High-vacuum oxide-cathode pulse modulator tubes

In Practically all pulsed oscillators such as those used in radar, some means must be provided to apply the pulse voltage to the oscillator circuit. In many early radars, a high-vacuum modulator was used for this purpose. The pulse was generated at low power level and then amplified by means of one or more stages employing high vacuum tubes. The final stage was required to block, or cut off the d-c supply voltage with no pulse applied, and to permit as much as possible of the d-c voltage to appear on the oscillator during the pulse. Since most radar oscillators operate at pulse voltages of from 5 to 20 kv and require currents of several amperes during the pulse, the requirements of the modulator tubes are quite severe. Standard transmitting¹ tubes were used at first, the higher power tubes having the necessary voltage rating and having in general a fair amount of cathode emission. Tubes were operated in parallel to provide the required amount of current. Practically all of these tubes were of the thoriated tungsten filament type. For example an early army radar, the SCR268,² employed 8 tubes in parallel having a total filament power of 1040 watts to provide a pulse current of about 10 amperes. The use of such equipment in portable or airborne service would be obviously impractical because of the large power consumption, bulk, and weight. In an attempt to provide tubes more suited to this type of service, those described in this paper were developed.