Silicon Fuzz Face Bias

[Drewmeyer]

So I've got a Fuzz Face built with BC108s and I'm looking to check the bias. I don't have much experience with transistor bias and all the information I could find was primarily for germanium Fuzz Faces. So anyways, what do I need to do to check the bias of the transistors and how do I bias them if they are off?

[jrod]

The easiest way to check the bias is to connect your DMM's + or red probe to the collector of Q2 and stick the - or black probe in the screw hole of the enclosure (or to ground somewhere on the board or battery). Turn your meter on voltage and you should get a reading close to 4.5V. If the bias is off and you used a trim pot in your build you can adjust the trim pot until you get the right voltage or it sounds good to you.

Do the same thing if you are checking the bias of Q1 collector, but IIRC, the voltage should be about 1.5V.

If you used stock values you should be pretty close already.

Forum member Gus just started a thread on a different way to bias a Fuzz Face. Also, Mac posted his ideas. Check it out!

Otherwise, the most popular way to adjust the bias is the adjust the resistor to the collectors of the transistors. It's done the same way as the Ge threads you saw, but Ground will be negative (if using NPN Si transistors) as opposed to PNP Ge transistor.

Hope this helps.

EDIT: Q1 Collector should be 1.5V!

[Drewmeyer]

The easiest way to check the bias is to connect your DMM's + or red probe to the collector of Q2 and stick the - or black probe in the screw hole of the enclosure. Turn your meter on voltage and you should get a reading close to 4.5V. If the bias is off and you used a trim pot in your build you can adjust the trim pot until you get the right voltage or it sounds good to you.

Do the same thing if you are checking the bias of Q1 collector, but IIRC, the voltage should be about .5V.

Oh, cool! Sounds easy enough. Only problem, I don't have a bias trim in the build. What can I do now?

[jrod]

Yeah, it's really pretty easy! I would suggest going ahead and checking the voltage and see where you're at. You may not need to make any changes.

Does the pedal not sound good to you as is?

[Drewmeyer]

Awesome, thanks a lot!

[LucifersTrip]

here're complete working voltages from GGG w/ 9V supply.

Q1
Collector
1.4v
Base
0.6v
Emitter
0.0v

Q2
Collector
4.5v
Base
1.4v
Emitter
0.8v

without a trimmer you can still adjust voltages with resistors in parallel across either the 33K or 8.2K or both.

have a blast