Posted by Jamie Tyson

June 03, 2012 - 01:17 pm

I'm happy to see the updates to this article. Again I'll say thanks for putting the time and effort into this testing. I don't use 7591's for hifi applications but I believe my theory of modern tubes applies to both HiFi and guitar amp use- it's helpful to view new tubes as something to be designed around. I'm sure this is frustrating for owners of classic equipment. The advantage of this thinking is that one can view a design with an open mind and appreciate new production 7591's for what they are- usable tonal choices. I've used and abused a pair of EH 7591's for about 5 years of weekly gigging. They've started to sound ratty but at no point in the last 5 years would I say they're bad sounding tubes. I'm happy to see that the transconductance numbers are a closer match for vintage tubes, of course. Again, thanks for the info!

Posted by Steve L.

January 04, 2012 - 05:08 pm

Hi Mike, Thank you for the kind words. We have considered offering a tube tester product. However, the market is small and there are one or two products out there which may do a decent job of it. There is no great secret to doing proper measurements on tubes. The problem is that all but a few of the testers from back in the day, used very crude test methods. For example, DC supplies were not filtered at all and grid AC levels were too high for meaningful results. Also, power tubes were tested at very low power levels. We just applied good engineering measurement practices to get accurate results. I guess that we hoped in the beginning, that we would find that classic tube tester results might correlate reasonably with the accurate measurements. Alas, that turned out not to be the case. Hence, we felt it was important to get some information (as crude as it is) out about the 7591's. Of course, the old adage about the actual operating equipment being the best tube tester, still applies. The measurements we present of actual amplifier performance, demonstrate that.

Posted by Mike

January 04, 2012 - 04:29 pm

Your data is MUCH appreciated simply because there is next to nothing on the 7591 tubes. If your tester engineering is so much superior to Hickok's why aren't you selling it? I've always understood a testers "GM" to be relevant and taken with a grain of salt. The 752a I own does fine for me.

Posted by Steve L.

October 25, 2011 - 12:47 pm

Hi Mo, Looking closely at EH's pictures of the 7868 and the 7591, it does indeed appear that they are different. In any case, our article is focused on tubes with the 7591 pinout and the 7868 would, of course, require rewiring. Nevertheless, I appreciate your bringing up the fact that EH uses a different structure in the 7868.

Posted by Mo

October 25, 2011 - 11:40 am

Steve- when you hold the EH7868 side by side with the EH7591 you can physically see that they are a different size internally. Unfortunately I can't attach a photo.

Posted by Steve L.

October 21, 2011 - 08:46 pm

Hi Mo, The original RCA 7868 was just the 7591 with a different pinout. Looking at the EH version of the 7868, there is no apparent difference in the internals, versus their 7591. What makes you think that the plate structure is different?

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.