2. amps-preamps
  3. 1403311208

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
wolf_garcia
6,608 posts
 
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.
rleff
1,065 posts
 
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.
ejman's avatar
ejman OP
62 posts
 
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...
wolf_garcia
6,608 posts
 
Jolida gives you 50mv of headroom anyway, and a dozen or so mv drift can be fairly normal.
jedinite24's avatar
jedinite24
1,503 posts
 
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.
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