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
6,606 posts
 
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.
jedinite24's avatar
jedinite24
1,503 posts
 
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.
ejman's avatar
ejman OP
62 posts
 
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?
wolf_garcia
6,606 posts
 
My KT120s have been in use for at least 3000 hours and are still going strong (over 2 years).
jedinite24's avatar
jedinite24
1,503 posts
 
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.
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