It is only necessary to reduce filament voltage by a few tenths of a volt, which is not going to hurt anything. The first thing you should do is check and monitor your power line voltage. Mine varies during the day by +5 volts! A variac is the easiest way to control it but you have to be able to monitor the filament voltage constantly for it to be of any value. Virtually all broadcast transmitters back in the tube days had filament voltage variacs and a meter to monitor the voltage, so it is important to know what is happening. Also back in the good old days, these and other operating parameters had to be logged every thrity minutes by FCC decree. A great after school job for us up and coming hams.
73, John, W4AWM
