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RE:Update

Posted by DAK (A) on August 23, 2015 at 21:21:01

In Reply to: Newly built amp was performing well...now oscillating posted by DAK on August 22, 2015 at 02:11:59:

I got the amp back from the owner to check on its condition and everything looks ok. I retested all the tubes and they are all good. So, I removed the 5v4 rectifiers and installed some SS rectifiers and did some voltage checks at 80 vac. Voltages look good EXCEPT; the bias circuit.
I have installed an "EFB" circuit which is the brainchild of Dave Gillespie. This uses an LM337 transistor along with an adjusting pot and some resistors and caps. With the EFB I was having a stable output of .56 vdc which translates into about 24 ma static dissipation per el84 tube. The problem is now I am reading 1.5vdc on the test point. Is it possible that the LM337 has failed? Anyway, I put back the 100 ohm 5 watt bias resistors for now and disconnected the EFB. I will run some sound checks and if the amp is stable I will probably just leave it at good ole cathode resistor bias. If the EFB is unreliable I would not consider it an improvement over a cathode resistor. regards, Dak

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