JFET tape-echo preamp, 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.
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.
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.
3Press Play
Press Play through the circuit to hear a picked note or chord. Play the string is the guitar by itself.
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.
5Tap a wire
Tap a wire to put the probe there. The Probe graph and the spectrum show the signal there.
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
- Not sure what a control does? Hover over it, or hold a finger on it, for a short note.
- For step-by-step walkthroughs, open Start here at the top of the page.
- To put together a pedal or preamp of your own, use the Builder tab.
- Your switches stay where you left them the next time you open that circuit, in this browser only.
- A tube amp opens whole, with its power amp. Pick Preamp only in the Power amp menu to hear the preamp alone, or another power amp to try this preamp on it.
- The graphs use a steady test tone. Play sounds a picked note that dies away.
- Part values are typical of their type. Each circuit’s notes say which schematic it’s based on.
Modeled after the Maestro Echoplex EP-3 (echo off).
The input stage of a solid-state tape echo, heard with the echo off. One JFET gain stage on a 22 V rail, then a passive mixer where the dry signal meets the echo side through 220 kΩ and 100 kΩ.
Try this
Common mods
Changes people often make to this circuit. Try it plays the note before and after, at the same loudness. Keep it adds the mod to Your changes, where it can go on a switch.
Input
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.
Your build
Stack blocks from the guitar down to the output. Tap + Add a block wherever you want one. The coupling caps and the supply are wired in for you.
Your blocks. These are blocks you saved with Save block, values and part changes included. Add one to any build of the same kind with + Add a block. They stay in this browser.
Your builds stay in this browser, on this device, and 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 to a file to keep a copy. Open a file, or drop the .bench file on the page, brings it back on any device.
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.
Tap where each leg goes. Tap both ends of a part to put the new one across that 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.
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.
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.
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. There isn’t much gain here. The JFET gains about five, and the mixer loses most of it, so as drawn the unit ends up a few dB below unity. Real units vary: some are reported slightly hotter than unity, and at least one measures below it at every setting. What people hear is the voicing. The input networks, the high-impedance JFET input and the output network tilt the response and shift the phase slightly, and the JFET adds some second-harmonic color. The early build has a small bypass cap across Q5’s source resistor, lifting the treble above a couple of kilohertz. The late build leaves it out and has a different input network. Echo volume is a balance control with its wiper grounded. With no echo, turning it up lifts the dry signal about 1 dB at half, then cuts it off at full.
Download.
Parts and what they do
Stock values for the Late version. Tap a part on the schematic to change it.
- C36, 36 µF electrolytic
- Filter cap on the 22 V rail.
- .047, 47 nF
- Late input cap. It isn’t in the early version on the conversion sheet.
- 100K, 100 kΩ
- Input series resistor. It and the shunt cap after it make a gentle low-pass on the guitar.
- CJ, 100 pF
- Input shunt cap. With the 100 kΩ it makes a low-pass near 16 kHz with the late 100 pF, or 7 kHz with the early 220 pF. The cable and pickup in front pull it lower still.
- 1M, 1 MΩ
- Gate resistor to ground. It holds the gate at 0 V and sets the input impedance near 1 MΩ.
- Q5, TIS58 (fitted)
- The only active stage in the dry path, a TIS58 JFET in common source. Real TIS58s vary widely, and so did the sound of different units.
- 22K, 22 kΩ
- Drain load. The schematic shows the drain at 14.4 V.
- 3.3K, 3.3 kΩ
- Source resistor, marked 1.1 V. It self-biases the JFET, and its local feedback holds the gain near 5.
- .1, 100 nF
- Coupling cap from the drain to the dry line.
- Rec. level, 47 kΩ
- The 47 kΩ record-level pot, end to end. It loads the dry line whatever it’s set to.
- 220K, 220 kΩ
- Dry signal into the echo-volume control. The sheet marks this part as changed but shows only the early value, so the late build is assumed the same.
- Echo vol., 500 kΩ pot, linear taper
- Echo volume, a balance control with its wiper grounded. Turning it up shunts the dry end to ground and lifts the echo end off ground. With the echo off, the dry signal comes up about 1 dB at half and is cut off at full.
- 100K, 100 kΩ
- Dry end of the control to the output.
- 100K, 100 kΩ
- Echo end of the control to the output. With no echo, this and the echo side form a divider that shunts some of the dry signal to ground.
- 100K, 100 kΩ
- 100 kΩ from node 10 to ground, late version only.
- 470P, 470 pF
- In the late version, this 470 pF sits across the 100 kΩ from node 10 to ground.
- 220K, 220 kΩ
- From the playback amp side into node 10.
- SOS recd, 220 kΩ
- Sound-on-sound record pot, end to end, as a fixed load.
- Echo sust., 500 kΩ
- Echo sustain pot, end to end, as a fixed load.
- .02, 20 nF
- Coupling cap from the playback amp.
- Q2 out, 10 kΩ
- The idle playback amp, modeled as its 10 kΩ collector load to the supply, which is AC-grounded.
Versions
- Late
- Serial 12961 to 28591, as printed: a .047 input cap, a 100 pF input shunt, the gate driven directly, the source unbypassed, and 470 pF and 100 kΩ at node 10. The 100 pF comes from a forum report, not the sheet.
- Early
- The early version on the conversion sheet has no input cap, a 220 pF input shunt, .02 into the gate, .022 across the source resistor, 470 kΩ to the playback head, and no network at node 10.
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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