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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?
128x128ejman
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
6,687 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.
rleff
1,065 posts
 
Wolf_garcia what has the KT120 tubes given you other than increased power vs the 6550's?
bombaywalla's avatar
bombaywalla
1,958 posts
 
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.
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