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RE: RH84 test amp: multiple problems found ...

Posted by DAK (A) on September 24, 2016 at 11:03:24

In Reply to: RH84 test amp: multiple problems found ... posted by deathtube 667 on September 23, 2016 at 17:24:17:

Hi DT, i have found that the LM317 to be less than reliable in the cathode bias duties. I have changed out almost everyone that i installed. I suspect that it is the heat from the tubes that makes them fail at a high rate but it maybe some other issue. Sometimes the power tube won't even red plate when they fail which can be tricky unless you have a test point. I was putting a heat sink on them to keep them from failing but even that did not seem to help as eventually some of them failed again. I mounted some on the outer chassis of an amp that i sold but i am not sure if that pair of ccs is still working. It is probably better to go back to a bias resistor in terms of reliability.
Thanx for posting your distortion measurements of the amp as many critics have dissed the rh amps simply because the driver tube selected is a 12at7 which they claim is a "distortion producer" and thus the design can not be "hi fi". I think your measurements show that discussion to be untrue. cheers, Dak

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