The Cause and Prevention of Hum in Receiving Tubes Employing Alternating Current Direct on the Filament

The various factors affecting the variation of plate current of a receiving tube when the filament is heated with a.c. are analyzed, and their composite effect considered. The variation in temperature of the filament is found to be of minor importance while the predominant factor is the voltage drop along the cathode. The smallest variation in I
p
is obtained from a tube having a properly designed V-shaped oxide filament. When straight filaments are used, their characteristics should be so chosen that the magnetic and electrostatic effects are neutralized.