JJ 7591?

[Luthierwnc]

Hi All,

Time to get the tubes for an Ampeg Gemini II rebuild. I liked the tone of the JJs a few years back but had terrible luck (meaning QC problems) with current draw. One tube in two of three pairs red-plated within hours of installation in different amps I was working on. One was a cathode-biased Gemini I and the others were fixed-bias with reasonable milliamps.

Does anyone know if JJ has straightened that out in the last few years?

Thank, Skip

[Andy Le Blanc]

dont know about the JJ... EH was good but the pin wire was a smidge too big

[Structo]

I don't normally use JJ's but when I did I always ordered from Bob at Eurotubes.

He sorts them and burns them in plus he warranties them.

Click or more preferably, give them a call and explain your concerns.

www.eurotubes.com

[VacuumVoodoo]

Had serious problems with JJ-7591S last fall, especially in combos. Rattling, pairs drifting apart within hours or just couple days of use. Tubes were not overtaxed, standard operating conditions 350V plates, 300V screen grids, 40mA idling plate current, fixed bias. Screen grids were collapsing causing beautiful display of sparks inside the bottle.
All in all, in the end it was cheaper to buy NOS than going through 5 pairs to match one that was holding up. Even though the dealer replaced them all without any discussion (which tells me he knew that there were problems) it still wasn't worth the frustration of having the customer close to 19th nervous break down....

[Luthierwnc]

Thanks Gents,

I'm figuring even some good-testing pulls would be fine. One of the JJ's flamed-out when I was playing at a local jam. I'd built a D-Lite with those tubes and it sounded great until it didn't make any noise at all.

"Oh, so you're the guy who makes amps?"

Of course, on a dark stage, you don't know what went wrong. Even if I had, I didn't have an oven mitt and a spare set of pre-biased tubes. Fortunately, there were more people playing than at the bar

sh

[Structo]

Skip, Eurotubes only sells JJ's so he may be able to give you the scoop on the problems those tubes are exhibiting.

What do you like about the 7591's compared to a 6L6?

[Luthierwnc]

This is a late 1960's Gemini with 7591s and a 7199 splitter. I'm just putting it back to stock (mostly).

As I understand it, the 7591 was a "modern" tube for the 1950's just before transistors ate their lunch -- sort of like the coolest dinosaur when the meteor hit. In our space, only a few amp makers incorporated them although the hifi guys used them a lot. Coatings, metallurgy, layout; all very high-tech at the time. That might explain why emerging-tech countries have a harder time reproducing them accurately than they do the 1930's-era pentodes. Most of the applications they were designed for are highish voltage. At no-load, the power trannie of the Gemini is 730VCT. The only 6V6s that can handle that are JJs so I'm back to square one. They have about the same Z-out as 6V6s, though.

It is a cool little tube for the right amp. Not much bigger than a 6V6 but almost the power of a 6L6 and pretty good lows. When the market was only NOS and pulls, a lot of Ampeg players converted to more available power tubes. And if you think this is bad, try coming up with a quad of 7027s.

I just got a pair of good Hammond pulls on Ebay for seventy bucks. They should last a long time.

Cheers, Skip

[Masco]

I have a Reverberocket II that I converted from 7591 to 6L6. The only thing I changed was the pinout wiring on the output sockets and the shared cathode R. Actually, I don't miss the 7591's at all, don't think they have any magical properties and they're damn expensive. Your Gemini is probably fixed bias, but you could adjust the bias for 6V6/6L6. Might be worth a try.

[Luthierwnc]

Hi Masco,

The new 7591s are on order so the die is cast. I have several 6L6 amps anyway.

I completely gutted the board and am putting in all new components (except pots). One not-completely-stock mod is adjustable fixed bias in case I was using eastern European glass. They don't always bias-up like the originals.

