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Suggestions for replacing a quad of 7591 power tubes?

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pabs911

Well-Known Member
I have no experience with 7591s other than the old weak quad that are in my Fisher X101C (which still sound decent BTW). Any suggestions as to what are good sounding new issue tubes or are vintage tubes the only way to go? Thanks.
:thmbsp:
 
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If you can find GOOD vintage 7591's on Ebay they will set you back $50 or more ea. I'm picky about the used ones I bid on. Some lower end tube testers will show a good tube even though it's shot and won't work too well in your Fisher. since the first of the year I've bought three NOS 7591's from AES (tubesandmore.com) I work about 8 miles from them. I call every three days or so to see if they have received more. If so, I blast over after work and pick up the tubes. The ones I bought this year are nice but over $50 ea. with tax.

Stay away from JJ 7591's. Do a search on this forum and you will see what I mean!

I bought a matched quad of Electro Harmonix 7591's from Jim McShane a year ago for my X-101-B, they are excellent tubes. Jim's matching for the quad was dead on when I put them in my amp. Mine have quite a few hours on them and they are fine.

Also search for the grid resistor mod that will have to be done on your Fisher to run any of the new production tubes. It's a must do or the new production tubes won't hold up.
 
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Foster Blair suggested the EH over the JJ's to me some time back, though as I remember they were too tall to fit in my LK-72 so I went with NOS (at a significant prememium). Are they producing tubes with the smaller envelope now?
 
drknstrmyknight said:
Foster Blair suggested the EH over the JJ's to me some time back, though as I remember they were too tall to fit in my LK-72 so I went with NOS (at a significant prememium). Are they producing tubes with the smaller envelope now?
Click to expand...

In fact, they fit if you do not have a case. There's however no space between them and the OPT's (I do not know if it could damage the tubes or reduce their heat dissipation).

I saw at least another picture on the web of an LK-72A fitted with EH's.

In my case, I returned back to my JJ's today to see if I could eliminate the little channel misbalance I had on my LK-72A:

The Daystrom's were more "matched" (31 ma on one side and 33ma on the other). The JJ's shows 35ma on one side and 30ma on the other... but they sound much better than my (probably) weak NOS. Channel misbalance seems to have disappeared too.

I'll try the EH's I keep as backup one day, just to compare (and see if the match is better).
 
EHs only fit in an integrated Fisher when you don't put the cabinet on. Probably the same with a Scott. I've got a 299D and I won't use EHs in it because the case won't fit. I won't use them in my Fisher X-200 either for the same reason.

That auction place is a funny thing. Sometimes you can get a deal and other times you can't on 7591s. But I'd suggest that if you have a tester, test yours. 7591s are funny tubes. They can look shot, like all the getter's gone or it's mostly brown and looks like it's dead but it can still perform fine. I've got one NOS one that went out on me after 10 hours with an infant mortality problem. Gm of less than a thousand, yet it looks brand new.

It appears you're testing across a 10 ohm cathode resistor. That's good and an accurate way to tell how they are. A 5 ma range between a pair is Ok from my perspective. There's been conjecture about whether you should have two tight pairs or spread them out across pairs. I prefer a tight pair. Don't confuse current dissipation with weakness in the performance area though.

I don't suggest EHs for your integrateds if you ever plan to put it in a cabinet. JJs get too many bad reviews. That leaves you with used ones.
I've collected a dozen spares over the last few years.
Don
 
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The Electro-Harmonix 7591s barely fit in the cabinet for my 800C. They are a great tube though, sound great in my 800C. You will need the mods though, Fishers run the 7591s hard when left stock. I have well over 1000 hours on my EH 7591s.
 
I am very close to pulling the trigger on the EH's for my 500-C, which came with two original Fisher's. I have no idea where the others went. First, can you guys confirm that the EH's will fit with cabinet on a 500-C? Second, can you be a bit more specific about the mod that's needed? I know I can look it up, but bear with me, I'm feeling lazy today...

Steve
 
Hey. Yes they will fit in the 500c and 800c. The difference is they are very close together, and they have to be inserted AFTER the unit is in the cabinet. They are so close together that a slice of cheese would have a hard time getting between 2 tubes.
They are about a half inch or more taller (taller than the output tranies) so it will not fit into the chassis with them in. Other than that they work great.
The mod for them requires replacement of 4 resistors and 4 capacitors. The 4 330K resistors leading to the 7591a tubes is changed to 180-200k and the 4 coupling caps are increased from .047uF to .100uF. If you are restoring an old unit these caps need to be replaced anyway.
John
 
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