Tube tape echo with a sliding record head, 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 Maestro Echoplex EP-2.

A tube tape echo with one playback head and a record head on a slider, after the Maestro Echoplex EP-2. Delay moves the record head along the tape, so the tape speed never changes; Echo Repeats sends the playback back to the record amp; Echo Balance 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 V2A, a 6EU7 half with its cathode bypassed only on the treble. Its plate feeds two places: the dry path to Echo Balance through R10, and Record Level, which sets how much reaches the record amp, V2B. V2B drives the record head through C13 and the bias trap, with more current on the treble (C11 across its cathode resistor), 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 playback head to 143 mm, so the repeat comes 65 to 750 ms behind the note (typical figures). The playback head drives V1A directly; C3 and R2 on its plate cut back the head’s rising treble, and V1B brings the echo up to several volts. Echo Repeats sends part of the echo back through two 1 M resistors and S2 to V2B’s grid, after Record Level, so each repeat is recorded again: at 4 a 1 kHz repeat comes back about 4.5 dB under the last, and above about 7.2 the loop runs away until the tape saturates. Echo Balance has its wiper grounded and pans between the dry end and the echo end. At the starting settings the first repeat comes back about 4 dB over the dry note. The bias oscillator (the 6C4) and the erase head aren’t modeled: bias is a setting of the tape. Moving Delay while notes are still on the tape slides the pitch of what it records, as the real slider does, though on this page the knobs only change between notes.

Download.

