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BudP 5th March 2008 07:55 AM

Eratic tube activity
Hi all,

I have an AudioPrism four EL 34 amplifier. We built the OPT's for a lot of these amps and I consider it one of the best sounding and most reliable designs available. Not because I supplied the transformers either!

Today, one channel went down. The 12AT7 driver tube heaters were lit for that side, but not the EL 34 heaters. So, I switched EL 34's side to side. The other side tubes worked fine in the new places and the original tubes, now on the other side, worked fine too. For a few minutes.

No distortion, no ultra bright glowing, no unusual noises, but that tube set that had quit before, quit again. Same strange circumstances.

Any suggestions about what is failing? I would hate to spend the money on new tubes, since these sound great while playing, but I haven't the skill or experience to figure out what has gone south here.



OzMikeH 5th March 2008 11:06 AM

I'm probably stating the obvious here...
I'm far from an expert.

Your problem might be undersized pins or corrosion products on the pins and or sockets. I've also had some very bizarre things happen where dissimilar metals connect, especially when you add a heap of hot/cold cycles and some humidity. It costs nothing to give them a good clean with some solvent then a light abrasion.

woodturner-fran 5th March 2008 11:38 AM

You need to have a look inside. See if you still have B+ on the affected side and then check if you have heaters on the affected supply. That will at least rule out one or the other. Report back after this.


Ps. if you are not comfortable with the high voltages and the dangers inherent, get a pro to have a look at it instead. **Discalimer over**

pedroskova 5th March 2008 11:55 AM


Thermal expansion is shorting out a heater of one of the tubes, and the heaters on each side are wired in series. Swap one tube from each side...if it stays, the bad tube is the one you didn't swap. If the behavior moves back to the other side, the swapped tube is the culprit.

BudP 6th March 2008 09:09 PM

Thanks guy's

Pedroskova had it in one.


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