10-13-2015, 05:43 PM
I am wanting to add a variable bias pot to my V4 and have some questions. The existing fixed bias is (ideally) -28.5V from a voltage divider consisting of 39K and 22K resistors. Wanting to use a Fender-style 10K bias pot, I am planning on switching the resistors to 33K and 15K respectively, with the 10K bias pot in series between them. I am assuming this will get the adjustment range about where it needs to be while staying pretty close to the current draw of the original fixed circuit. Is my thinking correct? Next question, I've noticed in my Mesa power amp that the fixed bias resistor is a blue metal oxide explosion-proof resistor, the only one in the amp. What is the reason for this? Last question, the only tidy spot I can see to mount the bias pot in the V4 chassis is fairly close to the hum balance pot on the back panel. Since both pots have a grounded metal can, can I assume this will shield the bias pot from any AC noise from the hum pot? I also intend to twist the wiring from the PC board to the bias pot to reject any induced noise in these wires. Am I on the right track here? Thanks all.