Best NEW 7591[A]s

Shrapnel · Post by **Shrapnel** » Tue 12/16/08 3:17 am

Out of the current production 7591s, what is the best brands? Well, best to good, and what to avoid anyway.

I've decided to go with these for my next project and want input before actually investing in a pair. I already know NOS on these is a pipe-dream, so that leaves this last bit of research before I invest.

jimipage33 · Post by **jimipage33** » Tue 12/16/08 5:41 am

I've been using EH's for a few years now -- I love 'em. Sturdy, tough, warm, and loud. Still haven't tried JJ's yet. Maybe next go 'round...

fzfwyv · Post by **fzfwyv** » Tue 12/16/08 10:38 am

From what I hear the JJ's seem to be fairly close to real 7591's. At least they are sized appropriately but I haven't had an amp in ages to try them in.

Can't remeber the exact brand but I believe you need to stay away from the EH 7591's. I thought I read they were just re-pinned 6L6's.

I've had these sitting here collecting dust (figuratively) thinking I'd build an amp for them someday. They are Raytheon 7591 dated from 1964. Yeah...a bit of spam I know...

but maybe you might be interested

-Rich
rszmagaj@comcast.net

Shrapnel · Post by **Shrapnel** » Tue 12/16/08 4:42 pm

From what I understand, I Not sure if I'd want these: Sovtek 7591XYZ.
They apparently require a cathode resistor modification. Anyone experience these?

Or should I stick to the more traditional 7591A releases... give up on them and go 6L6GC? (second guessing myself now.)

Gearfixer · Post by **Gearfixer** » Tue 12/16/08 6:33 pm

fzfwyv wrote:From what I hear the JJ's seem to be fairly close to real 7591's. At least they are sized appropriately but I haven't had an amp in ages to try them in.

Can't remeber the exact brand but I believe you need to stay away from the EH 7591's. I thought I read they were just re-pinned 6L6's.

I've had these sitting here collecting dust (figuratively) thinking I'd build an amp for them someday. They are Raytheon 7591 dated from 1964. Yeah...a bit of spam I know... but maybe you might be interested

-Rich
rszmagaj@comcast.net

Post by **zaphod_phil** » Tue 12/16/08 6:49 pm

Shrapnel wrote:Or should I stick to the more traditional 7591A releases... give up on them and go 6L6GC? (second guessing myself now.)

7591/7591As are similar to EL84s gain wise and therefore make good substitutes for higher power versions of 18W amps.

jimipage33 · Post by **jimipage33** » Tue 12/16/08 9:50 pm

fzfwyv wrote: Can't remeber the exact brand but I believe you need to stay away from the EH 7591's. I thought I read they were just re-pinned 6L6's.

I dunno, man. I've never heard a 6L6 in cathode bias that sounded like the EH 7591. You might be thinking of the Sovteks. Same manufacturer, different tube...

fzfwyv · Post by **fzfwyv** » Tue 12/16/08 11:32 pm

Yep, you're right. Did some searching and the EH's seem to have quite a favorable response on some of the other audio forums.

Looks like if the tube has "xyz" after it, that would be the re-pinned tube.

-Rich

Shrapnel · Post by **Shrapnel** » Wed 12/17/08 12:15 am

Guess the 7591XYZ is prob the repinned 6L6. the XYZ does require a mod to be used in a normal 7591 socket. Wasn't quite what I was looking for then. LOL.

JJs and EHs, seems to be good choices then.

Thx.

BTW: those NOS 7591s, SWEET! Cost an arm and a leg for 'em on the market, in addition to the first born.

