JJ-7591 Special Considerations?

[Tiago]

Hello,
I have an amp (2x6L6) in a rack and now I have to reduce one rack space in it. The only way is to do it is to change the power tubes to a smaller ones. I used 6L6 but now I want a higher transconductance octal tube and smaller so I decided to use the 7591 (B+ is 390Volt). I know that are some pinout differences.
But there are any other special considerations to take?
BTW, if someone can make a review of this tube I would appreciate it.

Thanks,
Tiago.

[Alex/Tubewonder]

Besides rewiring for different pin configuration you'll have to modify bias circuit to give you around -20+/-5V adjustment range.
This will also lower headroom of the power section a bit and you will notice earlier onset of compression and break up.

As for JJ version of the 7591: it's ok for amps in the 20-30W output power range so in your amp with 390V B+ it should work fine. The quality of these particular JJs is a bit questionable, at least that is my experience after going through about 20 pairs: some had idle current drift away by more than 30% after a few hours. Also, in my particular application with 450V plate voltage at full power output I only got about 35W, never 45W as with "genuine" NOS 7591.

I have measured JJs on a tube curve tracer and the curves look more like a slightly modified 6V6 (yes, higher gm). EH 7591 measure, perform and deliver power and sound as my NOS reference Westinghouse 7591.

Also, if this is of any value: I met with tech guys from JJ factory this March and made them aware of the problem - they were not overly interested to put it mildly and I haven't heard from them since.

This makes me think your amp might do well with 6V6 tubes in it unless it's the high gm you're after.

[Tiago]

Thank you very much for your reply. Very precise!

Well what gets me to want higher gm is that the phase inverter starts to break up before the power stage, so I'm thinking if I put some higher gm power tubes I might get more power.
I'm only getting +/- 15WRms with the current setup (2x6L6, fixed bias).
There is any other 7591 currently manufactured besides the EH (too tall) and the JJs (30% idle drift is weird) that you have tried?

Again, thank you very much,
Tiago.

[Rob Mercure]

Tiago,

I'm not sure what factors you're considering when looking for a higher transconductance tube - with power output tubes this is a bit more complicated than simply selecting triodes with different mu (and that's a bit more complicated than "tube rolling" mythology would make it). In any case what power output does your amp produce/claim and how much of that power do you use? The 7591 is only rated for around 19 W plate dissipation while the 6L6GC's usually good for at least 32W. But, if you're using your output tubes "modestly," and if you can find 'em "real" 5881s -6L6GWBs - pack a lot of dissipation capability in a small envelope and while rated at 23W can actually dissipate a bit more (and whatever output tube you're using in a rack mount you probably should have a dedicated fan to keep the air circulating). And the "whatever" tubes marketed as "5881/6L6GCs" (can't remember the cyrillic designation right now) are also a bit smaller than a "robust" GC.

Rob

[Tiago]

Hi Rob,
Thanks for your answer.
The amp does not claim to produce any power because I builded it from scratch. Made the schematic, pcb and got it working. The power stage, has a 390Volt B+ and the output transformer is wired for 8K plate-to-plate, I can also wire it for 4K, but the power drops a bit.
The power transformer is a 280VAC-200ma one, so I get +/- 140ma at 390VDC out of it.
I'm using the tube "modestly"...At some time ago, I used the same Xformers but for another board with a pair of EL84´s and they worked ok. I didn't do the tests that I wish I did with that board and now I dont have that board.
The point is, I got it sounding ok with 12watt tubes so tube power dissipation isn't a problem here. Even 6V6 could be used, altough, it appears that some of those 6L6xx are in some cases smaler than some 6V6's...
But my main problem with that kind of tubes, is that they have a "low" transconductance.
But, do you think that even I put in some higher Gm tubes I won't get more power?

[Snapcase]

Thanks a lot for your review.

As for JJ version of the 7591: it's ok for amps in the 20-30W output power range so in your amp with 390V B+ it should work fine. The quality of these particular JJs is a bit questionable, at least that is my experience after going through about 20 pairs: some had idle current drift away by more than 30% after a few hours. Also, in my particular application with 450V plate voltage at full power output I only got about 35W, never 45W as with "genuine" NOS 7591.

Now I understand why My Gemini I amp with JJs sound way weaker than a friend's '64 Fender Deluxe or my Soldano Atomic, both supposed to be 20 watters as well. It's hard to believe it's giving anything more that 15W. In fact compared with a Peavey Classic 20 (15W with two EL84) sounds about the same volume.

What about the EHs? Are NOS 7591 the only way to go with these Ampegs?

[Alex/Tubewonder]

Thanks a lot for your review.
Now I understand why My Gemini I amp with JJs sound way weaker than a friend's '64 Fender Deluxe or my Soldano Atomic, both supposed to be 20 watters as well. It's hard to believe it's giving anything more that 15W. In fact compared with a Peavey Classic 20 (15W with two EL84) sounds about the same volume.

What about the EHs? Are NOS 7591 the only way to go with these Ampegs?

EH are bigger/higher physically than JJ and NOS so if that's not an issue go for it.

[Snapcase]

I think there's room enough even for the huge 6L6GA bottles I plan to use.

Anyone has experience with EHs performance?

[Alex/Tubewonder]

Yes, my amps are fitted with EH7591 as standard. No problems, punch in the solar plexus sound.

[Snapcase]

Thanks. I'll give a try.

[Rob Mercure]

Snapcase,

There is very little difference in sound level between 15 and 20 watts - the human ear responds to sound pressure in a logarithmic manner, not a linear one, and the "rule of thumb" is that you have to increase the electrical power by about a factor of 10 to perceive a doubling of sound level. To put it another way: with the same speakers it takes about a 50 watt amp to sound twice as loud as a 5 watt Champ! So what you're probably perceiving is a difference in speakers, layout, etc. Feed your Gemini through your friend's Deluxe, or your Soldano, speakers before you condemn the amplifier - otherwise you're comparing apples and lichee nuts! And it's far easier to gain an apparent louder sound by adding external speakers or more efficient ones than by increasing sound level. I've been doing this a while and I couldn't judge the difference between a 20W and a 15W amp by simply listening - there are lots of 15W amps that sound "louder" than some 30W amps!

Rob

[Enzo]

Or the other way, doubling the power only yields a 3db increase in volume. A 30watt output is only 3db louder than a 15 watt output. That is output, not amplifier. A 15 watt amplifier can easily sound louder than a 30 watt amplifier.

[Snapcase]

Yes, guys. You're right. But the real thing, in a real scenario, (onstage), the Gemini I cannot keep up with the Deluxe or Atomic performance levels. With those two amps I can play along with a loud drummer without a problem. But not against my band mate with a Sovtek Miget 50, an Ampeg B25, an Ampeg V2 or a Hiwatt DR103 (tamed to 50w). I need my B25 or my Soldano SL60 in that band. But for my solo project, I want lower powered amps. 20 watters are just perfect but though I love the tone, the Gemini I falls short against a drummer. Do you think modern 7591s are guilty for that? Would things improve with NOS tubes all the rest beign equal? Any other idea? I need a little more volume from my Gemini.

[Alex/Tubewonder]

Perception of volume when comparing amps with output power roughly in same league is more dependent on midrange/treble voicing than just power. Also, speaker efficiency plays a major role. The amps you're mentioning are pretty different from Gemini in that respect.

[Gitarman]

7591's

Hello,
Did you ever get the pin configuration for a 7591?
I have an old Ampeg amp i'd like to restore back, from 6L6's to 7591's.
thanks for the help.
-mike