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7591a Alternatives

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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.
 
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.
 
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 :)
 
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...
 
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?
 
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
 
wrenchone said:
...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.

 
7591A alternatives

I've got a pair of EH 7591A s in a pp breadboard that I'm working on now - class A UL e-linear, 300 B+, 10vdc cathode bias, 50ma/tube. They biased up just fine and I very much like the way they sound, but what do I know - according to another recent thread, my 6SL7 ltp won't drive them, so the whole project is a loss.
 
7591A alternatives

My apologies, guys,

My post was inadequately-caffeinated, too-early-in-the-morning snarkiness, but thanks for your responses. As I posted on the other thread, I stole shamelessly from Poinz and put a neg. rail on the 6SL7, which worked just fine. I'd really like to try sand in the tail, but I have no idea at all how to implement that.

If 7591A s were in sexier bottles and relabeled as KT or EL Somethings, there'd be a huge run on them and the price would elevate seriously. I think the EH version compares well to the American ones I originally used. I haven't heard the JJ s and was scared off by the reported reliability problems.

Is the 6P3S-E maybe similar/same as New Sensor's 7591XYZ?
 
They DO look just like the Sovtek 5881WGC which is my favorite Russian 6l6-type for workhorse amps. They have worked consistantly for me in my Bassman for the better part of 10 years. That is to say, they have been consistent sounding, bias up nice, nice matching, etc.
If I'm not mistaken, the Sovtek 7591XYZ requires circut modification...a resistor change ? I once heard they are basically re-pinned 6L6s....:confused:
 
The Sovtek stuff does look very similar to a standard 6P3S-E. Given that Ebay is pretty flooded this tube, I wouldn't be surprised at all if a lot of re-marking is going on. The phrase "all you have to do is change a resistor" should read "all you have to do is redesign the bias and drive circuits". The 7591A was special in that it had a high transconductance. This is a blessing in that it's easy to drive. It's a curse in that the grid is closer to the cathode, making shorts more likely and grid emission more of a problem as well, as the control grid cant help but be heated up by the cathode if it's really close. The classic 6L6 tube requires more negative bias for a given bias current, and more drive level for a given output, making me suspicious that all the replacement parts like the Sovtek XYZ are re-marked surplus tubes.

Fortunately for myself, I'm designing a whole new circuit from scratch. It'd be nice if I could use the 7591A as in the classic Fisher circuit (especially as I have a matched quad in hand), but it's not a big deal if I end up using something else instead.

King Tut (dtut) - if you want to sprinkle some sand on the tail of your differential pair, it's a really easy thing to cob up a ring-of-two current sink using a pair of NPN transistors. This will most likely work well enough to evaluate your diff pair, and it's something that can be thrown together from the junk box in about 5 minutes (10 minutes without coffee).
 
I have used the 6n3c-E to replace the 7591 in a number of older amps and in new designs with out problems. The 6n3c-E in my experience is tougher than the 7591 and I have not seen a failure in over 5 years of daily use of the 6n3c-E in a Heath AA-100 amp that I converted to use the 6n3c-E. The AA-100 runs the plates at 425~450V.

I kept the screens of the 6n3c-E at or below 400V and this seemed to work well.
To go from a 7591 to the 6n3c-E (or any 6L6 type) do have to increase the bias voltage or you will cook the 6n3c-E. Also needed is to modify the drive circuits to provide about 3dB more gain to make up for the lower gain of a 6L6 type tube. If you skip adding more drive gain the bass may be a bit softer and the overall amplifier gain may drop off a small amount. This may be acceptable for you.
 
The 6Π3C-E (6p3s-e) is a 5881 equivalent. 5881 gm = approx. 5.5 mA./V. 7591 gm = 10.2 mA./V. Darned tootin more small signal gain is necessary.

Many vintage designs use the same small signal circuitry with 7591s and "12" W. types. That definitely will not work with 6L6 family types. A definite 6L6/7591 "common denominator" is 6.6 Kohm primary PP O/P "iron".

With a 25 W. design maximum plate, the 5881 is definitely tougher than the 7591, with its 19 W. design maximum plate.
 
I used to buy coin-based Sovtek 5881 from Mike Matthews (New Sensor) starting way back in the late 1980s. Mr. Matthews was selling them out of a large apartment on the Upper East Side of NYC at the time. The tubes I got from him then were identical to coin-based 6P3S-E tubes. They were the same thing.

He also used to sell the non-"E" 6P3S tubes. Groove Tubes used to sell that tube as a 'super' 6V6. That's a quite different tube. Definitely lower plate dissipation.

I and many other guitar amp repairers have used a lot of Sovtek 5881 (6P3S-E) tubes over the years.

In a Dyna ST70 I played with, you could drop in Sovtek 5881 instead of the stock EL34 and they'd bias up at very close to the same idle current (requiring about -32V on the grids). Put in 6L6GC or 6P3S (non-"E") and they would draw much higher idle current, requiring more negative bias (about -40V for Philips-ECG 6L6GC). So the Sovtek 5881 (6P3S-E) isn't the same thing as a 6L6GC, nor is it the same as an older 6L6WGB (aka 5881), or even a 6P3S (non-"E"). It's its own thing.

Maybe the 6P3S-E is the best currently-available replacement 7591? I don't know. Eli's right that it would need more drive voltage at its grids, but not so much as a true 6L6GC or 6P3S (non-"E"). Power output would be lower, due to lower gm.

What about EL34? The drawback might be the increased heater current (1.5A). Other than that, it should be closer.
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