Two-stage blend fuzz with a germanium second stage and followers, 1968 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 Acoustic 360 fuzz.
An emitter follower, a silicon gain stage and a germanium NPN gain stage after it. Attack blends the two stages’ outputs through small caps, and a second follower drives the output.
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. Q1 and Q4 are followers at each end, with no voltage gain. Q2 is a silicon stage whose feedback works at DC only: C5 grounds the middle of R8 and R7, so for the signal it runs at full gain, about 35 times. Q3, a germanium NPN, idles near saturation with its collector about 0.44 V and clips hard. Attack fades from Q2’s output, through 1 nF, to Q3’s, through 2.5 nF. Both caps are small, so both sides lose bass. The 260’s 100 nF into Q3 changes the output by under 0.3 dB in the model. The model leaves out the 24 V the amp’s followers ran on.
Download.
Parts and what they do
Stock values for the 360, bass amp version. Tap a part on the schematic to change it.
- R1, 22 kΩ
- Input series resistor.
- C1, 1 µF
- Input coupling cap.
- R2, 1 MΩ
- Top of Q1’s base bias.
- C2, 100 pF
- From Q1’s base to the supply. It trims radio pickup and the very top.
- R3, 4.7 MΩ
- Bottom of Q1’s base bias. With R2 the base sits near 7.4 V.
- Q1, 2N3904
- Input follower. No voltage gain. Its input impedance is near 800 kΩ, so it doesn’t load the pickup.
- R4, 100 kΩ
- Q1’s emitter load.
- C3, 1 µF
- Coupling cap out of the follower.
- R5, 22 kΩ
- With R6 a divider that drops the signal to about 0.8 of its level.
- R6, 100 kΩ
- Bottom of the divider.
- C4, 100 nF
- Coupling cap into Q2.
- Q2, 2N5088
- First gain stage, silicon, emitter grounded.
- R9, 22 kΩ
- Q2’s collector load.
- R8, 680 kΩ
- Upper half of Q2’s bias feedback, from its collector.
- R7, 470 kΩ
- Lower half of Q2’s bias feedback, to its base.
- C5, 100 nF
- From the middle of the bias feedback to ground. It keeps the feedback to DC, so the stage runs at its full gain for the signal.
- C6, 150 pF
- Collector to base on Q2. It rolls off the top end.
- C9, 1 µF
- Coupling cap into the germanium stage. 1 µF in the 360, 100 nF in the 260.
- R10, 1 MΩ
- Collector-to-base bias for Q3.
- Q3, 2N1306
- Second gain stage, a germanium NPN, emitter grounded. Most of the fuzz comes from here.
- R12, 100 kΩ
- Upper part of Q3’s collector load.
- C10, 100 nF
- From the middle of Q3’s collector load to ground, so for the signal the load is only R11.
- R11, 47 kΩ
- Lower part of Q3’s collector load.
- C11, 2.5 nF
- Couples Q3’s output to the top of the Attack pot. Against the pot and R13 it cuts the lows.
- R13, 22 kΩ
- From the top of the Attack pot to ground.
- C7, 1 nF
- Couples Q2’s output to the bottom of the Attack pot.
- Attack, 500 kΩ pot, linear taper
- Attack, 500 kΩ linear. Toward the top it picks up the germanium stage, toward the bottom the first stage.
- Gain, 500 kΩ pot, linear taper
- Gain, the output level, 500 kΩ linear.
- C14, 1 µF
- Coupling cap into the output follower.
- R14, 1 MΩ
- Q4’s only base bias, from the supply.
- C15, 100 pF
- From Q4’s base to the supply.
- Q4, 2N5088
- Output follower. It drives the cable and amp from a low impedance.
- R15, 10 kΩ
- Q4’s emitter load.
- C16, 1 µF
- Output coupling cap.
- R16, 100 kΩ
- Output pulldown.
Versions
- 360, bass amp
- Negative ground, 9 V. 1 µF into the germanium stage, as in the 360 bass amp. 2N3904 and 2N5088 stand in for the original MPSA09 and 2N3391.
- 260, guitar amp, modeled after the Acoustic 260 fuzz
- The 260 guitar amp: 100 nF into the germanium stage, which cuts more bass ahead of it. Otherwise the same. In the model it changes the output by under 0.3 dB.
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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