If you have access to a megger Id test the tube interelemrnt breakdown,
keeping within the tube specs.
It could also be a leaky electrolytic in the bias circuit, monitoring
the cut-off voltage may yield a hint.
Your symptom is rather common in other amps with a bad cap.
Carl
KM1H
----- Original Message -----
From: "Paul Christensen" <w9ac@arrl.net>
To: "Amps Amps" <amps@contesting.com>
Sent: Wednesday, January 21, 2009 3:21 PM
Subject: [Amps] Alpha 77Dx
>I had an unusual condition today that seems to be the prelude of my
>Alpha
> 77Dx's 8877 going bad.
>
> Shortly after applying AC power (about 20 sec.), the HV was reduced
> from
> what is normally 2600VDC during filament warm-up down to 2300 VDC and
> plate
> current ran about 40 mA where it is normally biased off until the
> 3-minute
> timer function completes. After the 3-minute timer completed, the
> amp's key
> line was working fine, but both LV and HV read much lower than normal
> and
> plate current was still idling around 30-40 mA where it is otherwise
> normally biased off until PTT is applied. Observing the amp's meters,
> there
> was no indication of RF drive from the transceiver whatsoever. In
> other
> words, there was no change between standby and either LV or HV. except
> that
> plate current idled at 40 mA and HV was lower in both positions.
>
> I re-started the amp a few times. On each successive attempt the
> fault
> condition as described above lengthened, beginning at 20 seconds, then
> about
> 60 seconds on the 2nd power-up, then 2 minutes on the third power up
> attempt, and finally everything was normal on the fourth and power-up
> attempt. Full RF power now and everything is running 100%. No snaps,
> crackles, or pops at any time.
>
> Thoughts?
>
> Paul, W9AC
>
>
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