Choosing a Transistor

In the Experimental F-Face kit, you can use any small signal NPN transistors. Each model has its own sound in the circuit, but the biggest differences come from the transistor's material (germanium or silicon) and gain factor (hFE).

The first Fuzz pedals were made in an era when only germanium transistors were available, and these are considered the most classic and often the best fuzz pedals. Therefore, if you're looking for the classic 60s fuzz sound, the choice is germanium transistors. On the other hand, with the sound comes a downside: germanium transistors have very large tolerances, and temperature variation greatly affects the transistor's performance (a germanium pedal carried from a cold car to the stage sounds completely different, usually worse, than one at room temperature). Germanium transistors must be measured and selected from a large batch: 35% are discarded due to high leakage current, and only a small portion of the acceptable ones are suitable for the FF circuit in terms of gain factor.

The advantages of silicon transistors are consistent quality, low cost, and temperature independence. Sound-wise, silicon transistors can make equally good (though slightly different) fuzz pedals, as long as the transistors used are chosen correctly. The most important thing is to look for types with low gain. Germanium transistors typically have an hFE of around 40–120, while silicon transistors range from 100–900. Older silicon transistor types are more suitable due to their lower gain. A silicon transistor fuzz often cleans up better when the guitar volume is reduced compared to a similar germanium pedal.

Based on tests and availability, we chose BC107A for the silicon version and AC127 for the germanium version. Both are still reasonably easy to obtain, and the price is still acceptable. BC108A also works well, as do other low-gain transistors. Check the pin configuration on Google if necessary (search: transistor type + pinout or data sheet). Experiment with different transistor types and combinations without prejudice. Let your ear decide which is best!

Below are images to help install the transistors correctly.

BC107 pinout:

AC127 PINOUT (viewed from below):

Soldering Transistors

The kit includes socket strips that can be installed on the circuit board as bases for the transistors. If you want to experiment with different transistors in the pedal, it's definitely worth using the bases. If you have already decided which transistors you will use, you can solder the transistors directly to the board. This makes the connection more secure and long-lasting. When soldering germanium transistors, you must be very careful: germanium components do not tolerate high temperatures very well, so soldering must be done quickly. A metal clip (e.g., an alligator clip, etc.) can be attached to the leg being soldered (on the component side of the board) to conduct heat away. With the right soldering technique, the transistors will not break, even if no additional heat sink is used. Always shorten the transistor legs as much as possible and ensure that the germanium transistor's casing does not cause short circuits with other components.

Transistor Biasing and Trimmer Adjustment Instructions

Assemble the device and check that everything is correct. Turn trimmer P6 fully to the right (i.e., to full), P5 to the middle position, and P4 fully to the left. The side trimmer P3 is turned fully to the right, i.e., to full. Set the Volume potentiometer P2 to halfway and the Fuzz potentiometer almost to full.

Turn on the power (by inserting the plug into the input jack) and measure the voltage at the test point (i.e., from the collector of transistor 2) with a multimeter. Place the red test lead on the test point and the black one, for example, in the screw hole of the casing or another ground point. Adjust trimmer P5 so that you get a 4.5 V voltage between the test point and ground. This biases the second transistor to a usable range. Then connect the effect between the amplifier and the guitar and listen. Adjust the volume and fuzz knobs and test the operation. The trimmer settings now correspond to the component values of the original FF. If the sound is not at all healthy, check the test point voltage again and, if necessary, go through the entire circuit again.

When the sound resembles a traditional Fuzz Face, you can continue testing the trimmer functions. P3 limits the incoming signal in the same way as the guitar's volume knob. Turning P3 lower reduces the maximum distortion amount but changes the sound more towards overdrive. This adjustment often works better for fine-tuning the amount of distortion than the Fuzz knob. Try different positions relative to the Fuzz knob's position.

Trimmer P6 adjusts the resistance between the power supply and the output. Turning P6 to the left increases the device's output volume and changes the character of the distortion. This adjustment is sometimes called the Contour adjustment in FF.

P4 adjusts the value of the first transistor's collector resistor. The effect is less pronounced with silicon transistors than with germanium transistors.

Before attaching the cover, ensure that the test point voltage is 4.5 V. Once the device is sonically familiar and other adjustments are under control, you can try different voltages. The best setting is the one where you get the best sounds.