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EH 7591 Opinions?

EH makes the proper size 7591 but it uses the 7868 base, physically they are different tubes inside and out. I've always wondered why they wouldn't give the EH 7868 a 7591 base.

I have EH7591's in my 340B, I like the way they fit, they are a bit more prominant but there is plenty of room for them in my 340B and I don't use a case. They have been dependable.
 
Thanks Larry,
I put the cover for my 1000a under the bench 30 years ago, I don't think anyone should leave the cover on without fans. I to have added a fan to cool the power transformer, power resistors, OPT, OPT area. I bought my 1000a in 1972 from a friend that bought it at a PX in Vietnam.
 
Destructor said:
EH makes the proper size 7591 but it uses the 7868 base, physically they are different tubes inside and out. I've always wondered why they wouldn't give the EH 7868 a 7591 base.

I have EH7591's in my 340B, I like the way they fit, they are a bit more prominant but there is plenty of room for them in my 340B and I don't use a case. They have been dependable.
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The 7591 is an OCTAL base, and the 7868 is a NOVAR 9 pin. Actually what you are thinking about is a 7591 with a Octal "COIN BASE". If you put a 7868 in an Octal base, you'd have to redo all the sockets on the unit that takes 7868's. Plus having to re-wire the base as the '91's and 68's aren't compatable pin for pin.

Larry
 
larryderouin said:
The 7591 is an OCTAL base, and the 7868 is a NOVAR 9 pin. Actually what you are thinking about is a 7591 with a Octal "COIN BASE". If you put a 7868 in an Octal base, you'd have to redo all the sockets on the unit that takes 7868's. Plus having to re-wire the base as the '91's and 68's aren't compatable pin for pin.

Larry
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No, he's wondering why EH doesn't release the 7868 tube (bulb and plates) with an octal base... as a "true" 7591.

Chances are, that the 7868 plates just aren't quite heavy enough to run under the conditions of 7591s (higher voltage)... that's probably why this never happened...

Regards,
Gordon.
 
Were they not interchangeable at one time other than basing, when the 7591 was out of production people converted amps to run NOS 7868's by changing sockets. If it so happens that most amps run a 7591 harder I could see that as a reason if the EH7868 is not quite as tough as an EH7591. I have EH7868's in my Fisher 400, they sound good and have been dependable.
 
Running them down at 25mA is just too low (unless it's a new production tube).

Using a fan to move the air across the tubes and trannies will generally keep the unit MUCH cooler, without having to shunt the current draw at the output tubes.

Depending on voltages/current at the plate and screen, you could run strong testing vintage 7591's up at 42mA under the right conditions. Just remember not to exceed the plate disappation wattage. I've run mine from 32mA to 44mA.

Obviously when you're running them up at the design max numbers their lifespan will be significantly reduced. Right now each of mine are set at 34mA@ idle.

. . Falcon
 
Mine are New production EH7591A from Jim McShane. 24-25 was all I could get out of the originals. basically they are shot. I put the new tubes in and balanced them out @ 25-26. I'm happy with the sound. I'm a bit leery of running them hotter right now. Think I'll keep an ey on them for a while and see. Might go up to 30-32 to see what's what.

Larry
 
Since the last post over 4 years ago......The tubes have been used in the SANSUI 1000A @ bias settings of up to 35ma @ 475V plate voltage(16.625W), and in a FISHER 800-C 34ma@430V(15.05w). I pulled them last weekend and they were less than 5% difference than original readings back in 2009. Both on an EICO 625 (emissions only) and a philco 9100(hickok 933) gm. I've also tested them on a Hickok 6000a and they showed somewhat weak, but Dave Gillespie's testing( http://www.tronola.com/html/7591a_tubes.html ....ADDENDUM's 1 & 2) actually shows that HICKOK testers will show 7591 gm lower than actual. So I'm not worried there. I've gotten two more sets of them for my 800-C's, one is in a console ('63 FISHER EXECUTIVE VIII) so it holds a bit more heat. TORTURE TESTING @ 34ma(430V plate) for 96 hours straight inside the console, did not produce any untoward distortion, imbalance, or precipitous drops in performance. They tested the same after the test as before.

Been hearing guys wanting to get hold of the NEW Tung-Sol "ReIssue" tube, lining up like smokers after a transcontinental flight. I'll wait until the Jury has released it's findings (namely McShane and Gillespie). Testing on a TV7 or a Mitey mite is one thing, testing them under real world conditions or worse is another. If they stand up to the abuse Jim and or Dave can dish out on their test rigs, I'll get a quad and see what they sound like. But for now I like the EH's (unless they are too big, then I'll drop back 10 yards and punt)
 
Plus the EHs have a blue halo if you're into that cool looking kind of thing.

