Germanium Darlington fuzz with a tone control, 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.

  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 Sola Sound Tone Bender Mk III.

A germanium Darlington pair boosts the guitar into a third germanium transistor biased by its own leakage, which does the clipping. A Tone pot pans between a full path and a treble path, and a Level pot follows.

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. Q1 and Q2 form a Darlington with its emitter bypassed, so the pair has a lot of gain and a high input impedance. Its collectors drive Q3 through C3 and the Fuzz pot. Q3’s base has no bias network: it runs on its own leakage, so the transistor sets the bias, and a cold or less leaky part leaves it nearly off. Q3 clips hard, and the germanium diode clips the other side of its base swing. Its load returns to a divider, and the Tone pot pans from a highs-only path at the collector to the divider’s full path. R10’s 220 kΩ into the 100 kΩ Level pot drops the output about 10 dB. Probe Q3’s collector to see the clipping, and try the Q3 part in the editor to hear the bias move.

Download.

Parts and what they do

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

C1, 100 nF
Input coupling cap.
R1, 47 kΩ
Lower leg of Q1’s base divider.
R2, 220 kΩ
Upper leg of Q1’s base divider, from the supply.
C2, 220 pF
From the collectors back to Q1’s base. It cuts the pair’s gain at high frequencies.
Q1, OC75
First half of the Darlington pair. Its emitter drives Q2’s base.
R4, 10 kΩ
Q1’s emitter to ground, so Q1 passes some current of its own.
Q2, OC75
Second half of the Darlington pair, collector shared with Q1.
R5, 3.3 kΩ
Q2’s emitter resistor. It sets the pair’s current.
C4, 10 µF electrolytic
Bypasses R5, so the pair’s gain is high above a few hertz.
R3, 10 kΩ
The pair’s collector load. The kit has 1 kΩ and a 50 kΩ trimmer, set to about 10 kΩ in all. The model uses one fixed 10 kΩ.
C3, 220 nF
Coupling cap into the Fuzz pot.
Fuzz, 100 kΩ pot, linear taper
A 100 kΩ linear pot as a variable resistor in series with Q3’s base. Turning it up lowers the resistance, so more of the pair’s signal drives Q3.
D2, 1N270 germanium
Germanium diode from Q3’s base to ground. It conducts on the swing that turns Q3 off, so it clips that side before Q3.
Q3, AC128, high gain
The fuzz stage, emitter to ground. Its base has no bias network, so it runs on its own leakage, and it needs a leaky transistor.
R6, 18 kΩ
Q3’s collector load, to the divider node.
R7, 10 kΩ
Top of Q3’s supply divider. The kit has 1 kΩ and a 50 kΩ trimmer, set to about 10 kΩ. The model uses one fixed 10 kΩ.
R8, 10 kΩ
Bottom of Q3’s supply divider.
C5, 100 nF
From the divider node to the Tone pot’s bass end. The node carries the collector’s whole signal, divided down.
C6, 2.2 nF
From Q3’s collector to the Tone pot’s treble end.
R9, 10 kΩ
From the treble end to ground. With C6 it passes only the highs, above about 7 kHz.
Tone, 100 kΩ pot, linear taper
A 100 kΩ linear pot between the two paths. Clockwise is the full path, so it’s darker, counterclockwise the treble path.
R10, 220 kΩ
Series resistor from the Tone wiper into the Level pot.
Level, 100 kΩ pot, audio taper
Output level, a 100 kΩ audio pot.

About this version

Germanium PNP set
Positive ground, so every voltage reads negative. OC75s for the pair and a high-gain AC128 for Q3, inside the selection advice, not the original parts. The pair’s collectors sit near -4.2 V and Q3’s near -3.9 V, inside the kit’s targets.

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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