Parts and what they do

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

47k, 47 kΩ
From the input jacks to V2A’s grid, 47 k. With the tube’s input capacitance it stops radio and the bias oscillator getting in.
470k, 470 kΩ
V2A’s grid resistor, 470 k: the input impedance the guitar sees.
V2A, 6EU7
Half a 6EU7, the input stage. It gains the guitar up for both the dry path and the record amp.
220k, 220 kΩ
V2A’s plate resistor, 220 k, +92 V on the drawing.
4.7k, 4.7 kΩ
V2A’s cathode resistor, 4.7 k, +0.9 V on the drawing.
.05µ, 50 nF
Across V2A’s cathode resistor, .05 µF. It bypasses it only above about 680 Hz, so the stage gains more on treble.
0.2µ, 200 nF
Coupling cap from V2A’s plate to Record Level and the dry path.
R18 Record Level, 500 kΩ pot, linear taper
Record Level, 500 k, taper assumed linear. It sets how much of the guitar reaches the record amp, so how hard the tape is driven and how loud the echo comes back. It doesn’t change the dry signal’s level or how long the repeats last.
220k, 220 kΩ
From Record Level’s wiper to V2B’s grid, 220 k.
220p, 220 pF
Across R19, 220 pF. It lets the treble past R19, a record pre-emphasis above about 3 kHz.
100k, 100 kΩ
V2B’s grid resistor, the top half, 100 k.
.01µ, 10 nF
From the middle of the grid resistors to ground, .01 µF. Above about 160 Hz it shorts the bottom half, so the grid’s divider passes less bass than treble.
100k, 100 kΩ
V2B’s grid resistor, the bottom half, 100 k.
V2B, 6EU7
The other half of the 6EU7, the record amp. It drives the record head through C13 and the bias trap.
100k, 100 kΩ
V2B’s plate resistor, 100 k, +82 V on the drawing. With the tube it sets how much current the record head gets.
2.2k, 2.2 kΩ
V2B’s cathode resistor, 2.2 k, +0.8 V on the drawing. On the real unit it returns through the playback foot switch jack, which grounds it with nothing plugged in.
.047µ, 47 nF
Across V2B’s cathode resistor, .047 µF. It bypasses it above about 1.5 kHz: more record current on the treble, which the tape loses first.
470p, 470 pF
From V2B’s plate to ground, 470 pF. It keeps the bias oscillator off the record amp’s plate.
.1µ, 100 nF
Coupling cap from V2B’s plate to the record head, 0.1 µF.
18mH, 18 mH
The bias trap’s coil, 18 mH (the drawing’s note 3). With C17 it is tuned to the bias oscillator, so the bias can’t get back into V2B. At audio it barely shows: about 1 kΩ at 10 kHz.
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-2 transport, typical)
The tape loop in its cartridge, the record head (160 Ω on the drawing) and the playback head (420 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 playback 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 playback head, about 65 ms. Full up it sits 143 mm away, about 750 ms (typical figures, assumed), in a straight line with the slider. Moved while notes are still on the tape, the ones it records slide in pitch.
V1A, 6EU7
Half a 6EU7, the playback preamp. The playback head drives its grid directly, with no grid resistor.
4.7k, 4.7 kΩ
V1A’s cathode resistor, 4.7 k, +0.7 V on the drawing.
50µ, 50 µF electrolytic
V1A’s cathode bypass, 50 µF: the stage’s full gain.
220k, 220 kΩ
V1A’s plate resistor, 220 k, +68 V on the drawing.
100k, 100 kΩ
A dropping resistor, 100 k, from the 122 V line to V1A’s plate resistor.
.1µ, 100 nF
Filters V1A’s own supply, 0.1 µF.
.01µ, 10 nF
With R2, from V1A’s plate to ground, .01 µF: the playback EQ. A playback head’s output rises 6 dB an octave, and this cuts the treble back from a few hundred hertz.
2.2k, 2.2 kΩ
In series with C3, 2.2 k. It stops the playback EQ’s cut near 7 kHz, so the very top is held at a level.
.02µ, 20 nF
Coupling cap from V1A to V1B, .02 µF.
3.3M, 3.3 MΩ
V1B’s grid leak, 3.3 M. V1B has no cathode resistor: its bias comes from the grid current through this resistor.
V1B, 6EU7
The other half of the 6EU7, the playback amp’s second stage, its cathode grounded.
220k, 220 kΩ
V1B’s plate resistor, 220 k, +60 V on the drawing.
470p, 470 pF
From V1B’s plate to ground, 470 pF: a treble cut above about 3 kHz, and the bias oscillator kept out.
.02µ, 20 nF
Coupling cap out of the playback amp, .02 µF.
R6 Echo Repeats, 500 kΩ pot, linear taper
Echo Repeats, 500 k, taper assumed linear. Its wiper sends some of the echo back through R22 and R22B 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.
1M, 1 MΩ
From Echo Repeats’ wiper to S2, 1 M: the first of two in the repeats’ path back to the record amp.
1M, 1 MΩ
From S2 to V2B’s grid, 1 M (also marked R22 on the drawing). The repeats reach the record amp here, after Record Level, so Record Level sets how hard the guitar is recorded but not how long the repeats last.
220k, 220 kΩ
The echo’s resistor into Echo Balance, 220 k.
220k, 220 kΩ
The dry signal’s resistor into Echo Balance, 220 k, from V2A’s plate side of C8.
100p, 100 pF
Across R10, 100 pF. It lifts the dry signal’s top end into the high-impedance mix.
R7 Echo Balance, 500 kΩ pot, linear taper
Echo Balance, 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.
470k, 470 kΩ
From the echo end of Echo Balance to the output jack, 470 k.
470k, 470 kΩ
From the dry end of Echo Balance to the output jack, 470 k. The two 470 k mix the ends. The output is high impedance, meant for an amp’s input.
T2 a, 202 V ac, 60 Hz
One half of T2’s center-tapped high-voltage winding, as a sine at the mains frequency. Typical, not on the drawing.
T2 b, 202 V ac, 60 Hz
The other half of the winding, half a cycle behind.
T2 R, 60 Ω
Resistance of one half of the winding. A typical value.
T2 R, 60 Ω
Resistance of the other half. A typical value.
SR1, 1N4001
One of the two silicon rectifiers, full wave.
SR2, 1N4001
The other rectifier, which conducts on the other half cycle.
100, 100 Ω
In series with the rectifiers, 100 Ω: it limits the charging current into C18A.
40µ, 40 µF electrolytic
First filter cap, 40 µF, 260 V on the drawing.
3.9k, 3.9 kΩ
Dropping resistor to the bias oscillator’s supply, 3.9 k.
20µ, 20 µF electrolytic
The bias oscillator’s filter cap, 20 µF, 170 V on the drawing.
Osc, 7.4 kΩ
A stand-in for the 6C4 bias oscillator’s current, which isn’t modeled: 7.4 k, about 23 mA, so the 260 and 170 V nodes sit near the drawing’s figures.
100k, 100 kΩ
Dropping resistor to the 165 V line, 100 k. The four triodes draw about 1 mA through it.
40µ, 40 µF electrolytic
Filter cap on the 165 V line, 40 µF.
47k, 47 kΩ
Dropping resistor to the playback amp’s 122 V line, 47 k.
30µ, 30 µF electrolytic
Filter cap on the 122 V line, 30 µF.

About this version

As drawn, serial 5939 to 9067
The EP-2 drawing for serials 5939 to 9067, in the Echo position with nothing in either foot switch jack. The power transformer, the bias oscillator’s draw and the tape figures are typical, not on the drawing.

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.

© 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.

Product and company names are trademarks of their owners. They are used here only to identify the circuits these models are based on; this site is not affiliated with or endorsed by any of them.

From Swell Amplification, swellamps.com.