Automatic volume swell with a JFET gain cell, 1979 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 Boss SG-1 Slow Gear.

A JFET across the signal mutes it at rest. Each new note is caught by a high-gain detector, which pulls the JFET’s gate down through a timing cap, so the note fades in.

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. With no note the JFET’s gate sits at its channel’s voltage, so the JFET is a few hundred ohms from the signal to Vd and, against R3’s 22 kΩ, mutes it. The detector amplifies the note about 2,500 times, splits it into two opposite copies and turns Q5 and Q6 on near each peak. They pull C6 down through Attack, the gate follows through R8 and C5, the JFET opens and the note comes up. Open, the circuit passes about 0.6 of the input, about 4 dB down. Between notes R9 pulls C6 back to Vd and the gate shuts. The bench measures steady notes, so the swell itself, which the Attack pot and C6 time, isn’t heard in these figures.

Download.

Parts and what they do

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

R20, 22 kΩ
Top of the Vb divider.
R21, 22 kΩ
Bottom of the Vb divider.
C17, 47 µF electrolytic
Filters Vb.
R22, 1 kΩ
Feeds the op-amp’s supply, Vc.
C18, 47 µF electrolytic
Filters Vc.
R23, 3.3 kΩ
Feeds the zener.
Z1, 5.6 V zener, in breakdown
A 5.6 V zener. It holds Vd, the gain cell’s reference.
C19, 47 µF electrolytic
Filters Vd, so the JFET’s other end is an AC ground.
R24, 1 kΩ
Input series resistor.
C1, 47 nF
Input coupling cap.
R1, 470 kΩ
Biases Q1 at Vb.
Q1, 2N5088
Input buffer, an emitter follower. It feeds the gain cell and the detector.
R2, 10 kΩ
Q1’s emitter resistor.
R3, 22 kΩ
Into the gain cell. Against the JFET’s channel it makes the attenuator.
C2, 1 µF
Coupling cap into the gain cell.
R4, 470 kΩ
Holds S at Vd for DC.
Q2, 2SK30A-GR
The gain cell, a JFET used as a resistor from the signal to Vd. With its gate at its channel’s voltage it is a few hundred ohms and mutes the note. Pulled a few volts negative it opens and the note passes.
R7, 1 MΩ
With C5, feeds half the signal back to the gate, so the JFET stays even on both halves of the wave.
C5, 33 nF
In series with R7.
R8, 1 MΩ
From the timing node to the gate.
R9, 47 kΩ
Pulls the timing node back up to Vd between notes, which mutes the JFET again.
C6, 680 nF
The timing cap. How fast it is pulled down sets the swell.
Attack, 20 kΩ pot, linear taper
Attack, a 20 kΩ linear pot used as a variable resistor between the timing cap and the detector. More resistance, a slower swell. Which way it turns isn’t on the drawing.
C3, 1 µF
Coupling cap into the output follower.
R5, 1 MΩ
Biases Q3 from 9 V.
Q3, 2N5088
Output buffer, an emitter follower.
R6, 10 kΩ
Q3’s emitter resistor.
C4, 1 µF
Output coupling cap.
R25, 1 kΩ
Output series resistor.
R26, 100 kΩ
Holds the output at ground.
C9, 22 nF
From Q1 to the detector.
Sensitivity, 100 kΩ pot, audio taper
Sensitivity, a 100 kΩ audio pot. It sets how much signal reaches the detector, so how soft a note still opens the gate.
R10, 470 Ω
Under Sensitivity.
C10, 22 nF
Coupling cap into the op-amp.
R11, 220 kΩ
Biases the op-amp’s plus input at Vb.
IC1, LM741 / µA741
The detector’s amplifier, an LM741 on Vc. Gain about 2,500, so a soft note drives it to clipping.
R12, 1 MΩ
Feedback resistor.
R13, 390 Ω
Gain leg, 390 Ω.
C11, 1 µF electrolytic
In series with R13, so the gain applies to the signal only.
R14, 390 kΩ
Into Q4’s base. It also sets Q4’s bias from the op-amp’s output.
C12, 1 nF
Across R14, so the signal reaches Q4 in full.
Q4, 2N5088
Phase splitter: equal and opposite copies at its collector and emitter.
R15, 4.7 kΩ
Q4’s collector load.
R16, 4.7 kΩ
Q4’s emitter load.
C13, 1 µF electrolytic
To Q5’s base.
C14, 1 µF electrolytic
To Q6’s base.
D2, 1N914 / 1N4148
Clamps Q5’s base, so C13 charges and Q5 turns on near each peak.
R17, 100 kΩ
Q5’s base to its emitter.
D3, 1N914 / 1N4148
Clamps Q6’s base, as D2 does for Q5.
R18, 100 kΩ
Q6’s base to its emitter.
Q5, 2N5088
Pulls P down on one half of the wave.
Q6, 2N5088
Pulls P down on the other half.
D4, 1N914 / 1N4148
From the emitters to K.
C16, 10 µF electrolytic
Holds K steady.
R19, 10 kΩ
Above the Trim.
Trim, 10 kΩ pot, linear taper
A 10 kΩ trimmer inside. It sets K, the level the detector pulls P down toward. Its setting isn’t on the drawing, so it’s set at 0.3 of its turn.
C15, 47 nF
From P to K.

About this version

2N5088, 2SK30A
Negative ground, 9 V. 2N5088 transistors and an LM741 as the kit drawing has them. The 2SK30A-GR stands in for the 2SK208-GR.

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.

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