As I have previously mentioned, I have two Alpha 3CX800A7LG's that appear to
have grid-cathode shorts that develop within a few minutes of applying filament
voltage. After seeing VE5RA's posting about shorts clearing themselves after 6
minutes, I did some experiments with the two tubes. I hope someone can explain
the physics/engineering for what I am seeing.
Apply 13.5 DC to the filaments. No other voltage applied to any pins. Initially
the filament-grid and filament-cathode voltages are zero. Also, the grid-cathode
voltage is zero.
As the filaments warm up, the filament-grid and filament-cathode voltage rises.
After a 20 minute warmup, I have these voltages:
+ filament to grid 9.5
- filament to grid 0.5
+ filament to cathode 8.9
- filament to cathode -2.9
The grid ring is extremely hot.
What's going on in the tube ?
73,
Steve, N2IC
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