Tube drum echo with a four-way head selector, 1960 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 Binson Echorec 2.

A tube echo that records on a spinning magnetic drum, after Binson’s first Echorec 2 drawing. A selector picks one of four playback heads, 74 to 296 ms behind; Mode plays it once, feeds it back, or brings in all four heads at once.

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 V4A. Its plate feeds the dry trim inside, whose wiper goes straight to the output mix, and a pad (R4 and R5) with Input across its bottom, which sets how hard the drum is recorded. V5A and V5B share one plate resistor, so the guitar and the repeats add there. V6A drives the record head through C11 and R17. Each playback head has its own triode. The head selector picks one. In Echo it goes once through R37 to V3A. In Repeat it also goes back through Length of Swell to V5B. In Swell V3A hears all four heads through their 330 k resistors while the selected one feeds back. V3A has R38 from its output back to its grid, so it mixes what comes in at a set gain. Its output goes to Tone and Volume, then through R24 to the output mix and V4B, a cathode follower.

Download.

Parts and what they do

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

47k, 47 kΩ
Across the input jack, 47 k. Each of the three input jacks has its own. One jack is modeled.
47n, 47 nF
Coupling cap into V4A, 47 nF.
1M, 1 MΩ
V4A’s grid leak, 1 M. The cathode is grounded, so the bias comes from the grid current through it.
V4A, 12AX7 / ECC83 / 7025
Half an ECC83 (4’A on the drawing), the input stage. Its plate feeds the dry line and the record side.
100k, 100 kΩ
V4A’s plate resistor, 100 k, +80 V on the drawing.
47n, 47 nF
Coupling cap from V4A’s plate, 47 nF.
1M, 1 MΩ pot, linear taper
The dry level, a 1 M trim inside. Its wiper sends the dry signal through a 330 k to the output mix. Set at 0.67, as on the PE 603 T (assumed).
100k, 100 kΩ
With R5, a pad in front of Input, 100 k. The pad drops the record side about 26 dB.
820p, 820 pF
Across R4, 820 pF. It lets the treble past the pad from about 2 kHz up, a record pre-emphasis.
4.7k, 4.7 kΩ
The pad’s bottom, 4.7 k.
Input, 500 kΩ pot, audio taper
Input, 0.5 M across the pad’s bottom, marked B (read as audio taper). It sets how hard the guitar is recorded on the drum, not the dry level.
10n, 10 nF
Coupling cap into V5A, 10 nF.
1M, 1 MΩ
V5A’s grid leak, 1 M.
V5A, 12AX7 / ECC83 / 7025
Half an ECC83, the guitar’s side of the record mix. It shares its plate resistor with V5B, so the two signals add there.
560, 560 Ω
V5A’s cathode resistor, 560 Ω, unbypassed. +0.7 V on the drawing.
V5B, 12AX7 / ECC83 / 7025
The other half, the repeats’ side of the record mix.
560, 560 Ω
V5B’s cathode resistor, 560 Ω, unbypassed.
47k, 47 kΩ
The shared plate resistor, 47 k from the 220 V line, +157 V on the drawing.
0.25M, 250 kΩ pot, linear taper
A 0.25 M trim inside, from the Swell line to ground. Its wiper feeds Length of Swell, so it sets the most feedback the knob can reach. Set at 0.097, so the repeats first run away at Length of Swell 7.2 (assumed).
Length of Swell, 250 kΩ pot, linear taper
Length of Swell, 0.25 M, marked A (read as linear). It sets how much of the selected head goes back to the record mix, so how long the repeats last.
47k, 47 kΩ
