Can anyone give a rookie some insight on how to set the bias on this pedal?
I'll start with my cliche advice: 'If it sounds good, it is good!' So, use your ears as the final test.
To start, socket the 2N5457 FETs, as there is a chance you will replace them until you achieve an in-tolerance match.
With no signal and all knobs off to reduce self-noise, I recommend shooting for an average of
-1.5v of Q1 & Q2, accepting up to a 10% tolerance (-1.35v to -1.65) between Pin #2 & Pin #3 (Vgss). If you get a positive voltage, you just have your DMM probes reversed, and absolute values will suffice (no need to panic).
Example: if Q1 measures -1.4v and Q2 -1.6v, you are ready to enjoy the effect.
I would personally follow up with sending a 1Khz sine wave signal into the pedal, and watch for linearity (top and bottom of the wave have equal voltage) and crossover distortion (a 'shelf' or dead spot which occurs in the middle reference point of the wave).
Here is my rationale:
- The trim appears to tuning both 2N5457 FETs which are operating in reverse polarity to each other (so that the compression occurs on both halves of the signal wave). So, any adjustment will be 'best compromise' between the two, and averaging is a good approach.
- The schematic suggests with no signal the voltage measurement between Pin#3 Gate & Pin #2 Source (Vgs) is -1.43v for Q1 and -1.5v for Q2. Looking at the schematic, this looks like what the reverse engineer measured and that the differences are the 2N5457's were close enough, but not perfectly matched. I surmise the original design was shooting for -1.5v, depending on the FETs Wampler was consistently sourcing (it is a memorable target spec a company can communicate to their supplier for acceptance and rejection, or vice versa).
- The more out of tolerance the FETs are to each other, the higher likelihood of crossover distortion (sounds harsh) at lower gain and compression settings. >10% difference between Q1 & Q2, I would audition 2N5457's until I achieved a closer match. The reverse engineer specs is about a 5% difference, which IMHO is great considering today's FETs. 2% is the best I've personally seen.
Hope this helps!