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    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

300B tube going dead after a few months

The 300B tube on (one of) my Cary 300B amplifiers goes dead after a few months' listening. There's no sign of burning (tube glows red), just eventually losing sound. It usually startsn when I notice what I think is a "phasing" problem from my speakers. Then one day, when I turn on my system, one side has no sound coming out of it (always on the same amp). Before I run out of money trying to replace 300B tubes on a regular basis (and the tube is not cheap), I thought I'd post it here to see if anyone out there is having the same problem, and if someone can offer some clue as to what is wrong with the amp. Thank you.

nl393a
 

ashok

Member
2002-06-06 4:43 am
3RS
Did you check the dc bias ( quiescent ) current on that defective channel ? Maybe it's running too 'hot ' and burned out due to that.
You have a good channel to compare by but I'm sure someone on the forum can tell you what it's been generally set at ........75mA ?
Depends on plate voltage also I guess.
 
You didn t mentioned which brand of 300b is in the case, but witihout measuring it on tube tester it s hard to tel what is the source of the problem. If you changed the tubes from one channel to other, and same tube isn t working, the tube is to blame. I had problem with sovtek 2a3 which I bought from one english distributor, one tube had about 80% lower sound from another, but tube tester (old hickok) measured that tube is OK. After I shortly rised voltages of filament and anode tube started to work well. I suppose that "shocking" of tube probably burned some obstacles on filament, and tube started with full emission. But "shocking" of tube is dangerous for its health, and I suggest to do it when you have nothing to loose...
 
Thanks to Ashok and Robot for the quick responses. I regularly tune the bias (on both monoblocks), so I am relatively positive that its not the bias.

As for brands, I have used sovteks, svetlanas, JJs, and EHs, and they all behave the same - one amp's 300B is fine, the "bad" amp's 300B tube goes dead.

I have also read in some old electronic book in how to "revive" a tube by burning its filaments with high current. I may give it a try. Thank you.
 

kevinkr

Administrator
Paid Member
nl393a said:
Thanks to Ashok and Robot for the quick responses. I regularly tune the bias (on both monoblocks), so I am relatively positive that its not the bias.

As for brands, I have used sovteks, svetlanas, JJs, and EHs, and they all behave the same - one amp's 300B is fine, the "bad" amp's 300B tube goes dead.

I have also read in some old electronic book in how to "revive" a tube by burning its filaments with high current. I may give it a try. Thank you.

Have you talked to Cary about this? Something is obviously wrong with that amplifier.. I've had the same pair of JJ 300B in my amplifier for several years now running at 80mA and no problems!
 
Hi Kevin,

Yes, I have talked to someone at Cary (I think his name is also Kevin), and he could not tell me what causes it (I have my JJs running at 82 mA, and the tube in the good amp is always fine. So I think something is causing the filaments to go dead - maybe from under heating - I don't know). Thank you.

BTW, I have looked at your 26 preamp and decided to make a 2A3 SE amp with the 26 as driver tube. What do you think?

David
 

Rod Coleman

Member
Paid Member
2004-07-25 6:55 pm
UK
had something like this with JJ 300Bs

worked OK two years, then anode current went dead.

Looked further & found the valve came back to life if the filament voltage was taken to 4.85V or above, below that, nothing.

If you're on ac-heat, as I was then, the line-voltage variation will take you too far below that required voltage with filaments that are like this. If the Cary is dc heated, measure the dc voltage across the two big pins of the 300B. It really needs to be as near to 5.0V as can be, and not less the 4.9V.

Keep those old 300Bs, though, since I solved that problem with some DIY:

http://www.diyaudio.com/forums/showthread.php?s=&threadid=38248&perpage=30&highlight=&pagenumber=2

The 'failed' JJ has been working again for the last four years, with no problems...

and current source heating is SO much better than ac OR dc with voltage regulation.
 
Hi Rod,

It's nice to know that someone else has had the same problem (that I'm not losing my mind!). Thank you, Rod.

I will check the filament voltage and proceed with your method (good thing I saved all those "dead" tubes). Thank you.

And, once again, my sincere thanks to Ashok, Robot and Kevin.

David