View Full Version : 7591 info?
Leonc
02-02-2009, 06:18 PM
I'm trying to figure out what the gm (mutual conductance) of a new 7591 should be. I have a blurry reprint of the charts for the Hickok 500 series tube tester and it looks like "New" is gm of 8000. Can that be right? My Hickok's (533a) chart doesn't include the 7591 and I've got two sets of new JJs that are only coming up to 5000 :(.
I'm trying them in an amp I don't really know...so I can't tell by ear if the tubes are really performing that well.
Trout
02-02-2009, 06:48 PM
I'm trying to figure out what the gm (mutual conductance) of a new 7591 should be. I have a blurry reprint of the charts for the Hickok 500 series tube tester and it looks like "New" is gm of 8000. Can that be right? My Hickok's (533a) chart doesn't include the 7591 and I've got two sets of new JJs that are only coming up to 5000 :(.
I'm trying them in an amp I don't really know...so I can't tell by ear if the tubes are really performing that well.
Be careful with the JJ 7591's, all the info/posts I have read the past 2 years note them as being failure prone. Most guys reported them failing within a few(under 10) hours.
My tester chart shows a GM of 10,500 Microhms, I had several NOS Westinghouse branded 7591A's that exceeded that spec. Not the same tester as yours though.
YMMV
Leonc
02-02-2009, 08:48 PM
Yeah true enough - I bought a pair for my old Gibson Invader a year or so ago and they failed within the first 3 or 4 hours or so. Pffft. But I got another set from the seller and they sounded fantastic and lasted very well. Inconsistency seems to be one of the problems with them.
10,000 gM you say?!! :eek:
VacuumVoodoo
02-02-2009, 11:29 PM
Original TungSol data sheet is here:
http://www.mif.pg.gda.pl/homepages/frank/sheets/127/7/7591.pdf
it's incomplete as pages with characteristic curves are missing.
The gm figure of 10200 is a typical value at specific operating conditions, see datasheet.
My experiences with JJs were not encouraging. NEver got full specified output power out of them in a circuit designed specifically for 7591. Direct contact with JJ at Frankfurt Musikmesse was, mildly put, less than satisfactory.
Now I'm using EH 7591, they are physically larger/taller then originals but electrically spot on specs. No failures so far, some post burn-in idle current drift but nothing like JJ. The amp sounds killer too ...
gldtp99
02-03-2009, 12:54 AM
I have a new matched pair of JJ 7591's waiting for a 35-ish watt build--- considering a vintage Orange circuit at the @35 watt level---- maybe i should have bought the EH's instead....................gldtp99
Trout
02-03-2009, 06:28 AM
Original TungSol data sheet is here:
http://www.mif.pg.gda.pl/homepages/frank/sheets/127/7/7591.pdf
it's incomplete as pages with characteristic curves are missing.
The gm figure of 10200 is a typical value at specific operating conditions, see datasheet.
My experiences with JJs were not encouraging. NEver got full specified output power out of them in a circuit designed specifically for 7591. Direct contact with JJ at Frankfurt Musikmesse was, mildly put, less than satisfactory.
Now I'm using EH 7591, they are physically larger/taller then originals but electrically spot on specs. No failures so far, some post burn-in idle current drift but nothing like JJ. The amp sounds killer too ...
In a lot of ways that does not surprise me, It seems on a lot of JJ's tubes, the original data sheets were thrown right out the window.
Try and find a current production 12AX7 that actually produces a gain factor of 100:dunno
It seems like they get them close based more on price point rather than traditional spec. Cheap is nice, but often self defeating.
Blue Strat
02-03-2009, 06:45 AM
Inconsistency seems to be one of the problems with them.
Inconsistency in the quality, or use, of the ones you bought maybe. 7591s are no less consistent than any other tube.
Trout
02-03-2009, 08:01 AM
Inconsistency in the quality, or use, of the ones you bought maybe. 7591s are no less consistent than any other tube.
That is a VERY broad statement,
In reference to JJ 7591's,
I agree, they are no less consistent because they are consistently BAD therefore they are very consistent.
A couple bad batches? possible, since demand for that tube is fairly low, a couple medium size batches will circulate for a heck of a long time.
Ronsonic
02-03-2009, 10:41 AM
The 7591 was introduced toward the end of the tube era and benefited from the technology available in the late 50s early 60s when it was designed. As always the desiderata were sufficient power, small footprint and ease of powering and driving. They used technology that was well beyond what was available for the tubes before it. The EL34 was a previous attempt in the same direction. 7591 was even smaller and easier to drive to full power.
When tube production shifted to former communist countries about all we saw was 30s to early 50s technology, 2A3, "5881" and the like. A true 7591 was pretty well out of the question. They're starting to get better but this is still a bit out of reach for a lot of these plants. I've had good functional results with the EH version of the 7591 but haven't looked hard enough at it to get proper measurements.
Leonc
02-03-2009, 10:47 AM
Very interesting info. I bought these from theTubeStore, who also carries the EHs. I've requested an exchange+$ for the EHs. Hopefully, they'll honor my request.
Leonc
02-03-2009, 02:52 PM
Ah, well at least TTS is being smart about this. They're taking the tubes back for full refund.
Leonc
02-03-2009, 02:54 PM
Inconsistency in the quality, or use, of the ones you bought maybe. 7591s are no less consistent than any other tube.Not tracking here Mike. What's inconsistent about use of 7591s?
Blue Strat
02-03-2009, 03:29 PM
Yeah true enough - I bought a pair for my old Gibson Invader a year or so ago and they failed within the first 3 or 4 hours or so. Pffft. But I got another set from the seller and they sounded fantastic and lasted very well. Inconsistency seems to be one of the problems with them.
10,000 gM you say?!! :eek:
Inconsistency in the quality, or use, of the ones you bought maybe. 7591s are no less consistent than any other tube.
Not tracking here Mike. What's inconsistent about use of 7591s?
Huh? ;) Oh, by "use" I meant, how "used" the ones you got were. Not sure if you were using new production or "NOS".
i've had 1 pair of eh 7591 start to show a very slight short in 1 position on my b&k 600 tester and these were burned in/screened and tested tubes from a big name midwest vendor.
this was after maybe 150-200 hours of use,,,,not encouraging imo...
Trout
02-03-2009, 05:10 PM
i've had 1 pair of eh 7591 start to show a very slight short in 1 position on my b&k 600 tester and these were burned in/screened and tested tubes from a big name midwest vendor.
this was after maybe 150-200 hours of use,,,,not encouraging imo...
Being a less popular tube compared to EL84's, EL34's, 6L6's and so on, it seems these manufactures just sort of go through the motions on QC and Development.
IMO, the best bet is to go with NOS from a known vendor that takes the time to properly test, burn in and grade his tubes, NOS, or ANOS.
(See member signatures above)
Sometimes ANOS are actually a better way to go in many cases. Save a few bucks, and get a tube with a known history of service reliability.
This might be a better route especially in vintage gear where a failed tube can produce disastrous results taking out power or output transformers that are near impossible to get.
the eh 7591 i bought were burned in and tested and i tested them in my b&k 600 when i received them with no shorts showing up, so i installed them and they actually sounded pretty good in my 67 ampeg gemini II (for 150 or so hours)..
i didnt have any problems, but for some reason i decided to pull them and retest and lo and behold up came a short after i warmed the tube up good in the amp and then put them back into the b&k 600..
no way am i reinstalling them in the gemini !!
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