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

Posted by Jim McShane (D) on January 18, 2005 at 05:51:45

In Reply to: Re: 7591 JJ Tesla or EH in Scott 299c. posted by smollett on January 17, 2005 at 18:53:43:

"How can the difference in performance between the EH 7591As and the NOS GE/Sylvania/Westinghouse/ and other branded tubes, such as Fisher, Hammond, RCA and Amperex, be explained? Let us put aside the sonic differences that a number of people, including me and my tech, a tube afficionado, heard, all of which favored the NOS. He changed all the resistors so that all would be within Fisher's original design specs. All the capacitors were checked, as well as every diode. He could not find a problem with the amp which would have caused the EH tubes to glow cherry-red."

What exactly is the internal difference in the tube is unknown to me. All I can tell you is what I found. I think it's safe to say the EH tube is built somewhat differently than the NOS, but what exactly those differences are (other than the obvious bottle size) I just don't know.

In the engineering world, a term that is heard occasionally is "an unfavorable combination of tolerances". My guess is the combination of a bit higher than recommended grid resistor combined with tubes that have a bit higher than NOS grid emission resulted in your problem.

I only offered the explanation so you and other readers would know that the tube itself is okay. If all EH 7591As ran away it would have been much more widely reported, wouldn't you think?

In my own experience, I was able to make a small circuit change to accommodate the tube in the case where it acted up. I just wanted to pass that along. My original post was written when there was no JJ 7591, and the NOS supply had dropped to near extinction levels. In was in everybody's best interest (IMHO) to work with the EH tubes, since they were the only game in town. Remember too, Svetlana had been "studying" making them for about 3 years, obviously there was no help coming from them.

I thought I was helping. Now I'm sorry I said anything.

This post is made possible by the generous support of people like you and our sponsors:

Schiit Audio

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