Under Length of Swell, 47 k. At 0 a little still comes back.
10n, 10 nF
Coupling cap into V5B, 10 nF.
1M, 1 MΩ
V5B’s grid leak, 1 M.
470p, 470 pF
From V5B’s grid to ground, 470 pF. It darkens the repeats each time round.
47n, 47 nF
Coupling cap from the record mix, 47 nF.
0.5M, 500 kΩ
The level meter’s trim, 0.5 M to ground. Its wiper feeds the meter amp and the EM81, which aren’t drawn.
330k, 330 kΩ
From the record mix to V6A’s grid, 330 k.
150p, 150 pF
Across R12, 150 pF. It lets the treble past R12, a record pre-emphasis above about 3 kHz.
120k, 120 kΩ
V6A’s grid resistor, the top part, 120 k.
560k, 560 kΩ
V6A’s grid resistor, the bottom part, 560 k.
10n, 10 nF
From the middle of the grid resistors to ground, 10 nF. Above about 30 Hz it shorts R14, so the grid’s divider passes less bass than treble.
V6A, 12AU7 / ECC82
Half an ECC82, the record amp. It drives the record head through C11 and R17.
1.5k, 1.5 kΩ
V6A’s cathode resistor, 1.5 k.
22n, 22 nF
Across V6A’s cathode resistor, 22 nF. It bypasses it above about 5 kHz: more record current on the treble.
47k, 47 kΩ
V6A’s plate resistor, 47 k from the 280 V line.
47n, 47 nF
Coupling cap to the record head, 47 nF.
27k, 27 kΩ
In series with the record head, 27 k. It sets the record current from V6A’s plate swing, near constant across the band.
Drum, Magnetic drum (Echorec, typical)
The magnetic drum, the record head and four playback heads 34 mm apart. The drum turns at one speed, so the heads play 74, 148, 222 and 296 ms behind. Every drum figure is typical and can be changed.
220p, 220 pF
Across playback head 1, 220 pF. With the head’s inductance it sets a resonance near the top of the band.
2.2n, 2.2 nF
From head 1 to its preamp’s grid, 2.2 nF. Into the 10 M grid leak it passes everything above about 7 Hz.
10M, 10 MΩ
Grid leak, 10 M. The cathode is grounded, so the bias comes from the grid’s own current through this resistor.
V1A, 12AX7 / ECC83 / 7025
A triode of the two ECC83s in positions 1 and 2, the preamp for head 1 (74 ms).
47k, 47 kΩ
Plate resistor, 47 k from the 160 V line, +120 V on the drawing.
330p, 330 pF
From the plate to ground, 330 pF. It trims the top end and keeps the 50 kHz bias out of the rest of the unit.
20n, 20 nF
With the 10 k trim, 20 nF from the plate to ground: the head’s treble cut.
10k, 10 kΩ pot, linear taper
The head’s tone trim, 10 k, its wiper and one end grounded: a variable resistor under the 20 nF. Near 0 Ω the 20 nF cuts this head’s treble hard. Turned up, it cuts less.
10n, 10 nF
Coupling cap out to the head selector, 10 nF.
0.5M, 500 kΩ pot, linear taper
Head 1’s level in Swell, a 0.5 M trim. In Swell the echo amp hears all four heads, each through its trim’s wiper and a 330 k. Set at half (assumed).
330k, 330 kΩ
From the trim’s wiper to the four-head line, 330 k.
220p, 220 pF
Across playback head 2, 220 pF. With the head’s inductance it sets a resonance near the top of the band.
2.2n, 2.2 nF
From head 2 to its preamp’s grid, 2.2 nF. Into the 10 M grid leak it passes everything above about 7 Hz.
10M, 10 MΩ
Grid leak, 10 M. The cathode is grounded, so the bias comes from the grid’s own current through this resistor.
V1B, 12AX7 / ECC83 / 7025
A triode of the two ECC83s in positions 1 and 2, the preamp for head 2 (148 ms).
47k, 47 kΩ
