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 Post subject: Substitute for 7591 tubes
PostPosted: Apr Tue 20, 2004 4:26 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 653
Location: Rochester, NY USA
I recently picked up a 1960s Sansui stereo that needs four 7591s, which are very costly! Is there anything that will reasonably sub for this type in the output stage? Thanks!<P>David<P><P>------------------<BR>


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 Post subject: Substitute for 7591 tubes
PostPosted: Apr Tue 20, 2004 4:43 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4102
Location: Rochester NY USA
New Russian made 7591s are $65-$75 for a matched quad. They're taller though - more like a 6L6. If they will fit, they're the best bet.<P>You CAN rewire the sockets for 6L6s - but you'll need to increase the bias voltage (or add a cathode resistor) and probably will have less output, since they require more drive voltage. Same size problem.<P>------------------<BR>


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 Post subject: Substitute for 7591 tubes
PostPosted: Apr Tue 20, 2004 5:13 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 20659
Location: Detroit, MI USA
Hi David,<P>The 7868 which is a 9-pin Novar, and the 6GM5, which is a large envelope 9-pin miniature, are both the exact electrical equivalent of the 7591. Either would require the construction of an adapter, or the replacement of the sockets in your amp. Most amps which take 7591's are low profile and there is usually not enough room to fit any larger tubes inside the cover. 7868's are usually almost as expensive as 7591's, but if you shop carefully you may be able to get a set of 4 new 6GM5's for less than the cost of a couple of good used 7591's. <P>All other replacements which have been used over the years have different characteristics, as Tom already mentioned the 7591 was specially designed for high power output from a smaller input signal. While you can alter the circuit to use other tube types, in most amps the gain and power output suffers noticeably, and in my experience you are wasting your time trying to modify the circuit. I haven't tried the new Russian tubes because my Eico ST-70 has no room for the larger tubes.<P><BR>------------------<BR>Dennis


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 Post subject: Substitute for 7591 tubes
PostPosted: Apr Thu 22, 2004 7:44 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 653
Location: Rochester, NY USA
Hi everyone,<P>Thanks for your suggestions. I borrowed a Russian 7591A, and found that it will fit into my unit. The top of the bulb is less than 1/2 inch from the amp cover, however - any problem if I go that route? There are ventilating slots over the tubes.<P>David<P>------------------<BR>


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