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?
ejman

Showing 1 response by bombaywalla

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.
More to discover