Filament Connections and Voltages

Author: N.H. Crowhurst

Thus far, we have not discussed how we get electrons into the tube. This requires heating of either filaments or heaters. Most modern tubes employ a cathode with a separate heater. This is a considerable help in the construction of a complete amplifier, because it allows the cathode to be biased to any suitable voltage, while permitting all of the heaters in the different tubes to run from the same supply in any convenient manner.

Usually, with a-c-operated amplifiers, all of the heaters are connected in parallel to a winding on the power transformer. The potential across this winding is chosen to maintain the correct temperature in the tube. This potential is usually 6.3 volts, because this particular voltage happens to coincide with the battery voltage at one time used on almost all automobiles. This heater voltage became standard, because it was then possible to operate the tubes alternately from a battery or a 6.3-volt winding on a transformer.

Filaments, voltages and connections