[Amps] 3-500z Silly question

[Bill, W6WRT]

ORIGINAL MESSAGE:


>> Is the filament of the 3-500z 5V-DC or 5V-AC??
>> The last two .PDF iv read its not stated as wich ....

REPLY:

This may be an old wive's tale, but I read years ago that DC should not be used
on a filament of a high power tube. The reason was that it creates a bias
condition where one end of the filament (in your example) would have a -5 volt
bias with respect to the other end. As a result the emission would be greater
from the negative end compared to the positive end, and the tube would age
unevenly. 

This of course does not apply to indirectly-heated cathodes.

I have never seen any documentation of experiments done to prove the theory, but
it makes sense and has stuck in a back corner of my head for years. 

73, Bill W6WRT