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Re: 7591 JJ Tesla or EH in Scott 299c.

Wow, I can't believe I found this thread!
I've been going thru the same "Hair Pulling" problems that have been described here!. I purchased an Eico ST70 two months ago and loved the sound until I noticed one of the EH7591's going critical on me. It was intermittent and difficult to diagnose. One tube this time, another tube the next. I'd shut down the amp, let it cool, turn it on and it would be good again for days, then a different tube would go critical on me! I then bought a tube tester to check my tubes. They checked OK, so I figured it was the Amp. I purchased all new Caps and replaced them all. I got out the meter and checked the resistor values, nothing out of the ordinary. Now I figured I had the problem licked and started using the amp regularly, until one evening I could smell that sick odour of melting tube!!! Sure enough, one look at the amp confirmed a glowing red EH!
OK, I've had enough, so I ordered a quad of the JJ's figuring this to be THE solution. Got the new tubes and for days I'm in tube heaven. Then one night, that same sick smell!! Like burnt dust or something. Good thing my work desk is close to the amp. Now I'm back to square one. It sounds great as long as I keep my eye on it!

There's got to be a permanent solution. Oh, by the way, I've got the original EICO (sylvania or something) tubes which test borderline. Before my JJ's arrived, I used the old tube for two weeks without one single incident.
I have a hard time believing consumers back in 1962 (when this amp was made) would tolerate a self destructive tube condition.
Anyway, Thanks for reading my gripe.
Darrell

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