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Vintage Asylum Classic gear from yesteryear; vintage audio standing the test of time. |
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Vintage Asylum Classic gear from yesteryear; vintage audio standing the test of time. |
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In Reply to: Important and probable cause for failure posted by Russ57 on April 26, 2007 at 05:58:45:
Russ is correct. Check the link below for a look at the resistors in question. They were 330K from the factory, and many have drifted up to 360K or more over the many years of use.There have been some cases where the new production 7591A tubes (EH and JJ) have had trouble in Fisher circuits using a very high value grid resistor. The best solution is to change all the grid resistors to 150K to 220K as Russ mentioned (from the old value over 300K - they even exceed the old RCA tube manual recommendation!), and change the coupling caps to .1 uf.
Your amp might have been right on the ragged edge and just the small disturbance kicked it into failure.
The diagnostic advice from Mike Samra is very good too, don't overlook it!
Also, if the main B+ filter caps are tired it is time for those to go too. Reduced capacitance from age/heat/etc. can reduce the amp's stability as well due to poorer decoupling.
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Follow Ups
- Re: Important and probable cause for failure - Jim McShane 06:57:27 04/26/07 (2)
- I have 7591S (not 'A') - do these also need mods? - PeterI 21:29:10 04/26/07 (1)
- Re: I have 7591S (not 'A') - do these also need mods? - Eli Duttman 23:31:03 04/26/07 (0)
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