Newbie with tube problems (long)

Tubes Asylum

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I have a problem that I - as a tube novice - cannot understand, therefore I turn to you, the tube experts at the AudioAsylum for help.
There is something strange happening with my Edison 60 amp (6922/EL34 in PP ultralinear mode), I will try to explain the whole chain of events (forgive me if I include stuff which is not relevant for the "investigation"):
After about 50hrs of use a hum appeared at the end of a listening session (4hrs). I noticed it after the CD I was listening to ended so it was not loud enough to disturb the music while it was playing. The hum was only in the left speaker. I immideately turned the amp off but switched it on again after a minute or two: the hum was still there. Changing the 2 left output tubes over to the right side moved also the hum over to the right side.
From this I deduced that a tube must be bad (strange after only 50hrs of use) and ordered a new one.
Changing first one of the tubes in the pair I suspected to be faulty lowered the hum but it was still there, the same happened when switching for the other in the pair. Hmm, could they both be bad?
I again went to my local store and got a new EL34 and low and behold, now the hum was gone!
Happy with this (the sound of the Edison 60 is fantastic!) I could again listen with pleasure. Until a few days after the tube replacement when after some hours' listening the dreaded hum appeared again. The new tubes were now in the left channel and again it was the left channel humming. Looking at the amp I found that the right tube in the left pair was not glowing. Wiggling the tube (amp off) made it glow again and with the return of the glow the hum disappeared. This continued to happen several times, always after a longer period of listening, and would go away with a slight wiggle of the tube. At this point I had the idea that perhaps the amp had to work too hard with my speakers connected to the 8Ohm tap on the amp, the speakers are supposedly 4Ohm 90-91dB. So I moved them over to the 4Ohm tap. Now I might be imagining things but it felt like I could listen longer before the hum appeared and the tube stopped glowing.However after a few more nights of listening the tube said good-night for good!
I have now put the speakers on the 2Ohm tap and bought another new tube and the system sings again. But I am not confident that the problem is cured (what would be the reasoning behind that?) just by connecting the speakers to another tap on the output transformer.
So dear inmates, please help a tube newbie: what is wrong here?
Can I have made a mistake when assembling the unit? Bad soldering? Overheating a component? All SMC's were soldered by a professional and the pcb's were tested so should be ok.
Thanking you in advance for an answer.
mikke

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Topic - Newbie with tube problems (long) - mikke 11:38:39 10/10/01 (4)

Re: Newbie with tube problems (long) - Chris Spafford 15:51:49 10/10/01 (1)

Re: Newbie with tube problems (long) - mikke 23:04:01 10/11/01 (0)

Re: Newbie with tube problems (long) - steven oda 12:38:48 10/10/01 (1)

Re: Newbie with tube problems (long) - mikke 22:46:48 10/11/01 (0)

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