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Tube won't bias, but works

71.191.43.98
Posted on October 31, 2009 at 12:50:47
bdgregory
Audiophile

Posts: 180
Location: Mid-Atlantic
Joined: July 30, 2005
 Can someone tell me what's going on here? this is a little related to my earlier post.

I'm running a quad of KT88's that have been in use for just under a year, and has maybe 200-300 hours on them. I went to rebias the amp today and one would not bias at all (each tube has a separate bias pot). I was getting a reading of 2ma, and no matter what I did to the bias pot, it would not change. However, there is nothing wrong with how the amp sounds. Before, and after the amp sounds fine. I replaced the KT88's with a quad of EL34s and all bias fine.

Does this make sense? Is it possible that if I reinstall the KT88's it will bias up fine?

thanks (again) in advance!

It may or may not bias again...., posted on October 31, 2009 at 17:00:12
doodlebug
Audiophile

Posts: 1741
Location: Little Rock, Arkansas
Joined: July 27, 2006
 I'd suggest you tag the the offending tube and put it into another position in the amp and see if you get the same symptoms. Odds are that you will. If so, there's your problem.

If this is an older amp, I'd also want to be sure the tube sockets are nice and clean with tight pin receptacles.

Ron O's advice is good, however, but understand that not all tube testers will report a problem. Putting the tube 'under torque', as in full operating parameters, is a good way to know if the tube will stand up to the 'torque'.

Cheers,

David

RE: It may or may not bias again...., posted on November 1, 2009 at 07:26:07
bdgregory
Audiophile

Posts: 180
Location: Mid-Atlantic
Joined: July 30, 2005
 thanks David, the amp is fairly new but the sockets don't have a super tight fit so this was part of my concern when I posted the question. As I said in my other post the tube does indeed test bad (very bad), and I learned something about P-P amp operation in the process.

thanks again to both you and Ron.

RE: Tube won't bias, but works, posted on October 31, 2009 at 13:41:52
Ron Oehlert
Audiophile

Posts: 4649
Joined: December 18, 2003
 No matter how many times you trial, the result will likely remain the same. Push-Pull circuts (as you probably have) can still operate with one tube of the pair failed or even pulled & even tho you did not immediately notice a differance in sound, you should not contiunue to operate the amplifier in this manner. A tube tester can verify condition of that suspect tube before you toss it, but do not use it until such testing. To YOU, with your present amplifier, that tube IS unusable.

RE: Tube won't bias, but works, posted on November 1, 2009 at 07:22:33
bdgregory
Audiophile

Posts: 180
Location: Mid-Atlantic
Joined: July 30, 2005
 thanks Ron, I did test the tube after I posted this question and it's bad. I'm ordering a replacement for it, and in the mean time am running a different set of tubes.

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