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

Posted by Jim McShane (D) on January 17, 2005 at 07:58:40

In Reply to: Re: 7591 JJ Tesla or EH in Scott 299c. posted by smollett on January 16, 2005 at 18:13:24:

I only wanted to point out that I've seen the mismatch issue come up from "reputable" vendors too. If you've had a hard time with the tubes then by all means take another approach.

Believe me, the 3 reputable vendors all got their tubes from the same source I do.

If you look through the archives you'll also find a post of mine regarding the value of the grid resistor with the EH 7591s. Here's a section of it:

"Of the 7591A EHs I've sold, I can say I have run into only minor variations in heater current. But I can tell you to pay close attention to the value of the grid resistor. I really think 150K to 200K is the max. based on my experience.

Some older Fisher, Sansui, etc. use (IIRC) 330K as the stock value. Add 30+ years of drift up in value and you can easily have 375K of grid circuit resistance. I have seen this cause the EH tubes to run real hot as a result. A change of the resistors (probably a good idea anyway) is all it takes to fix it. Maybe even a fresh NOS tube would do the same thing, but I've never tried it to see."

That's why you had trouble as well.

I use under 200K for the EHs. That is a difference between them and the old production. FWIW, with fixed bias the RCA tube manual calls for 300K max, which the Scott exceeded with the 330Ks in there. The old tubes would work fine, but the new ones don't.

The fact is that eventually the NOS supply will be totally gone, and at some point the new stuff will be all there is. So working with the new tubes is important IMHO.

I'm sorry you had trouble, but I can promise you the tubes work very well, and would have worked very well for you too if one resistor value was changed. I wish you'd had a better experience, I hate the kind of hassle you had to endure!

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