Util DialAFuzz

util-DialAFuzz

A plug-in NPN transistor with a dial for hFE. Fuzz circuits are picky about transistor gain, and modern Si transistors are usually too hot. This board plugs into a TO-92 socket in place of the transistor and lets you trim the effective hFE of an ordinary transistor down from its native ~300 to wherever the circuit sounds right, without hunting for a rare low-gain part.

How it works

Q2 is the transistor the pedal sees, wired straight across the header. Q1 is diode-connected in parallel with Q2's base-emitter junction, with the trimmer in its emitter leg, so it steals a settable share of the base current (a Widlar split). Effective hFE rises roughly linearly with the trimmer resistance, and scales with the stage's bias current, so the same setting reads higher in a 1 mA stage than in a 50 µA one. Trim it in circuit.

LEAK is an optional 2.2M collector-base resistor (leave it off unless wanted). It injects a small base current that the transistor multiplies, which mimics the collector leakage of a germanium part, useful when running silicon in a circuit designed around leaky germanium.

Header order is E, B, C. That matches 2N3904 / 2N5088 style sockets directly; for a BC-series (C, B, E) socket, plug it in reversed.

Example: Fuzz Face

PedalPCB's SiliSmile Fuzz is a silicon NPN Fuzz Face with socketed transistors, a good host.

Drop it into the Q1 socket with a BC847 (hFE ~300) and the trimmer covers roughly hFE 9 at minimum to 240 at maximum. The classic germanium Q1 range of 70 to 100 lands around 200K to 300K. Set it the same way you would bias any Fuzz Face: adjust for Q2's collector near 4.5 V, or by ear. Nothing is at risk at either end of the travel; at the bottom the stage just biases wrong.

A companion board, util-BleedAFuzz, does the same job with a single transistor and a base-emitter bleed, which suits collector-feedback fuzzes better.

Gerbers

GERBER-util-DialAFuzz.zip is the JLCPCB-ready gerber and drill set; no KiCad install needed to order the board. Two SOT-23 BC847s (or whatever ~matched transistors you have), one 0805 resistor (optional), a 3-pin header and a through-hole 1M trimmer on the back.

License

Licensed under the CERN Open Hardware Licence Version 2 – Permissive (CERN-OHL-P v2). See LICENSE and NOTICE.

You may make, modify, and sell boards from these files, with no obligation to publish your changes.