Tube amp bias question or not?

Hi EJman

I wish I could tell you. I was very bad with my Jolida JD502P from the get go. I read a lot of reviews about the different KT88/6550 tubes and then decided to go with the Tung Sol KT120 tubes right from the get go. I didn't even bother with the stock tubes. By the time I got the 502P I had the KT120s, the Mullard CV4024s and different pairs of 12AX7 type tubes in hand. I sold the stock tubes to offset the cost.

I say enjoy the stock tubes in your 502P until after you have hit the 3000 hour mark and then start tube rolling.

Wolf_garcia what has the KT120 tubes given you other than increased power vs the 6550's?

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.