After 20 years of service, I experienced my first problem with my Alpha 77Dx
amplifier.
I noted that the input VSWR into the amp was very high, zero plate current,
and only 40mA of grid current regardless of exciter drive level.
I narrowed the root cause to a broken 8877 filament....although it was
difficult to find beacuse it is intermittent. At the onset of filament
current, it opens and after powering down the amp, and checking with a DVM,
the filamant resistance reads fine. I finally got it to read an open
resistance right at the pins with the transformer Molex connectors
unplugged. I can repeat the sequence and by putting the DVM in series with
the filament, I can see the DVM read AC current for a brief instant, then
it goes to zero. When this occurs, I can then see the filament read open
resistance right at the pins. Once I pull the tube and measure the filament
resistance, 75% of the time, I can then see good resistance. The other 25%
of the time, it will read open. I do not have a spare 8877 at the moment.
I'm not thrilled about a new Eimac replacement. Are there reasonable
sources of good pulls with ample life? If so, where should I begin looking?
Thanks in advance.
-Paul, W9AC
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