Posted by Mo

October 21, 2011 - 05:08 pm

Too bad you didn't include the EH-7868 in your test. These have a different plate structure than the EH-7591's. I use these in my Fisher 400 and they sound better to me than the EH-7591 that I'm running in a few other amps.

Posted by Steve L.

October 18, 2011 - 08:19 pm

Hi Butter, Thank you for the kind post and for the DIY fix on making the JJ pins fit. Yes, I imagine that acoustic feedback really would degrade sound quality. Sorry that the one Sylvania developed microphonics. However, you might find a hifi amp owner that would want the pair. Microphonics would be far less of a concern in that kind of application.

Posted by Butter

October 18, 2011 - 05:25 pm

I had one bad sylvania 7591 in a pair and it was causing microphonic feedback in my Audio Guild guitar amp. I tried to change out the Sylvanias for a set of JJ 7591S but could not figure why they would not go into sockets. That turned out to be non-tapered pins. After a session with moto-tool, I got them tapered and they fit. You are probably right about lower wattage produced by the JJ's but, happily, the sound quality improvement is awesome. This is the best amp I've ever played through. Judging by your figures it is now about 30 watts compared to about 45 w/ Sylvania tubes. I don't need 45 watts so I'm happy. Just hope the JJs last a fair amount of time. If not I may buy a pair of NOS Sylvanias (if I can afford them--the amp only cost me $200. Thanks for the info. Very useful.

Posted by Steve L.

October 07, 2011 - 07:27 pm

Hi Richard, Thank you so much for your message. It is indeed true, that the new manufacture 7591s cannot equal the originals. Your mention of the 6GM5 is interesting. You can see one which I installed in an octal base, in the picture at the top of the page (on the left). It did perform well in the Eico ST-40 amp, as expected. I intend to write up the rebasing, eventually. However, the rebasing was a Pyrrhic victory of sorts, because it took a lot of work (surprisingly). I join you in hoping that some tube maker will be able to produce a true 7591, someday. Perhaps, in shedding light on the situation, this webpage can help bring that about, in some small way.

Posted by Richard Sherman

October 07, 2011 - 05:15 pm

Thanks for taking the time to analyze the current production 7591 tubes. I have been consistently disappointed by these tubes and your results indicate they are at best a close approximation to the real thing. I survive with 6GM5's and watch $400 quads of NOS Sylvanias on Ebay go by with no bidders. We are paying for devices that like a bouquet of flowers, will wither with time and fail after hours of use. The proof is almost always in how a tube performs in a real amplifier versus a tube tester. Your findings with the EH 7591A are the same, the tube does not have the smooth sound of a real 7591A. The JJ tubes in my amps just turned cherry red and one lost it's vacuum after 10 hours of use. All in all to date, no one has been able to equal the real 7591A. I am waiting for that moment to arrive.

Posted by Steve L.

September 19, 2011 - 09:41 am

Hi Rob, I really appreciate your kind words. ---Steve