Cayin A50t biasing problem

The pot travel of 3/4ths of a turn is not unusual. Three of my amps do this, including a Cayin. The bias curcuit not functioning at all is a different matter. I don't own a A50t so I don't know but is it possible the bias circuit is seperately fused and a fuse has blown? Is it possible that the previous owner blew out a resistor(s) in the bias circuit that might affect all of the pots? You might take off the lid and see what is there. After its fully discharged of course.

Larry, are you saying that you have a 15 to 20ma reading regardless of the rotation of the bias pot(s)? Or that is the maximum reading available? Is there any difference in the reading between the 4 tubes/locations? You know it is entirely possible that your have some seriously exhausted tubes. Do you know their history? Do you have any spares - you could always sub in a good one and see what happens.

Larry, There is something in the bias circuitry that is common to all four pots. The meter! I'm not thinking it is broken, though it might be. What I'm thinking is that something between it and the pots is defective, for example the switch where you select the pot you want to adjust or, more likely, the connections between the switch and the meter. Just a guess. Maybe you need a techie to take a look.

Larry, I doubt that this is an issue, but are you taking your readings off your meter using the DC ma setting? That would make a big difference in your readings. Other than that I'm baffled right along with you.