Plate resistor, 47 k from the 160 V line, +120 V on the drawing.
330p, 330 pF
From the plate to ground, 330 pF. It trims the top end and keeps the 50 kHz bias out of the rest of the unit.
20n, 20 nF
With the 10 k trim, 20 nF from the plate to ground: the head’s treble cut.
10k, 10 kΩ pot, linear taper
The head’s tone trim, 10 k, its wiper and one end grounded: a variable resistor under the 20 nF. Near 0 Ω the 20 nF cuts this head’s treble hard. Turned up, it cuts less.
10n, 10 nF
Coupling cap out to the head selector, 10 nF.
0.5M, 500 kΩ pot, linear taper
Head 2’s level in Swell, a 0.5 M trim. In Swell the echo amp hears all four heads, each through its trim’s wiper and a 330 k. Set at half (assumed).
330k, 330 kΩ
From the trim’s wiper to the four-head line, 330 k.
220p, 220 pF
Across playback head 3, 220 pF. With the head’s inductance it sets a resonance near the top of the band.
2.2n, 2.2 nF
From head 3 to its preamp’s grid, 2.2 nF. Into the 10 M grid leak it passes everything above about 7 Hz.
10M, 10 MΩ
Grid leak, 10 M. The cathode is grounded, so the bias comes from the grid’s own current through this resistor.
V2A, 12AX7 / ECC83 / 7025
A triode of the two ECC83s in positions 1 and 2, the preamp for head 3 (222 ms).
47k, 47 kΩ
Plate resistor, 47 k from the 160 V line, +120 V on the drawing.
330p, 330 pF
From the plate to ground, 330 pF. It trims the top end and keeps the 50 kHz bias out of the rest of the unit.
20n, 20 nF
With the 10 k trim, 20 nF from the plate to ground: the head’s treble cut.
10k, 10 kΩ pot, linear taper
The head’s tone trim, 10 k, its wiper and one end grounded: a variable resistor under the 20 nF. Near 0 Ω the 20 nF cuts this head’s treble hard. Turned up, it cuts less.
10n, 10 nF
Coupling cap out to the head selector, 10 nF.
0.5M, 500 kΩ pot, linear taper
Head 3’s level in Swell, a 0.5 M trim. In Swell the echo amp hears all four heads, each through its trim’s wiper and a 330 k. Set at half (assumed).
330k, 330 kΩ
From the trim’s wiper to the four-head line, 330 k.
220p, 220 pF
Across playback head 4, 220 pF. With the head’s inductance it sets a resonance near the top of the band.
2.2n, 2.2 nF
From head 4 to its preamp’s grid, 2.2 nF. Into the 10 M grid leak it passes everything above about 7 Hz.
10M, 10 MΩ
Grid leak, 10 M. The cathode is grounded, so the bias comes from the grid’s own current through this resistor.
V2B, 12AX7 / ECC83 / 7025
A triode of the two ECC83s in positions 1 and 2, the preamp for head 4 (296 ms).
47k, 47 kΩ
Plate resistor, 47 k from the 160 V line, +120 V on the drawing.
330p, 330 pF
From the plate to ground, 330 pF. It trims the top end and keeps the 50 kHz bias out of the rest of the unit.
20n, 20 nF
With the 10 k trim, 20 nF from the plate to ground: the head’s treble cut.
10k, 10 kΩ pot, linear taper
The head’s tone trim, 10 k, its wiper and one end grounded: a variable resistor under the 20 nF. Near 0 Ω the 20 nF cuts this head’s treble hard. Turned up, it cuts less.
10n, 10 nF
Coupling cap out to the head selector, 10 nF.
0.5M, 500 kΩ pot, linear taper
Head 4’s level in Swell, a 0.5 M trim. In Swell the echo amp hears all four heads, each through its trim’s wiper and a 330 k. Set at half (assumed).
330k, 330 kΩ
From the trim’s wiper to the four-head line, 330 k.
330k, 330 kΩ
