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-   -   6GT5 distortion problem (http://www.diyaudio.com/forums/tubes-valves/30193-6gt5-distortion-problem.html)

imix500 14th March 2004 03:08 AM

6GT5 distortion problem
 
Hi, I have a precision electronics amp with 4 6GT5 outputs that has been dormant for a year. After plugging it in I find that I can only get low volume that's mostly distortion. It appears that the B+ rail is being taxed heavily as the 0A2 regulator dims when signal is applied. All caps were replaced 1 year ago. Any ideas? Thanks!

imix500 16th March 2004 02:52 AM

Also, the unit seems to be drawing several solids Amps from the AC line when input signal is applied. Could the output stage be saturating way early? Thanks!

fdegrove 16th March 2004 02:59 AM

Hi,

Quote:

All caps were replaced 1 year ago. Any ideas?
Did you reform the caps when you put the unit back into service?

It sounds as if you have a considerable amount of leakage current somewhere along the PS which could be a dead cap.

Cheers,;)

imix500 16th March 2004 03:15 AM

Frank, when I recaped the amp nearly 2 years ago actually, I didn't know about cap reforming and just checked polarity and powered it back up when I was done. The amp worked well for about a year and it has sat in a closet for about a year. Could one of the caps died that quickly? Does reforming them increase thier life when put into service? Wouldn't a bad filter cap also come with some 60 cycle hum? Thanks!
Ricky

fdegrove 16th March 2004 03:23 AM

Hi,

Quote:

Could one of the caps died that quickly? Does reforming them increase thier life when put into service? Wouldn't a bad filter cap also come with some 60 cycle hum? Thanks!
It's certainly not impossible. Electrolytics need reforming after a long period of inactivity.
60 Hz or actually 120Hz hum is one indicator of a faulty cap but since you're using a regulated supply I suspect it rejects quite alot of PS hum.

Mind you, I could well be wrong in my analysis of the problem.
It just seems a good place to start looking...

Cheers,;)

imix500 16th March 2004 05:58 AM

It's a good starting point to check the caps then, hadn't suspected them. Thanks Frank.

imix500 19th April 2004 06:20 AM

Ok, found the problem. There was a direct short to ground in the output connector, so the distortion was the transformer saturating and possibly the output tubes as well. It's the simple things like those that are frustrating. My question now is, could I have damaged the outputs by driving them into a dead short?- through a transformer of course. They never glowed red and niether transfromer got warm during testing. It sounds ok but I won't know for sure until I put it through it's paces. Hoping I just got lucky this time. Thanks,
Ricky

dhaen 19th April 2004 06:28 AM

Quote:

could I have damaged the outputs by driving them into a dead short?
I doubt it. Not unless the anodes (plates) ran red for any length of time.
According to your description of the symptoms, the power supply sag limited dissapation.


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