Octave fuzz with blend, 1969 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, not measured from one unit. Each circuit’s notes say which schematic it’s based on.
Modeled after the Ampeg Scrambler.
A Darlington buffer, then one high-gain transistor stage. A diode network steers the positive and negative halves of the wave to opposite sides of a Darlington differential pair, and a pot blends the result with the clean buffer.
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. Both halves of the wave push the pair’s output the same way, so the output is close to a full-wave rectified copy of the input, its fundamental an octave above the note. Probe T and B to see the two halves split apart, then OUT. Texture sets how far the positive half can move Q3’s base before D4 clamps it. That changes how evenly the halves match, and uneven halves let some of the original pitch back in. The pair runs on under 1 mA through a 1 kΩ load, so the effect side swings only about 0.35 V peak to peak however hard you drive it. With a hot pickup the clean side comes out louder.
Download.
Parts and what they do
Stock values. Tap a part on the schematic to change it.
- C8, 100 pF
- Small cap across the input against radio pickup.
- R1, 4.7 MΩ
- Input pulldown, added in GGG’s version against switching pops.
- C1, 10 nF
- Input coupling cap. Against the 500 kΩ bias divider it sets a corner near 32 Hz.
- R2, 1 MΩ
- Top of Q1’s bias divider.
- R3, 1 MΩ
- Bottom of Q1’s bias divider. The base sits at half the battery.
- Q1a, 2N5306 (half)
- First transistor of the Q1 Darlington, driving the second’s base, so the pair has the product of the two gains and two base-emitter drops, about 1.1 V.
- Q1b, 2N5306 (half)
- Second transistor of the Q1 Darlington. The pair is an emitter follower with a gain of one and an input impedance of many megohms, so the bias divider sets what the guitar sees.
- R4, 4.7 kΩ
- Emitter resistor of the buffer. About 0.7 mA flows through it.
- C3, 1 µF electrolytic
- Couples the buffered clean signal to the blend pot.
- R6, 47 kΩ
- Holds the clean side of the blend at ground for DC.
- C2, 4.7 µF electrolytic
- Couples the buffer to the gain stage.
- R5, 2.2 kΩ
- Input resistor of the gain stage. With R7 it sets the gain, 470 kΩ over 2.2 kΩ, about 210. That’s close to Q2’s full gain with no feedback, so the stage runs near flat out.
- Q2, BC169B
- The gain stage. Common emitter, with its emitter bypassed and its bias set by feedback from collector to base.
- R8, 4.7 kΩ
- Q2 collector load.
- R7, 470 kΩ
- Feedback from collector to base. It sets Q2’s bias and, with R5, its gain.
- R9, 220 Ω
- Q2 emitter resistor.
- C5, 100 µF electrolytic
- Bypasses R9, so the emitter is at ground for signal down to about 7 Hz.
- D1, 1N456
- Runs from emitter to base. It only conducts if the base gets pulled below the emitter, which limits how far the feedback through C6 can turn Q2 off.
- C4, 4.7 µF electrolytic
- Couples Q2’s collector to the rectifier.
- D2, 1N456
- Passes the positive half of the signal to T, Q3’s base.
- D3, 1N456
- Passes the negative half of the signal to B, Q4’s base.
- Texture, 10 kΩ pot, linear taper
- A variable resistor from T back to the midpoint. More resistance lets the positive half swing Q3’s base further before D4 clamps it. Less holds T near the midpoint.
- R11, 8.2 kΩ
- The fixed resistor from B back to the midpoint. It works like the Texture pot, but on the negative half and at a fixed value.
- D4, 1N456
- Clamps T to one diode drop above the midpoint.
- D5, 1N456
- Clamps B to one diode drop below the midpoint.
- R12, 220 kΩ
- Top of the midpoint divider. Both pair bases rest at half the battery, through the Texture pot and R11.
- R13, 220 kΩ
- Bottom of the midpoint divider.
- C6, 4.7 µF electrolytic
- Feeds the midpoint back to Q2’s base. When D2 or D3 conducts, the midpoint moves with the signal and Q2 turns it upside down, so this is negative feedback only while the diodes conduct.
- Q3a, 2N5306 (half)
- First transistor of the Q3 Darlington, one side of the differential pair.
- Q3b, 2N5306 (half)
- Second transistor of the Q3 Darlington. Its collector goes straight to the battery. Q3 just steers current away from Q4.
- Q4a, 2N5306 (half)
- First transistor of the Q4 Darlington, the other side of the pair.
- Q4b, 2N5306 (half)
- Second transistor of the Q4 Darlington. When T rises, Q3 carries the shared current and Q4’s collector rises. When B falls, Q4 lets go of it and the collector rises too, so both halves come out the same way up.
- R15, 4.7 kΩ
- Shared emitter resistor for the pair. About 0.7 mA flows through it at rest, split between the two sides.
- R14, 1 kΩ
- Q4 collector load. At 1 kΩ with under 1 mA the output swings only about 0.35 V peak to peak.
- C7, 1 µF electrolytic
- Output coupling cap from the pair.
- R16, 47 kΩ
- Holds the effect side of the blend at ground for DC.
- Blend, 50 kΩ pot, linear taper
- Blends the clean buffer (down) with the octave side (up). Its wiper is the output. There’s no volume control.
- R18, 4.7 MΩ
- Output pulldown, added in GGG’s version.
About this version
- Original transistors
- Negative ground. Q1, Q3 and Q4 are 2N5306 Darlingtons, each drawn as its two transistors. Q2 is a BC169B. GGG suggests a 2N3904 for Q2 and any Darlington for the others. The transistor models are typical of these parts, not measured.
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.
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