MOSFET clean booster, 1990s 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 ZVEX Super Hard-On.

One small MOSFET with a 5.1 kΩ drain load. Two 10 MΩ resistors, one from the drain and one to ground, hold the gate at half the drain voltage, and a single knob sets the resistance from the source to ground.

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 knob sets the resistance from the source to ground. Less resistance means more gain and more current. The bias sets itself: R2 and R1 hold the gate at half the drain voltage, so when the current rises and the drain falls, the gate falls with it and holds the current back. Probe the drain and turn the knob. It runs from about 7.4 V at 0 to about 4.5 V at 10, so there’s room to swing both ways at any setting. The gain runs from about 1 at 0 to about 9 at noon and about 60 at full, fairly even in dB across the knob, since the pot is a C taper. From about 7 up, hard-picked notes flatten at the top and bottom of the drain’s swing. Even before that, the MOSFET’s square law adds some second harmonic, a few percent at full gain. The feedback through R2 also lowers the input resistance as the gain goes up, from about 3 MΩ at 0 to about 900 kΩ at noon and about 150 kΩ at full (less in the treble), so at high gain it loads the pickup slightly. At full gain the drain settles at about twice the MOSFET’s threshold. Tap Q1 on the drawing and try a 1.5 V or 3 V threshold to hear how the headroom splits. The 2N7000 build has about half the gain at the top.

Download.

Parts and what they do

Stock values for the BS170 version. Tap a part on the schematic to change it.

C1, 100 nF
Input coupling cap. Against the input resistance it passes everything down to about 2 Hz at noon and about 11 Hz at full gain.
R1, 10 MΩ
Gate resistor to ground. With R2 it holds the gate at half the drain voltage.
R2, 10 MΩ
Feedback bias, from the drain to the gate. If the current rises and the drain falls, the gate falls too and pulls the current back, so the drain stays near the middle of the supply at any gain. It also feeds a little output back to the input, lowering the input resistance as the gain goes up.
Q1, BS170
N-channel MOSFET as a common-source amplifier. The gate draws no current, so R1 and R2 alone set the bias.
R3, 5.1 kΩ
Drain load. The gain is about this resistor over the source resistance plus 1/gm.
Gain, 5 kΩ pot, reverse audio taper
Source resistance to ground, set by the knob: a 5 kΩ reverse-audio (C) pot with its wiper and one end grounded, so the resistance falls fast at first, about 500 Ω at noon, near 0 at 10. Less resistance means more gain and more current. The bias shifts as it turns, so a real pot crackles when you move it.
C2, 10 µF electrolytic
Output coupling cap. It goes straight to the jack in the single version, and into the second stage in the two-stage one.

Versions

BS170
Negative ground, 9 V. Thresholds spread from about 0.8 to 3 V between parts. The feedback bias keeps any of them working but moves where the drain sits.
2N7000
The common substitute, with its pins the other way around. It has a quarter of the BS170’s K, so at full Gain it gains about 30, against 60 for the BS170.
Two stages, modeled after the ZVEX Super Duper
Two identical stages in series, the second with its own gain knob and a 50 kΩ volume control after it. With both gains up the second stage clips.

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.

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