The amp has Tungsol 6550 power tubes and being that I am a tube newby I check the bias every couple of weeks or so.after how long after the amp is on do you check the bias voltage?
if you check it sooner than 30-40 minutes after the amp is on, you will get a bogus reading as it takes 30-40 mins for the tubes to settle down (re. the bias/operating condition) before their resp. bias voltages will be stable.
if you are testing the bias voltages after 30-40 mins after the amp is on & you still get a drift then it could be the bias ckt inside the amp OR the tube degrading.
Brf made a good suggestion - move one of the varying bias tubes to another location & see if it still drifts. If it does, it's the tube; if it does not , it's the bias ckt inside the amp.