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?
128x128ejman

Showing 4 responses by ejman

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?
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?
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 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?