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Old 1st June 2009, 07:05 AM   #1
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Default 7591a Alternatives

I'm laying out the chassis for a 7591a amp using the iron (both power and output) from a Fisher 500B receiver. I have a quad of EH 7591a that I'll be using for initial playing around. Are the JJs worth the trouble of comparison or should I just not bother, given the bad luck people have reported with JJ octals? The EH tubes are the coin base style.
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Old 1st June 2009, 07:36 AM   #2
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Hey Wrenchone,
Stay away from the JJs! Tried 5 pairs for the first run of guitaramps but had to threw them out as they where totally inconsistent. They where also very noisy. EHs work great and are reliable.
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Old 1st June 2009, 07:39 AM   #3
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On the other hand, I've had problems with the EH tubes.
Premature failure several times.
I'm now using the JJ version. They've been in my HH Scott 299C amplifier for over a year.
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Old 1st June 2009, 04:16 PM   #4
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Beware of JJ 7591's ! I usually don't test new tubes but wanted to check these to compare them with genuine N.O.S 7591's. From a lot of 10 new JJ 7591's I found 5 (yes, that's 50%!) with a screen to cathode (G2 to K) dead short right out of the box ! I e-mailed JJ about this problem and their (lack of) quality control but never got any reply. Remember a G2 to K short could (an very likely will) damage your amp. Thus,better to test these tubes for shorts before putting them in your amp. The good ones performs very well (so far) in a FISHER X-101 amp. The EH 7591's are much more reliable but doesn't perform (nor sound) like genuine 7591's, in many circuits they doesn'bias correctly and you'll need to change the biasing circuit resistors to get the correct plate current. Not really a "Plug and Play" substitute... From my experience none of the currently produced 7591's are recommendable and I would stay away from this type for a new amp project ,except of course I you have plenty of N.O.S 7591's at your disposal.
BTW, the 7868 is exactly the same tube with a different (novar) basing,is cheaper, and runs cooler thanks to its all-glass construction.
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Old 1st June 2009, 07:18 PM   #5
pchw is online now pchw  United States
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I had a better luck - only 25% failure (1 out of 4). That was approximately 1.5 years ago. Since then, the replacements have been solidly running inside an Eico st-70.

Sonically, the JJ is on par of the Westinghouse if not better. I also have a quad of EH in the ST-70's little brother st-40. They have worked reliably from day one but not as preferable to my ears as the JJ.

BTW, I also have a Fisher waiting to be restored. Wrenchone, I will be watching your 500B as they are similar enough for me to steal some of your good ideas :-)
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Old 1st June 2009, 07:54 PM   #6
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I agree JJ 7591's are sonically very close to genuine N.O.S 7591's but reliability is still a problem and even a 25% failure rate is not acceptable for a new product. 7591's were pushed very hard in some Fisher amp's with excessive screen dissipation (look through the plate holes and you'll often see the screen grids glowing brightly which lead to premature tube failure). Even original 7591's had a very low life expectancy in the FISHER X-202 and later FISHER lowered plate/screen voltages to prevent repeated tubes failures. Don't ask too much from this "little bottle" (derating,derating...) and select (test) your JJ 7591's before use . Fisher iron is quite good and you'll end up with a very nice sounding amp...
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Old 2nd June 2009, 03:23 AM   #7
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The JJ 7591A is definitely out of the running - I have no use for unintended fireworks or wrangles with vendors regarding tubes being DOA right out of the box. If the company I work for had 25% initial failure rate, we'd be out of business. Our customers get overwrought with failure rates in the PPM range (parts per million).

The 7868 is not a real option, either. NOS tubes are almost as expensive as the 7591As. The only one making a new 7868 is EH, and they use magnoval pins, making it a PITA for socketing. Having said that, though, it appears from reports I've seen around the Web that the EH 7868 is a different beast than their "7591A" and sounds better.

I have pretty much complete freedom regarding drive and bias levels, as the only thing in common with my circuit and that of the Fisher 500B will be the iron. Those pathetic 12AX7s at the input were the first to go.... What I'll probably be using is a hybrid long-tail input/splitter stage using JFETs cascoded with the husky half of a 6CS7. The other triodes of the pair will be pressed into cathode follower service to drive the output tubes, making possible modest excursions into class AB2 without blocking or other mishaps.

I may just give up on the 7591A and use 6P3S-E outputs instead, as I have a lot of them, and I can get a lot more for next to nothing (relatively speaking). One question remains - will these stand up to 400V at the plate and 350 at the screen without antisocial behavior?
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Old 2nd June 2009, 06:05 AM   #8
chrish is offline chrish  Australia
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I am just finalising a 7591 design at the moment. I have a quad of JJs, but have not tested them yet. Due to the reports I am hearing, I am going to put a switch in to be able to change from 7591 to 6L6 and similar (different grid and cathode pinout). I too have a bunch of 6P3S-E tubes to try so will be interested to see if they are up to it. Voltages sound the same as mine : 400v.

Cheers,

Chris
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Old 2nd June 2009, 07:02 AM   #9
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Quote:
Originally posted by wrenchone
...use 6P3S-E outputs instead, as I have a lot of them, and I can get a lot more for next to nothing (relatively speaking). One question remains - will these stand up to 400V at the plate and 350 at the screen without antisocial behavior?
In my experience, the 6p3s-e will be perfectly well behaved at the conditions you described, provided you keep the overall dissipation under 20 watts. I'm running them in a single ended amplifier with about 450 volts plate and UL connections to the screens. They haven't given me any problems yet.

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Old 2nd June 2009, 07:06 AM   #10
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Looks like the same beasts I have, coin bases and all - love the blue glow, an extra incentive to use the tube...
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