Fuzz Face with Bias Trimmer

From the post

The Arbiter Fuzz Face is probably the most iconic guitar pedal in history, and for good reason. With just a handful of components, a well-built Fuzz Face can deliver gorgeous tones from a subtle edge-of-breakup sound to full-on "Hendrix at Woodstock" dirt. At the heart of the original Fuzz Face circuit are two germanium transistors, which are mainly responsible for the inconsistent nature of germanium Fuzz Faces. Though Fuzz Faces can sound fantastic, they can also sound terrible and anything in-between. Arbiter built the germanium Fuzz Face with set resistor and capacitor values. The 8k2 resistor was always 8k2, the 100k resistor was always 100k, etc. While component tolerance can contribute to some inconsistency, it is a minor difference compared to the varying specs of germanium transistors. Because the germanium transistor specs in Fuzz Faces varied substantially while the resistors stayed the same, the bias point of the transistors could vary drastically among different Fuzz Face builds, resulting in varying (and sometimes undesirable) sound. One of the ways this issue is mitigated in modern builds is by using a trim pot in place of one or more of the resistors. Dunlop uses this method in some current production versions of the Fuzz Face. In this build, the 8k2 resistor normally found coming off of the collector of Q2 is replaced with a 10K trim pot. Using a trim pot here allows you to change the bias point of Q2, which allows for a wider range of usable transistors. This is not the only location a bias trimmer can be used, but it is one of the most common. Transistor sockets are not included in this build, but if you would like to have the option of swapping out transistors, P-SGP-001 pin sockets can be press-fit into the holes on the vintage-style stripboard used in this build. Note that the included stripboard should be cut down to 14 columns to fit in the enclosure. If you would prefer to not cut the board, you can use a larger enclosure to fit the full board inside and only use part of it. The full board will fit well in a 1590BB, S90, or YY enclosure. The trim pot should be used to set the bias point of the transistors. Some builders use a multimeter and aim for a voltage around -4.5V on the collector of Q2, which is centered between the power rails. This is likely to result in a pleasant sound, but it is not essential for a great-sounding Fuzz Face. Feel free to experiment and trust your ears when setting the bias point. The Fuzz Face will most likely sound poor and may temporarily stop producing sound entirely if the voltage on Q2 is near the rails (0V or -9V), but it doesn’t hurt anything to experiment with extreme settings. The bias point varied among vintage units. The below layout diagrams have three drill holes marked which can be used for mounting holes. One hole also functions as a trace cut, cutting the second row at column 7. This cut is necessary for the circuit to work properly. It is important that the trace is cut entire