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Old 2nd Jun 2020, 11:06 am   #7
Herald1360
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Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,526
Default Re: Ferguson 384U (Finesse)

The low anode volts but expected cathode volts on the UL84 plus the hot rectifier suggest that the rectifier is going high resistance. They could also suggest, since the UL84 IK is about right, that there is a problem with the valve and internal leakage. That could explain extra hum, too. Does the hum come up as the grid volts rise? Disappear if the grid is shorted to chassis?

Maybe the dying UL84 has hastened the demise of the rectifier? They seem to be failing in a sort of lethal embrace. At least the output transformer will have benefited from the rectifier failings!
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