Tom says:
>With no bias, you must NEVER have anode or screen voltage.
Even with bias, you don't want to have screen volts without plate
volts. I know of an old (now) military tx which had a 5894 driving a
pair of 4-250As. The plate voltage for the 5894 was the screen for the
4-250A, and was fused, with a fuse not very generously rated (150mA). No
interlock. Once the fuse blew, the 5894 had screen volts, but no plate
volts. The 5894 screen (fed from the 300volt 400mA supply common to the
low level stages) would glow bright orange (but not for long) and
collapse into the control grid, shorting the bias supply to 300 volts,
and leading to the 4-250's getting 300 volts on the control grids. These
then showed a tendency to collapse into the filament. Sometimes the
electrolytics in the bias supply blew first and put a short on it, but
not always. How this horror story ever got through a design review, I
don't know - maybe things were laxer in the early '60s. It was a well
known thing in the Test department, but for some reason, there was never
any pressure for a post design mod.
73
Peter G3RZP
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