[AMRadio] 100 V filament transformer primary

[Bob Bruhns]

A rheostat is cheaper and smaller than a variac, offers a softer start on the filament, and because of the temperature
coefficient of the filament, there is a small reduction in filament voltage variation over line voltage fluctuations, although
that isn't much.  A rheostat generates more heat, though.

   Bacon, WA3WDR