PULSE MODULATOR TUBES 821 



plate capacitance is of little importance as long as it does not store much 

 energy, there being little chance for oscillation in such a circuit. While it is 

 desirable to operate with as low a minimum plate voltage as possible, it is of 

 little additional advantage to bring the plate voltage below the screen volt- 

 age if the screen voltage is about 1000 volts and the supply voltage 15 kv. 

 It was therefore thought permissible to increase plate to screen spacing 

 beyond the optimum for best characteristics in the interest of high voltage 

 and screen dissipation ratings. 



The insulation in the tube between plate and other electrodes must be 

 capable of withstanding the full supply voltage plus a comfortable margin. 

 This dictates that if internal insulators are used they must have long path 

 and that the bulb must have sufficient length to prevent flash-over ex- 

 ternally. 



The 701A Vacuum Tube 



At the time of this development a tube was very urgently needed for a 

 Navy radar application.^ Since speed was of prime importance it was de- 

 cided to take parts of a standard oxide-cathode beam-power tetrode, 

 Western Electric 350A, and mount them in a stiucture capable of with- 

 standing the required voltage; 12 kv in this case. Accordingly a cruciform 

 structure was designed in which four sets of 350A electrodes were mounted 

 on ceramic members attached to a molded glass dish-stem as shown in 

 Figure 3. The four cathodes have a total coated area of approximately 14 

 square centimeters. A molybdenum plate of cruciform section mounted 

 from a lead-in at the top of the bulb was used. This construction elimi- 

 nated internal insulators between plate and grids other than the bulb. 

 The control-grid of the 350A is normally gold plated to inhibit primary 

 emission. This feature was retained in the 701A and the screen-grid also 

 gold plated. The plate to screen-grid spacing was -increased over that 

 normally used in the 350A in order to allow somewhat better cooling of the 

 grids and to allow greater clearance for high voltage reasons. This made the 

 characteristics depart from good "beam tube" performance but at the high 

 voltage condition of operation this was of little consequence. Character- 

 istic curves of the 701A indicating performance under both high voltage 

 and low voltage conditions are shown in Fig. 4. Since no experience was 

 available at the time of this development to indicate what currents could 

 safely be drawn from the cathodes under pulse conditions, the matter of 

 rating these tubes was mainly guesswork since time was not available to 

 await the outcome of life tests under various conditions. The ratings put 

 on the 701 A are as shown in Table I. 



For the immediate application in hand, which required 12 ampere pulses 

 at about 10 kv, it was decided to specify two 701 A tubes operating in paral- 



