Amps will not hold bias.

Thanks for asking.
Bias will drift up or down dramatically, for instance if adjusting up to desired reading they will initially hold and then continue to drift upwards subsequently if adjusting down they will initially hold then drift downward.

If you are for sure that your tubes are not bad and the socket which holds these tubes are check and inpected then your answer would be:


There could be two things.

1) Your bias pot are going bad. Clean and do measurement on both channel.  See if you get a reading.  Turning clock wise or counter clock wise will either increase or decrease.  If both has the same reading ( you are good).

Next step. 

2) one of the resistors to ground could be open or bad.