, the erase head and the Sound on Sound side of the switch aren’t modeled: bias is a setting of the tape.

Download.

Parts and what they do

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

.047µ, 47 nF
Input coupling cap, .047 µF.
100k, 100 kΩ
In series with the gate, 100 k. With C2 it rolls off radio and the bias oscillator above about 16 kHz.
100p, 100 pF
From the gate to ground, 100 pF.
1M, 1 MΩ
Q5’s gate resistor, 1 M: with R1, the input impedance the guitar sees.
Q5, TIS58 (fitted)
The input stage, a TIS58 JFET. It feeds both the dry path and the record amp. Its gain is about 5.
3.3k, 3.3 kΩ
Q5’s source resistor, 3.3 k, unbypassed: 1.1 V on the drawing.
22k, 22 kΩ
Q5’s drain resistor, 22 k: 14.4 V at the drain on the drawing.
.1µ, 100 nF
Coupling cap from Q5’s drain to Record Level and the dry path.
Record Level, 500 kΩ pot, linear taper
Record Level, 500 k, a small dial on the front panel, taper assumed linear. It sets how hard the guitar is recorded, so how loud the echo comes back. It doesn’t change the dry level or how long the repeats last.
47k, 47 kΩ
From Record Level’s wiper to the record amp, 47 k.
.002µ, 2 nF
From the top of Record Level to the record amp, .002 µF. It passes treble around the pot, so the record amp gets more treble as Record Level goes down.
.0015µ, 1.5 nF
In the repeats’ path to the record amp, .0015 µF. With R7 it cuts the bass of each repeat, so the repeats thin out as they go round.
470k, 470 kΩ
The repeats’ resistor into the record amp, 470 k.
100p, 100 pF
From the repeats line to ground, 100 pF.
.1µ, 100 nF
Coupling cap into Q3’s base, .1 µF (an electrolytic on the drawing).
47k, 47 kΩ
Q3’s base to ground, 47 k. With R9 it sets Q3’s bias, 1.0 V at the base on the drawing.
470k, 470 kΩ
Collector to base, 470 k: Q3’s bias, and negative feedback that holds its gain down and its input impedance low.
Q3, TIS98
The record preamp, a TIS98.
22k, 22 kΩ
Q3’s collector resistor, 22 k: 12.2 V on the drawing.
1k, 1 kΩ
Q3’s emitter resistor, 1 k: 0.4 V on the drawing.
100µ, 100 µF electrolytic
Across Q3’s emitter resistor, 100 µF: full gain at audio.
.1µ, 100 nF electrolytic
Coupling cap from Q3 to the record amp, .1 µF.
22k, 22 kΩ
Into Q4’s base, 22 k. With C11 and R14 it divides the signal down at middle and low frequencies.
.002µ, 2 nF
Across R12, .002 µF. Above about 3.6 kHz it passes treble around R12: the first part of the record pre-emphasis.
4.7M, 4.7 MΩ
Q4’s bias from the 22 V line, 4.7 M: 2.2 V at the base on the drawing.
.33µ, 330 nF
With R14, from Q4’s base to ground, .33 µF.
10k, 10 kΩ
The bottom of the divider into Q4’s base, 10 k, in series with C11.
Q4, TIS98
The record amp, a TIS98. Its collector current drives the record head, so its emitter network sets the record current at each frequency.
10k, 10 kΩ
Q4’s collector resistor, 10 k: 14.3 V on the drawing.
2.2k, 2.2 kΩ
Q4’s emitter resistor, 2.2 k: “16V” on the drawing, read as 1.6 V.
.003µ, 3 nF
From Q4’s emitter to the coil and R17, .003 µF. With them it lowers the emitter’s impedance at the top of the audio band, so the record current rises there: the second part of the pre-emphasis.
18mH, 18 mH
In Q4’s emitter network, 18 mH. With C12 it is tuned near 22 kHz, which peaks the record current at the top of the band.
3.3k, 3.3 kΩ
Across L1, 3.3 k: it sets how sharp that peak is.
1µ, 1 µF
Coupling cap from Q4’s collector to the record head, 1 µF.
18mH, 18 mH
The bias trap’s coil, 18 mH. With C14 it is tuned to the bias oscillator, so the bias can’t get back into Q4.
250p, 250 pF
The bias trap’s trimmer, 90 to 400 pF on the drawing, here at 250 pF (assumed). With L2 it tunes near 75 kHz.
