7591, 7868 and 6GM5 tubes

quadaptor

Active Member
I have read that 7591, 7868 and 6GM5 tubes are identical in their electronic characteristics but have different pin configurations. I have an 800B receiver and was wondering if it could be rewired for the 7868 since 7591 tubes are getting hard to find and expensive. Also, what are the pros and cons of making this conversion, assuming it is doable.
 
Not adviseable.

From a standpoint of OLD STOCK tubes, 7591's are more readily available than 7868's as the 7868's were made for fewer years and less models used them. And the 7591's are generally less expensive. Last quads of each (7591EH and 7868EH) I bought around the same time went $80 and $110 respectively. The prices for both are slightly higher now. So even the new production follow the old stock conventions for pricing.

ALSO The novar/magnoval sockets are very hard to find. It may take months to find 4 matching sockets. Then they have to be rewired in the unit.

When you get to NEW production, you have one choice. The 7868EH. Pretty good tube but only about 80-85% as good (durability wise) as the originals. And the pins are slightly larger than the originals which makes for an extremely tight fit, with the possibility of fracturing the envelope when installing.

7591EH's are about, if not the best NEW Production tube out there. Get them from a reputable dealer that will match them under operating conditions. Jim McShane is about the only one I know of that tests them under operating conditions and then notes the results on the tube and box for future reference. His prices are in line with everyone else who just passes them on without testing or in a gm tester.

Larry
 
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Thanks, this is good to know. I have questions about how EH tubes compare to NOS, but perhaps that's a subject for a new thread.
 
Prices are as found on ebay last 6 months. I usually monitor daily. At times NOS goes higher than shown below.

NOS 7868- $99 to $165 a quad
NOS 7501- $250 to $350 a quad

New manufacture Electro Harmonic 7591 are selling as low as $18 on ebay as of last night. Unless one is wanting the last degree out of 7591s, the Electro Harmonic are just fine. Lower the 7591 grid bias resistors to 200-220K with the Electro Harmonic 7591 tubes.
 
I can think of only 1 truly good reason to change your sockets over to 7868...the EH7868 sounds better than the EH7591 and it takes up less space. If you plan to only use old or NOS tubes, you are better off spending your time putting in individual tube bias pots (IBAM it) than going through the fire drill of resocketing.
 
I can think of only 1 truly good reason to change your sockets over to 7868...the EH7868 sounds better than the EH7591 and it takes up less space. If you plan to only use old or NOS tubes, you are better off spending your time putting in individual tube bias pots (IBAM it) than going through the fire drill of resocketing.


Ditto!

A little while back I too thought about using 7868's by making up some 7591 adapters. I even started putting together pieces that I needed. Then an AKer offered up a bunch of 7591's for a decent price and I decided, with almost 20 tubes, I'm good in the power tube department till I croak.

Instead of spending time resocketing, you're better off IBAMifying your unit and picking up wayward 7591's and individually biasing them. Works like a dream :thmbsp:
 
7868s have less power than 7591s.
Two vendors make new production 7591s.
One vendor makes new production 7868s.
The EH 7591 is a good sounding tube. The bottle size is larger (6L6 size) but that's only a problem in certain situations, such as in integrated amps inside cabinets.
Overall, I'd leave it as is, unless you have a stash of the tubes you'd want to convert to already hidden away.
 
The size issue is why I converted my Sherwood to Magnoval sockets in order to accept an EH7868. I wasnt interested in the prices of NOS 7591, and new ones would not fit. The only practical choice was changing the worn out sockets for something else. I was able to find 4 NOS Cinch (or was it Amphenol ? I forget) sockets for a reasonable price and I got a matched quad of EH7868's for around a C note. It works just fine and sounds excellent, and it cost me about half what I'd have been into it for NOS 7591 tubes. If not for the fact that new 7591's didn't fit, I'd have probably not bothered changing the sockets though.

The 6GM5 isn't currently made as far as I know. Its also got a very small 9 pin miniature base. Its like a fat girl in stilletos. Not very solid feeling in the socket at all. The sockets are also small enough that they won't swap for an octal easily. The Magnovals and Novar will. Magnoval is new EH7868, Novar is old stock 7868. The pins are fatter on the Magnoval base, but you can force them into a Novar, especially an old tired Novar.
 
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