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, not measured from one unit. Each circuit’s notes say which schematic it’s based on.
Two inverters from a CD4049UB, each run as an amplifier with a resistor from its output back to its input. Driven hard they round off and clip unevenly, top and bottom.
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
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 playedEach note now rings until the string dies away, up to 12 s. It can take five times as long to start.
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.
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.
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.
Your changes
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.
Put in this wire:
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.Each inverter is a P and an N channel MOSFET in series across VA. With a resistor from its output back to its input it settles where both conduct, slightly under half of VA, and there it amplifies about 30 times. The first stage’s gain is its feedback over R1: 1 with Drive at 0, about 9 at full (the inverter’s own gain keeps it under the 11 the resistors ask for). The second stage is fed straight from the first one’s output, so it runs near its full gain, about 28, and clips first. Both MOSFETs conduct at rest, so each inverter pulls about 2 mA through R6, and VA sits near 4.9 V. Driven hard, the output swings from near ground to near VA, and the bend into each end isn’t the same, so the clipping is uneven top and bottom. VA rises as the inverters clip, since a stage swung to one end pulls less current. The CD4049UB is fitted to TI’s typical curves, not measured.
Download.
Parts and what they do
Stock values for the Red Llama version. Tap a part on the schematic to change it.
R6, 1 kΩ
Supply resistor. The two inverters pull a few mA through it, so VA sits a few volts under the battery.
C6, 220 µF electrolytic
Supply filter.
C7, 220 µF electrolytic
Second supply filter, beside C6.
R1, 100 kΩ
Input resistor. The first stage’s gain is its feedback over this 100 kΩ.
C1, 68 nF
Input coupling cap.
IC1F, CD4049UB
First inverter, a P and an N channel MOSFET in series across the supply. The feedback holds its input near the point where both conduct, about half of VA, and there it’s an amplifier with a gain of about 30.
R4, 100 kΩ
Feedback resistor, with Drive in series.
Drive, 1 MΩ pot, linear taper
Drive, 1 MΩ as a variable resistor in the feedback. Turned up, more resistance, more gain: from 1 at 0 to about 11 at full, before the inverter clips.
C2, 47 pF
Across the feedback. It lowers the gain above a few kHz, more as Drive goes up.
C3, 33 nF
Couples the first stage to the second. With nothing in series, the first stage’s output resistance is the second stage’s input resistor.
IC1D, CD4049UB
Second inverter. Its 1 MΩ feedback against the first stage’s low output resistance leaves it nearly its full gain, so it clips first.
R5, 1 MΩ
Second stage feedback. It biases the input and sets the stage’s input resistance.
C4, 100 pF
Across R5, cutting the top end.
C5, 10 µF electrolytic
Output coupling cap.
Volume, 10 kΩ pot, audio taper
Output level, 10 kΩ audio.
Versions
Red Llama
The Red Llama values, as the kit redraw shows them. The CD4049UB is typical of its data sheet, not measured.
Tube Sound Fuzz values
The values the kit notes list for Craig Anderton’s 1978 book version: 100 Ω supply resistor, 100 nF and 47 nF coupling caps, 10 pF across each stage.
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, not measured parts. 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.
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