From the head selector to V3A’s grid in Echo and Repeat, 330 k. With R38 it sets V3A’s gain for the selected head.
V3A, 12AX7 / ECC83 / 7025
Half an ECC83 (3’A), the echo amp. R38 from its output back to its grid makes it a mixer: what comes in through R37 or the four 330 k resistors adds at the grid.
1.5k, 1.5 kΩ
V3A’s cathode resistor, 1.5 k, +1.4 V on the drawing.
25µ, 25 µF electrolytic
Across V3A’s cathode resistor, 25 µF: full gain across the band.
47k, 47 kΩ
V3A’s plate resistor, 47 k from the 220 V line, +170 V on the drawing.
22n, 22 nF
Coupling cap to Volume and Tone, 22 nF.
470k, 470 kΩ
From the echo amp’s output back to its grid, 470 k. It holds the stage’s gain near R38 over R37 and is the grid’s only path to ground.
100p, 100 pF
Across R38, 100 pF. More feedback on the treble, so the echo amp rolls off above about 3 kHz.
Tone, 250 kΩ pot, audio taper
Tone, 0.25 M, marked B (read as audio taper). Its wiper sets where C14 and R21 meet it. Turned down, C14 goes nearer ground and the treble with it. As drawn it hangs on V3A’s output, inside R38’s feedback, so end to end it moves the echo only about 2 dB at 3 kHz.
22n, 22 nF
From the top of Tone to its wiper, 22 nF.
47k, 47 kΩ
From Tone’s wiper to ground, 47 k.
Volume, 500 kΩ pot, audio taper
Volume, 0.5 M, marked B (read as audio taper). It sets the echo’s level at the output. The dry level is a trim inside.
470k, 470 kΩ
The echo into the output mix, 470 k.
330k, 330 kΩ
The dry signal into the output mix, 330 k.
47k, 47 kΩ
The output mix’s bottom, 47 k.
10n, 10 nF
Coupling cap into V4B, 10 nF.
1M, 1 MΩ
V4B’s grid resistor, 1 M, returned to the bottom of its cathode resistor, so the follower barely loads the mix.
V4B, 12AX7 / ECC83 / 7025
The other half of V4A’s ECC83, a cathode follower driving the output jacks.
1.5k, 1.5 kΩ
V4B’s cathode resistor, 1.5 k: the grid’s bias.
47k, 47 kΩ
The follower’s load, 47 k.
270n, 270 nF
Coupling cap to the output jacks, 270 nF.
47k, 47 kΩ
Across the output jack, 47 k.
T1 a, 250 V ac, 50 Hz
The 250 V winding into the bridge, as one half cycle. The motor and heater windings aren’t drawn.
T1 b, 250 V ac, 50 Hz
The other half cycle of the bridge.
T1 R, 1.25 kΩ
Resistance of the winding and the selenium bridge, 1.25 k. Set so line 1 sits near the drawing’s 280 V.
T1 R, 1.25 kΩ
The same for the other half cycle.
B250, 1N4001
The B250 C60 bridge (selenium on the unit), here as two silicon diodes.
C60, 1N4001
The bridge’s other half.
40µ, 40 µF electrolytic
First filter cap, 40 µF.
5H, 5 H
The filter choke, 5 H (typical, not on the drawing).
200, 200 Ω
The choke’s winding resistance, 200 Ω. A typical value.
40µ, 40 µF electrolytic
Filter cap on line 1, 40 µF, 280 V on the drawing.
4.7k, 4.7 kΩ
Dropping resistor to line 2, 4.7 k.
40µ, 40 µF electrolytic
Filter cap on line 2, 40 µF, 220 V on the drawing.
Osc, 56 kΩ
A stand-in for the bias oscillator’s and the level meter’s current, which aren’t modeled: 56 k, about 3.9 mA, as on the PE 603 T.
10k, 10 kΩ
Dropping resistor to line 3, 10 k.
40µ, 40 µF electrolytic
Filter cap on line 3, 40 µF, 160 V on the drawing.

About this version

As drawn
Binson’s Echorec 2 drawing of 1960 (revised 1961). The power transformer, the choke, the oscillator’s draw and the drum 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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