Tube amp bias question or not?

I purchased a Jolida 502P amp as an experiment back in January for use with my Magneplanars (1.7's) and it has been absolutely wonderful. Love the sound and much to my surprise my very powerful SS amp (CJ) is not seeing much use these days.

Now to my question. 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. Three of the tubes have held rock steady bias - never varies - 500 mv all the time. One of the tubes tends to very slowly drift down to maybe 495 mv, then I tweek it up to 500mv and then maybe a month latter it's back down to 495 or so. What causes this? is it the tube? is it normal, is it anything to worry about?
Dont worry about it. 495 - 500 is close enough to be considered "matched" for biased purpose. If you want to mark sure that it is the tube and not the circuit, move the tubes around and see if the lower bias tube follows the new position.
Thanks. I'll just keep an eye on it and see what it does. If it is in fact the tube, is there some bias reading differential that would indicate the tube may cause some mischief?
As long as the tube can't hold steady bias there is no worry. When you see a tube that cannot hold bias it is time to swap out the tube. Bias levels will fluctuate with the power grid. Your amp is running fine, enjoy!
I bought a new (factory upgraded) JD502P a couple of years ago and a Radio Shack bias meter just to keep the Jolida LEDs honest, and they have been. Swapped out the 6550s for a quad of KT120s and kept the bias at 500mv, and they've been steady for over 2 years with minor drift mostly due to when I test them (typical drifty house AC)...well within spec. Running a set of great sounding Psvane 12AX7 T-IIs with NOS Mullard CV4024s (12AT7WA). I could get by with just the Jolida LEDs for bias but then the hair on my knuckles would grow back from not using the meter to burn it off.
Ha ha. Very true wolf. With the Tung Sol KT120s in place on the Jolida JD 502P I too would singe my knuckles when I would check/adjust the bias of each tube. I'm now GE 5751s along with the Mullard CV4024s. I like the lesser gain of the 5751s.
Thanks guys. Very pleased with the JD502P. I figure at about 6 hours/day, I'm probably up to about 1000 hours on these output tubes (tungsol 6550) probably time to start thinking about replacements? Although the manual says they should last on average 3000 hours?
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.
My KT120s have been in use for at least 3000 hours and are still going strong (over 2 years).
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.
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...
Hi Ejman

You may be going a bit overboard with the bias. For me I let my Jolida 502P stay on for about 45 mins to an hour. I then set the bias for each tube to around 550mv. This way depending on the voltage spikes at your place you will always be above 500MV and not have to worry about the bias going under 500. I didn't like seeing those red lights flicker. Also after I set the bias for each tube when I'm done I check them all again just in case. After that I put the cage on and check the bias again in a couple of months.
Jolida gives you 50mv of headroom anyway, and a dozen or so mv drift can be fairly normal.
Hi Jedinite, Yes you are right about my going overboard on this bias business. But I guess I'll keep gnawing this bone a bit longer... Why is it important for the bias not to drop below 500? Wolf in the post above yours (and the Jolida manual I just checked) say that 500 +- 50 mv is within range. But you are right, the LED's come on at exactly 500 so when listening to music they do flicker.

Did you notice any difference in sound when you biased higher to the 550 level?
Hi Ejman

For me I didn't like seeing the red lights flicker a lot on the 502P. When I was striving for the exact 500MV there wre times I saw the lights really flicker a lot. That and I was under the impression for the amp to sound the best the power tubes should be biased at 500mv or a little above.

To your question about differences in sound when the tubes were biased above 550MV. I didn't notice any differences but I didn't have my power tubes biased above 550MV for very long. I had the 502P on for over and hour and played a couple of songs. Then checked the bias and it was 700MV on the tubes.

How this happened was I was power tube rolling between NOS 6550 tubes and Tung Sol KT120 tubes. I thought I had dialed down the bias enough before making the changes. I make the switch and then check the bias later and it was very very high.

So please be careful when you are power tube rolling and don't make the mistakes I did.

Lastly when I called Jolida to discuss the 502P they told me if you wanted to really make audible sound differences with the amp was to roll 12AX7 tubes.
The "blue glow" in my veteran 120s is this finally a sign of impending tube death? They still sound fine...hmm...
Wolf I read this on the web it might give you some idea of what your are seeing:
The blue glow is back (I have read about it someplace)...noticed it last night, so I assume it merely went on vacation for a while.