Build - Pedal Project: Fuzz Face with Bias Trimmer
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 entirely, leaving no continuity between the two halves of that row.
The other two drill holes are not necessary. If you are not using them for mounting the board, they can be ignored.
This is a PNP build and uses battery power only. With no LED indicator, this pedal will last a very long time on battery power. If you would like to build this pedal with a DC jack, please see our PNP "Positive Ground" Pedal Considerations tech article.
If you do not need the bias trimmer and would prefer to use a set resistor, the circuit can be built as seen in Figure 3. The Parts List for this build includes a 10K trimmer, so if you would like to use a set resistor it will need to be provided. If using an 8k2 resistor like the original Arbiter Fuzz Face, we recommend part R-PR02-8D2K.
Last, if you’d like to build the circuit with an external bias control (rather than the internal trimpot control), the circuit can be built as seen in Figure 4. A 10K bias potentiometer and 1k8 series resistor will need to be provided. We recommend parts R-VAM10KL-SS and R-PR02-1D8K. Although it’s still possible, adding a third knob will make it a little trickier to squeeze everything into a 1590N1/125B, so a 1590BB, S90, or YY might be a better enclosure choice in that case. The 1k8 series resistor can be left out, but depending on the transistors used, it is likely to give a better range of controls on the bias pot if it is included. Germanium transistors are sensitive to temperature, and their specifications will change with temperature, varying the bias point and sound of the Fuzz Face. Some germanium transistors are more susceptible to this than others. An external bias control can be useful for compensating for extreme changes if you have a sensitive transistor set or if the build is likely to be used in drastically varying temperatures.
Though it can be a way to compensate for temperature changes, external bias controls are sometimes used even on silicon Fuzz Faces, which will not noticeably change with temperature. A Fuzz Face will sound bad or not work if it is biased poorly. However, there is no one “optimal” bias point for a Fuzz Face. Rather, there is a range where the Fuzz Face will sound nice. Within that range, the sound can vary somewhat due to the way the clipping characteristics and other aspects of the sound change with varying bias. Because of this, an external bias knob can be used for tone-shaping and can be a desirable feature even if the transistors used never need to be rebiased.
See Figure 4 in our Guitar Pedal Footswitch and Jack Wiring article for the recommended footswitch and I/O wiring.
|2u2||1||C-BC-2D2-63||Electrolytic||2u5 in originals. 2u2 used here.|
|10k trimmer||1||R-VT29U-10K||Bias trimmer||8k2 resistor in originals|
|100k||1||R-PR02-100K||Vishay PR02 metal film resistor|
|470R||1||R-PR02-470||Vishay PR02 metal film resistor|
|33k||1||R-PR02-33K||Vishay PR02 metal film resistor|
|1M||1||R-PR02-1M||Vishay PR02 metal film resistor|
|2N1309||2||P-Q2N1309||PNP Germanium transistor||NKT275 used in originals|
|Soft click 3PDT||1||P-H501-L||Footswitch|
|Rigid frame battery snap||1||P-BATC-SN-1S||Battery snap|
|Low profile stereo jack||2||S-H534||1/4" Jacks|
|Adhesive battery cushion||1||S-H165||Battery holder|
|Adhesive standoffs||1||S-H190||Board mounts|
|Adhesive cable tie mount||1||P-HTIEMOUNT-BLK||Battery strain relief|
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