Was it you, Don, who also said (at one time) that running too little of
voltage
on filaments can have the same undesireable effect on tube life?
That's right. The manufacturers say the filaments need to be within 5% of
the nominal value. I think the story of the 6C21 is that they willingly
re-spec'ed a 450T for increased peak emission at the expense of life. I
would run the tube with a variac on the filament transformer, crank it up
class-C plate modulated AM, and see what the postitive peaks do at 8.2 volts
and at 7.5. If the emission holds up at 7.5, I'd leave it there. The
thing that ruins a tube is to starve the filament emission capability by
using too low a voltage. If the plate current, or peak output just begins
to drop off due to reduced filament voltage, the voltage is low enough to
shorten tube life. You can try reducing voltage just to that point, then
increase it just enough to restore full peak output, and that would be the
minimum filament voltage it would be safe to run the tube at.
Don K4KYV
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