Another not especially stock hack I'm considering is two 10s instead of one 15. I've already got a tweed Pro that covers a lot of 15 stylings and think this would be a good fit (physically and sonically) for some WGS G10Cs. A nice feature of the Gemini II is a multi-tap OT. Mine measured in at a shade over 6k for an 8 ohm load but there is also a 16 ohm tap that goes to the NFB. A little creativity with the NFB resistor and I can use a wide range of speaker impedances. Decisions, decisions!

Thanks for your comments and good luck with your builds, sh

[skeezbo]

Luthierwnc,
The Gemini II is a cool amp that doesn't seem to have gotten the appreciation that the 1X12" Reverberockets have attained. Converting one to 2X10" sounds like a great idea; I'll bet it will be an awesome blues machine, if that is what you are into.
Since NOS is getting so hard to find, it sure would be good to hear what you think about how the JJ 7591s sound.
Skeez

[Luthierwnc]

I can't do it with this amp since I have the USA glass coming but I do have a pair of JJs in a T-Lite build (search for the schematic on that). It is a Dumble based amp with a 5879 pentode OD and a one-tube reverb. Sounds great. I've got a WGS Invader in the combo.

The plates are seeing around 425VDC fixed-bias. This is the one set of three that has held up quite well. I rechecked the current draw after about 10 hours of heavy wanking and they were almost where I started. These were purchased three years after the red-platers, which was why I wanted some independent verification whether JJ had tightened their tolerances or this was just one of the unacceptably rare good sets.

Getting back to your question, the tubes sound good to my ears. The question has always been whether you can trust them. sh

[10thTx]

I do have a pair of JJs in a T-Lite build (search for the schematic on that). It is a Dumble based amp with a 5879 pentode OD and a one-tube reverb.

Maybe something like this? Not sure what the cathode resistor would be?

With respect, 10thtx

[Luthierwnc]

Hi Jeff,

Pretty close to your amp on Hoffman. Fixed bias and no tone controls on the OD. PAB is just a 180k resistor on the bottom of the Mid control switchable N/C to ground.

I used a Hammond M series chassis and power transformer (great PT's BTW -- perfect for this kind of project). My camera is making everything yellow but this is blonde tolex with a faux alligator top. You can't see it in this shot but the chassis is shock-mounted to help the 5879. I used wooden blocks under the flange of the chassis with heavy 1/2" rubber gaskets. Those are bolted into the side of the box with a little clearance at the top -- an idea stolen from the V-series Ampegs. The bottom piece that attaches to the cab is 1/8 wider than the one that bolts to the chassis so there is an eighth inch of wiggle room to either side. There are actually large holes in the thinner piece for capture nuts to hold it onto the bolts coming through the stationary piece but after I test fit it, that chassis wasn't going anywhere.

BTW, the 5879 PentoPlexi is underway. Iron is on order from Edcor and I spec-ed the chassis for the machine shop last night. With that and this Gemini about 2/3 done and an ODS in the debugging stage I've got a lot of new noisemakers in the pipe.

sh

[10thTx]

That is a nice looking amp! And I sure like your shock mounting idea. Excellent.

Thanks for sharing ! With respect, 10thtx

[Luthierwnc]

sorta gotten off track on the 7591 discussion but the shock-mounting idea for combos seems to make sense. I was a little worried that with the chassis bolts in the side, the lack of rigidity from the chassis on the top would put a lot of stress on the finger joints. That is until I remembered I'd stolen the idea from the Ampeg VT-22 -- the heaviest guitar amp ever made. If you can pick up one of those suckers without cracking the top, this little unit won't break a sweat.

Attached is a pic from another shock-mount idea that could come in handy for pentodes or other vibration-adverse tubes. I've got some old stainless guitar side bending heat shields in the "someday" pile that could probably be made to do something like this with a little snipping and bending. The rubber based ones that are available from old equipment pulls look like they would pass on more wiggle than they absorb. Maybe not, but this looks easy enough to try on spec.

Skip