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Old 2nd June 2009, 04:15 PM   #11
dtut is offline dtut  United States
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Default 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.
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Old 3rd June 2009, 02:25 AM   #12
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I don't quite understand that. The Fisher 500B uses a lousy 12AX7 for input and concertina splitter duty to drive a pair of 7591As. They have a high gm, so they are fairly sensitive. Inserting a 6SN7 after the 6SL7 as a cathode follower might help.
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Old 3rd June 2009, 02:44 AM   #13
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Or if you just can't fit any more tubes the tiniest of mosfet followers should be able to do it. 7591s are not hard to drive. Don't give up yet.
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Old 3rd June 2009, 03:51 PM   #14
dtut is offline dtut  United States
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Default 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?
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Old 3rd June 2009, 11:58 PM   #15
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Default Re: 7591A alternatives

Quote:
Originally posted by dtut
Is the 6P3S-E maybe similar/same as New Sensor's 7591XYZ?
The visual similarities are undeniable: Sovtek 7591XYZ.

They also bear a startling resemblance to the Sovtek 5881.
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Old 4th June 2009, 12:19 AM   #16
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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....
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Old 4th June 2009, 05:34 AM   #17
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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).
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