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In Reply to: RE: Troubleshooting a JE Labs 300b amp posted by ddelaney on July 10, 2017 at 08:49:55
Did this occur all at once, quickly, or gradually? Have you checked the tubes? Easy enough to switch he tube from one am to the other, one at a time, to see if the problem remains with the amp, or switches to the other. Any other symptoms, buzzing, humming, crackling?
It has been a gradual fading. I did not mention that I have tried virtually every combination in my system - like swapping tubes from the good amp to the troubled one, swapping speakers between sides, swapping cables between sides, and the bass and midrange always sound diminished on the one amp. There is no humming or crackling; I have not had to adjust the variable resistor between the 300b heater legs.
Possibly a PS electrolytic cap that has leaked over time.
Would a degrading capacitor in the PS necessarily cause voltage to fall? I saw 4V difference between amps at the PS capacitor-output transformer junction, and the same 4V difference (or very close to that) at the 300B plate. This seems pretty consistent from when I built the amps (circa 2002).
The PS caps are paper-in-oil, and I have not seen any physical indications of leaks.
The 6SN7 cathode cap is a Black Gate. The coupling cap is an oil filled cap (I believe .22uf). All the iron is Hammond (300BX power, 1627SE output, 193J choke)
Besides the .22 uF 300B grid cap replacement, I would replace the electrolytic cathode resistor bypass caps with DC Link poly caps. Panasonic, Wima, Vishay brands.
I have a Kemet and a Vishay cap ordered for each amp. I'm replacing the coupling cap with a V-cap.
I should be soldering in a week or two.
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