I have also heard that dc on the filaments causes migration. It makes
sense. However, practicall experience leads me to question whether the
conclusion may not be relevant.
My old GE Progress Line 2 meter rig was all tubes and it ran on dc. It was
in my car for years without replacing any tubes and without reversing
polarity of the battery. Finally replaced the Prog Line by little devices
that do not have filaments.
I have run the tube vfo's in many of my rigs on dc filaments. Left the tube
on for years with no failure of the filament.
I have never had a failure of a power tube filament - except for one I
dropped off the table and one that failed while in the hands of United
Parcel Service. So, even if the dc causes premature failure of the
filament, it may still outlast the operator or other failure.
After all this, would I occassionally change polarity if dc was applied to a
filament of an expensive power tube - probably.
Colin K7FM
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