Fuzz pairs

How the pair works

The first transistor of the pair has no bias network of its own. Its base is fed from the second transistor’s emitter through the 100 kΩ feedback resistor, and its collector drives the second transistor’s base directly. If the second transistor pulls more current, its emitter rises, the first transistor turns on harder, its collector falls, and the second transistor is turned back down. The pair settles where those balance, and the second collector voltage is the one number that says where.

The collector voltage. Builders set the second transistor’s collector with a trimmer in place of the 8.2 kΩ (or by changing the first collector’s resistor). About half the battery is the usual starting point: −4.5 V on a germanium PNP fuzz, 4.5 V on a silicon one. Away from the middle, one side of the wave runs out of room first and clips before the other, which adds even harmonics. Players set it by ear from there. The dashed line on the chart is half the battery.

Gain and leakage. On the Fuzz Face the first transistor’s gain moves the bias the most. The second one’s gain matters far less, because the feedback holds its current. Germanium transistors leak: some collector current flows with no base current at all, and it about doubles every 9 °C (16 °F). A little leakage acts like more gain. A lot of it turns the first transistor on hard and shoves the second collector toward the battery. That’s why vintage germanium fuzzes were matched by hand and why they drift on a hot stage.

The guitar volume. The pair’s input impedance is only a few kilohms, so the guitar’s pickup and volume pot are part of the circuit. Turning the guitar down puts resistance in front of a low impedance, which cuts what reaches the first base much more than the level at the amp. That’s the cleanup these fuzzes are known for. Most of it comes between 10 and 7, where an audio-taper pot already has three quarters of its track in series. It works less well after a buffer.

The Tone Bender Mk II. Its Q1 is a booster in front of the pair with only a resistor from its base to ground. It runs on its own leakage times its gain, so it needs a leaky transistor. With a less leaky part it barely conducts.

Warnings. Red flags the second collector within half a volt of the battery or of its emitter at rest, the first transistor saturated, the Tone Bender’s Q1 nearly off or saturated, or the solver not settling.

How this tool works. The circuit is built from the same engine parts as the fuzz on the bench: the battery with 5 Ω inside, each transistor with its base resistance and junction capacitances, the pots on the bench’s tapers, the guitar, the cable and the amp’s 1 MΩ input. The transistors come from the bench’s presets. Gain and leakage are the part editor’s hFE and Iceo, with leakage as a tester reads it at 77 °F. They’re typical of the parts, not measurements. We solve the bias point with the bench’s engine, and each point on the chart is another DC solve. The sine runs 120 ms so the caps settle after clipping moves the bias, then the last four cycles are kept. It starts at 25 mV, about the fundamental of the bench’s played note at its usual 0.2 V pick. That note peaks at 0.2 V, but most of the peak is the pick’s attack, so a 0.2 V sine is much hotter than the note and clips at any guitar volume. For one transistor stage at a time, try the transistor and op-amp stage tool. To play the whole pedal, open it on the bench.

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