PeterS · Post by **PeterS** » Wed 12/17/08 11:58 am

The tube with xyz in the number is the repinned 6L6 and it came out before there were any other new 7591 on the market. No reason to use those now. I have a pair of the JJs and they sound good but they are somewhat microphonic. NOS is hard to find but pulls may not be bad, 7591 seems to be a very long-lasting tube so buying used ones might be OK.

klingo · Post by **klingo** » Wed 12/17/08 12:27 pm

a while ago, i asked the EH/JJ question on another BBS, a guy from sweden answered:
"Since NOS 7591 are very rare and expensive I have to rely on new production. I tried a batch of JJ and was really disappointed: they didn't deliver the power, drifted in bias after warm up. I went to the trouble of taking full measurements and drawing characteristics - they don't look like original 7591 curves but more like sort of scaled up 6V6GT.
EH 7591 on the other hand perform as specified to the dot. And they deliver both power and punch. I run them at 475-500V plate and pull 45W output power before clipping. (fixed bias)

Yes, 7591 PP cathode bias could interesting, a quick calculation tells me it might be something like EL84 on steroids putting out around 22W in real class A. And pretty early break-up. Need to do some more math on this..."

Shrapnel · Post by **Shrapnel** » Wed 12/17/08 2:37 pm

I've seen the specs posted for the 7591As from the RCA RC-25 manual:

28watts with 450v-p, 400v-g2, 200R-k, 9k p-p load (p-p AB1)

get 45w with the same p & g2 voltage, -21v-g1, 6k6 p-p load.

neither one good as-is for 36w or 18w. BUT it can be done with a little creative biasing and B+ adjustments.

Ok, sounds like the EH-7591As in particular may be the way to go. NOW if I can just nail the real current pull of an AX7. I've heard 10mA, I've seen manuals quote 1.2mA (@250v).

Alexo · Post by **Alexo** » Wed 12/17/08 2:41 pm

Current pull of an AX7? It really depends on it's cathode resistor, plate voltage, etc., but usually it's somewhere around 1 ma per triode.

Shrapnel · Post by **Shrapnel** » Wed 12/17/08 3:10 pm

That sounds better to me... puts the use of a 5U4G or maybe a GZ34 into a more usable choice.

PeterS · Post by **PeterS** » Wed 12/17/08 4:42 pm

It really depends on it's cathode resistor, plate voltage, etc., but usually it's somewhere around 1 ma per triode.

Yes, 10mA might be a maximum or something like that, but in a typical amp circuit 1 mA per triode is a pretty good rule of thumb.

Shrapnel · Post by **Shrapnel** » Wed 12/17/08 5:37 pm

Ok thx on the AX7 currents

Hmm... now 5u4 (1.2) @ 375 = 450 (274BX) OR gz34(1.3) @ 350 = 455 (273BX). 175ma Should do the trick for them, with a little room to spare.
(Whoa is me with these if I get 10mA on all triodes though. -> 250-300mA would be needed then for max output on the 7591A, depending on preamp config)

Thinking of a SLO type front end with perhaps 1 or two more channels (for 2-4 channels ttl) 30-45w out, prob aim for 36w

though.

Post by **zaphod_phil** » Wed 12/17/08 7:34 pm

Sounds like it might be more of a project for PPwatt.com then....

Shrapnel · Post by **Shrapnel** » Wed 12/17/08 7:42 pm

Only if I don't go for the 36watt point, or squeeze it down to the 18watt point. I should probably investigate the details before commenting further then. After all, these should be decent for 36watts.

[now to go do the math and get the data figured out and then post it

]

Shrapnel · Post by **Shrapnel** » Thu 12/18/08 3:13 am

Someone check my math plz.

I believe the 7591A will hit 36w at the following:

Vp=400v
Vg2=350v
Vg1=-17v OR Rk=260R
Zpp= 6k6

IF I am in err, plz show me how you came up with your answer so I can compare.

I'll calculate for 18w later (Vp~300v).

[edit] found data for a 37w output... Vg1 was -16v Vp, Vg2, & Zpp was same.

klingo · Post by **klingo** » Thu 12/18/08 1:38 pm

i think the datasheet ask for a 6K6 Zpp load in fixed bias (Vg1=-17v) and a 9K Zpp load in self bias (Rk=260R)
i haven't done the math but you can adjust the fixed bias or swap the cathode resistor to keep your 7591 happy.
as Zaphod said, with the same amplification factor than EL84, the 7591 seems to be perfect for a bigger 18W.

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Best NEW 7591[A]s

Best NEW 7591[A]s