The trick with EHs is getting some that bias up tightly without using an individual bias tool. It's difficult. They tend to vary, even when bought as matched pairs from a reputable seller, but they do not drift with grid bias runaway once set up properly. If I'm going to use them, I always use an individual bias setup such as IBAM (documented in the Fisher forum on this website).
 
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audiodon said:
Plus the EHs have a blue halo if you're into that cool looking kind of thing.

The trick with EHs is getting some that bias up tightly without using an individual bias tool. It's difficult. They tend to vary, even when bought as matched pairs from a reputable seller, but they do not drift with grid bias runaway once set up properly. If I'm going to use them, I always use an individual bias setup such as IBAM (documented in the Fisher forum on this website).
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Blue halo? Are you referring to them glowing blue like a 6L6GC, or the color of the glass? So far as I know only JJ makes tubes in blue bottles.
 
I know the EH's sounded better in my Sansui 1000A than the original Toshiba's and by a goodly margin.
 
I've used them in several MC225s, a couple 500c's, a sansui 1000, and a scott unit. All with good luck. I agree on individual bias adjustments per tube where you can but on those that didn't have that feature, I've used good matched quads with good success. Not expensive, sound good, haven't had reliability issues, why not...

Mark
 
Yeah, a blue glow. It's a cool sight and rare in 7591s. It makes sense that a repinned, repurposed 6L6GC turned into a 7591 would have a blue glow.
 
Postscript on this old thread . . .

I bought several 7591EHs for various gear including a couple of Eico ST-70s. One of the ST-70s got regular use -- the other, not so much.

After perhaps 2 years (I'm uncertain of the exact time), out of 8 tubes, two lacked emissivity and two were plain bad. Both the much used and the lesser-used amps had bad tubes. Very disappointing.
 
Dave C said:
Larry,
How do those EH's fit next to the Sansui transformers? I would think the larger size would create heat issues in that unit. I also have a 1000a and skipped over the EH tubes because of size. Maybe I should reconsider them.
Thanks,
Dave
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Not ALL the "transformer heat" comes from FROM the transformers!, some is absorbed from the power resistors\OP tubes because their surfaces are black in color.

I would get some aluminum (the REAL metal, NOT just the fabric tape!) duct tape, remove the output tubes and clean the transformer sides and put a strip on the transformers where they face the OP tubes and the topside power resistors (putting two computer fans at the back of the case works wonders too, don't forget the 1/4 inch holes in the shield at the back of the transformers.

Mark T. :music:
 
BuzzK said:
Postscript on this old thread . . .

I bought several 7591EHs for various gear including a couple of Eico ST-70s. One of the ST-70s got regular use -- the other, not so much.

After perhaps 2 years (I'm uncertain of the exact time), out of 8 tubes, two lacked emissivity and two were plain bad. Both the much used and the lesser-used amps had bad tubes. Very disappointing.
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Um... The Stereo 70 does not use the 7591. It uses EL34s. I'm guessing that you might be thinking of 7199s? If so, that's a totally different discussion.

Edit - My mistake. I was thinking of the Dynaco Stereo 70
 
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I bought a quad of the new Tung-Sol ReIssue 7591's from Jim McShane shortly after my last long post above, and installed them in the 1000A. Bias'ed them to 34ma with 470v on the plates (15.98W). I had replaced the power resistors with Dale surface mount high wattage resistors, and moved them to the front of the unit on the tuner dial cover plate. Also had cut down the baffle plate behind the #1 and 3 tubes for airflow. The EH's sounded real good on it, but the Tung-Sols sound a lot better. More Low end, smoother mids and high's. Heat is no longer a problem with regard to the transformers. After 1 hour with the top ON the POWER transformer hits 134* with the outputs hitting 140-144*. With the EH's and the power resistors in the original positions, temps wre hitting 160*, which is well below max temp of 190-195 for the transformers. So the Tung-Sols, with the power resistors moved, has dropped the temps noticibly with a decidedly pleasant increase in sound quality due to the Tung-Sols. But they are 1/2 again more expensive than the EH's. Biased conservatively they should last 8-10 years.
 
Larry-I've yet to get my hands on the TS 7591,but I have used the EH's with good results,although I don't feel it's a ''real'' 7591. As you have both,I'm curious to know if the internals appear to be different between them.
 
Art; The internals on the the T-S are very close if not totally true to the original 7591. The EH's with the Larger bottle, are internally larger, they look more like a 6L6 than a 7591.
 
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