High-Vacuum Oxide-Cathode Pulse Modulator Tubes 



By C. E. FAY 



Introduction 



TN practically all pulsed oscillators such as those used in radar, 

 -*■ some means must be provided to apply the pulse voltage to the oscil- 

 lator 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 employmg 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 transmitting^ 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,^ 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 imprac- 

 tical 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. 



Tube Requirements 



The function of the high-vacuum modulator tube essentially is to act as a 

 switch U) turn the pulse on and off at the transmitter in response to a con- 

 trol signal. The best device for this purpose will be the one which requires 

 the least signal power for control and which allows the transfer of power w ith 

 the least loss, from the transmitter power source to the oscillator. 



If the oscillator must be supplied with a pulse of voltage E„ and current 

 /;„* or power Epfp, then the voltage w-hich must be supplied by the trans- 

 mitter power su[)ply will be E^ = Ep-\- c,,, I'ig. 1, if e'p represents the voltage 



• It is assumed here that the pulse is rectangular in shape. This is usually the desired 

 shape and is fairly well ajjproximated in most cases. 



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