A range of single and double transmitting tetrodes was introduced in the early 1960s for use in mobile equipment. These valves are of the directly heated type but have a very low filament voltage of the order of 1.1 Volts and cannot be operated directly from a 6 or 12 Volt car battery. The valves themselves have characteristics similar to the normal indirectly heated types.
The filament is energized from additional centre tapped windings on the transformer of a transistor DC to DC converter. These windings have a rectangular output waveform, the RMS value of which is half the peak to peak amplitude.