2. amps-preamps
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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?
ejman
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
6,744 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.
ejman's avatar
ejman OP
62 posts
 
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?
jedinite24's avatar
jedinite24
1,503 posts
 
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.
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