Tape, Tape cartridge (EP-3 transport, typical)
The tape loop in its cartridge, the record head (70 Ω on the drawing) and the echo head (395 Ω). The tape runs at one speed. Delay slides the record head along it, so the repeat comes later the further the head is from the echo head. Every tape figure is typical and can be changed.
Slider, 10 V
A stand-in, not a part of the unit: 10 V across Delay, so the pot’s position can set where the record head sits.
Delay, 10 kΩ pot, linear taper
Delay, the record head’s slider, drawn as a pot. At 0 the record head sits 12.4 mm from the echo head, about 65 ms. Full up it sits 143 mm away, about 750 ms (assumed), in a straight line with the slider.
1µ, 1 µF
From the echo head to Q1’s base, 1 µF.
Q1, TIS98
The playback preamp’s first transistor, a TIS98. Its collector drives Q2’s base directly.
1k, 1 kΩ
Q1’s emitter resistor, 1 k: 0.6 V at the emitter on the drawing.
100µ, 100 µF electrolytic
Across R18, 100 µF, so at audio Q1’s emitter sees only R19.
100, 100 Ω
Under Q1’s emitter, 100 Ω. The playback EQ’s feedback comes back into this point.
22k, 22 kΩ
Q1’s collector resistor, 22 k: 3 V at the collector on the drawing.
10k, 10 kΩ
Feeds Q1’s rail from the 22 V line, 10 k.
35µ, 35 µF electrolytic
Filters Q1’s rail, 35 µF.
Q2, TIS98
The playback preamp’s second transistor, a TIS98.
10k, 10 kΩ
Q2’s collector resistor, 10 k: 15 V on the drawing.
3.3k, 3.3 kΩ
Q2’s emitter resistor, 3.3 k: 2.4 V on the drawing.
100µ, 100 µF electrolytic
Across Q2’s emitter resistor, 100 µF.
470k, 470 kΩ
From Q2’s emitter back to Q1’s base, 470 k: the pair’s DC bias, 1.2 V at Q1’s base on the drawing.
.003µ, 3 nF
In the playback feedback, .003 µF. A playback head’s output rises 6 dB an octave. This cap lets more feedback through as the frequency rises, so the gain falls 6 dB an octave to match, up to about 2.9 kHz.
18k, 18 kΩ
In series with C19, 18 k. Above about 2.9 kHz it holds the feedback, so the very top is left at a level.
.02µ, 20 nF
Coupling cap out of the playback amp, .02 µF.
SOS Record, 220 kΩ pot, linear taper
SOS Record, 220 k, inside the unit. In Sound on Sound it sets how much of the playback is recorded again. In Echo its wiper goes nowhere, and it’s only a 220 k load on the playback amp.
Echo Sustain, 500 kΩ pot, linear taper
Echo Sustain, 500 k, taper assumed linear. Its wiper sends some of the echo back to the record amp, so each repeat is recorded again. Turned up far enough, each repeat comes back louder than the last and the loop runs away.
220k, 220 kΩ
The echo’s resistor into Echo Volume, 220 k.
100k, 100 kΩ
From the echo end of Echo Volume to ground, 100 k. With R28 it sets the echo’s level into the mix.
470p, 470 pF
Across R29, 470 pF: a treble cut on the echo above about 5 kHz.
Echo Volume, 500 kΩ pot, linear taper
Echo Volume, 500 k, its wiper grounded, taper assumed linear. Turned up, the wiper moves toward the dry end and shorts it, leaving the echo. Turned down, it shorts the echo.
100k, 100 kΩ
From the echo end of Echo Volume to the output jack, 100 k.
100k, 100 kΩ
From the dry end of Echo Volume to the output jack, 100 k. The two 100 k mix the ends.
100k, 100 kΩ
The dry signal’s resistor into Echo Volume, 100 k, from the top of Record Level.
1k, 1 kΩ
Feeds the record side’s rail (Q5 and Q3) from the 22 V line, 1 k.
35µ, 35 µF electrolytic
Filters the record side’s rail, 35 µF.

About this version

As drawn, serial 12961 to 28591
The EP-3 drawing for serials 12961 to 28591, in the Echo position with nothing in either foot switch jack. The supply is drawn as the 22 V line, 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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