Tube amp bias question or not?

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.

The KT120s are larger so they have more physical mass (this isn't important really, but I like it), and they were so glowingly reviewed I had to try them. I doubt they add much, if any, output power (any amp has other factors determining output limits), but they do seem to have a certain dynamic quality where all the fundamental aspects of the amp's sound are revealed without seeming to be trying too hard. A wider range of somewhat more accurate tone from tighter bass to clear highs. That make sense? I have stuck the stock 6550s back in (or a set of Sovtek KT88s) just to see if the 120s are still up to par, and the 120s always go back in after a couple of days. This amp, which I rave about ad nasuseum as being a bargain in the tube amp world, seems to sound great with any of these tubes and personal taste could dictate sticking with 6550s anyway, but tube rolling can be educational and fun.

Wolf_garcia I have audiovalve challenger 180 monos and have been thinking of trying the 120's for a while; using 6550s now and have gold aero kt88's in reserve but always have gone back to the 6550s. They seen to have a nicer tone but the kt88's have more punch and control.Thanks for the feedback on the kt120's.

Thanks for the reminder Bombaywalla. I guess I'm really confused now. After turning the amp on for 30 minutes with no preamp signal and then testing I found the left channel tubes at 492 and 505. The right channel tubes at 500 and 512. The tube I was suspecting before is the one on the right channel that came in right at 500 after the 30 minute warm up. I set them all at 500 and will check again in a couple of weeks. Maybe I'm obsessing over this too much...

Jolida gives you 50mv of headroom anyway, and a dozen or so mv drift can be fairly normal.