A silicon fuzz that finally behaves at low battery voltage
Most silicon fuzzes are tuned at 9.0V and then left to fend for themselves. The Coalpit is trimmed the other way round: Marrow set the second-stage bias so that collector voltage tracks the supply, and the pedal keeps its character down to about 6.4V before the sputter turns into a gate.
1 · Circuit & teardown
Two BC239C stages, no buffer, a 68 kΩ input impedance that will interact with whatever sits in front of it — which is the point, and which is documented on the box rather than hidden. The bias trimmer is accessible without desoldering, a courtesy that costs nothing and almost nobody offers.
2 · On the bench
Measured at unity output into a 1 MΩ load, 1 kHz sine, fresh alkaline cell and then again on a bench supply walked down in 0.5V steps.
| Supply | Q2 collector | Output at max | Character |
|---|---|---|---|
| 9.0 V | 4.6 V | +11 dB | Tight, full sustain |
| 8.0 V | 4.1 V | +10 dB | Slightly softer attack |
| 7.0 V | 3.5 V | +8 dB | Sag audible, still musical |
| 6.4 V | 3.1 V | +6 dB | Sputter on note release |
| 6.0 V | 2.8 V | +3 dB | Gates — the useful floor |
Sag is a feature here, not a fault: the failure mode arrives late, and it arrives gradually.
3 · In the room
Recorded through a 5W valve amp, one mic, no post EQ. Clean-up on the guitar volume is real — at 6 the pedal turns into a thick overdrive rather than a thinner fuzz, which is the behaviour you want from the 68 kΩ input.
4 · Against the alternatives
Against the two obvious rivals in the database, the Coalpit gives up a little raw output for a far wider usable bias window. If you play at one setting forever, that trade buys you nothing. If you swap batteries mid-set, it buys you the set.
5 · Verdict
A well-documented, serviceable, deliberately voltage-tolerant fuzz at a fair price. The only real complaint is the LED, which is bright enough to read by.
