PULSE MODULATOR TUBES 



833 



of a moderately high current through an inductance and then suddenly to cut 

 off the current and withstand the resulting voltage which built up across the 

 circuit. A schematic of this type of circuit is shown in Fig. 9. In this cir- 

 cuit the tube is required to pass about two amperes peak plate current 

 which builds up over a period of about 150 microseconds. The d-c. volt- 

 age under these conditions is about 1000 to 4000 volts and the screen volt- 

 age may be obtained from the same source through series resistance. The 

 grid is driven only slightly positive. Screen-grid dissipation is one of the 

 limiting factors in this type operation. Primary emission from the screen 



50 100 150 



GRID POTENTIAL 



200 250 

 N VOLTS 



S 300 



200 400 600 600 1000 

 PLATE POTENTIAL IN VOLTS 



Fig. 11 (Continued) 



grid, being present when the plate rises to its very high potential, tends to 

 discharge the circuit prematurely, the energy wasted appearing as heat at 

 the plate. 



The 426XQ Tube 



Since there was considerable demand for tubes capable of operating at 

 voltages as high as 25 KV, a lube was developed to operate at this voltage. 

 The limit of the 5D21-715B type structure seemed to have been reached at 

 about 20 KV. It was also desirable to increase the current rating of the 



