[Amps] 8877 HF amp cathode wiring
w9ac at arrl.net
Mon Aug 20 09:05:29 EDT 2018
>"I've done it both ways and either seems to work fine. There are 2 schools
of thought on connecting the filament to the cathode to keep arcing from
happening between the filament and the cathode."
On my Alpha 77Dx, a bifilar filament choke was added, while keeping
isolation between the filament and cathode. In the event of an internal
cathode-to-filament short, the amp will continue to run normally. The 8877
currently in my 77Dx has an internal short. Had I not modified it this way
and kept the stock Alpha design, that 8877 would be useless because Alpha's
design uses chassis ground as a filament return. I have no idea how long
the tube will last this way but it in my case, I'm able to get some extra
operating hours from the tube.
At the same time, I also added some low-voltage Siemens GDTs between the
cathode and chassis, and both filament sides to chassis. I don't recall the
value I settled on. It may be whistling in the dark, but at least they
haven't added any new problems to-date.
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