These things are supposed to be biased at 30mA per tube from the factory and the bias is adjusted by two pots, one for each pair of tubes, so readings of 60mV per side (teh connector is off a 1 ohm shunt) should be about right. The designer recommends setting in the region 30-35mA.
Take off the back cover, and the bolts from underneath the headcase which bolt the chassis to the case while the amp is lying on it's face. My feet look horrible I know.
Lie the amp headcase on end end carefully and then grab the top transfomer casing, lift the chassis slightly and slide it out of the headcase. Set the case aside and lean the amp against something (like a filing cabinet), we're going to have to connect a speaker cabinet (for safety) and power so keep those wires accessible and not dangling over the tubes which will get hot. I'm using my little Marshall 1x10" cab with an 8 ohm speaker.
This is the bias pots and probe connector on the PCB. Middle pin is ground, the side pins are for each pair of power tubes, along with their associated bias pot.
Turn master volumes to 0. Plug in amp and put it in standby, let tubes warm up for a few minutes and then measure the bias: I measured 60mV (60mA) per side so 30mA per tube - perfect as factory setting. I decided to up it to 35mA. Do this with very small incremental changes to each pot, measuring the bias each time until you get there.
Some dodgy photos of the glowing toobs.
Had a play afterwards and it still sounds great, and I know I'm running at a slightly higher level now - will know more next time i get to crank it at rehearsal.
This method will help you for most PCB probably amp designs but the calculations for the bias voltages will differ with design.
