I've got a small variac now on the filaments of my 250TH's, and always
run a variac on the plate transformer primary. But, just to test the tubes
in the final, I reduced the filament variac down to where the 5v meter was
showing around 3.5v, before any emmission was lost in the 250TH's in
the final of that rig. So, I'm wondering if there would be any harm in
dropping the filament voltage to 4.0v (as opposed to 5.0v) if there is no
reduction in emmission of the tubes?
That is a good way to test transmitting tubes. A good tube will have high
reserve emission, and it will take a substantial filament voltage reduction
to reduce emission. A marginal tube will show a dropoff of emission with
even slight undervoltage. A weak tube will often show full emission with
filament voltage above the rated value, but that proformance is likely to be
short lived.
For actual operation, be sure to carefully check the PEAK emission of the
tube. You can have substantial peak limiting before it shows up on the
meters in the transmitter, since mechanical meter movements are
average-reading devices. Such operation will reduce tube life. Better to
use a scope or LED peak reading device.
Don k4kyv
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