From MetroAmp Wiki
Bias pots come in many forms. Most often in the 10-50k range.
Marshalls typically use a slide pot on early amps (where there is an adjustable pot at all) and later a PC board mounted vertical bias pot.
Fenders more often used a chassis mounted type of pot with a recessed, screw driver adjusted shaft. These were often wired as balance controls as opposed to bias voltage adjustments.
Vertical mount Marshall type:
Horizontal mount Marshall type:
