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BELL SYSTEM TECHNICAL JOURNAL 



lei. The pulse in this case was trapezoidal having a base width of about 

 4n seconds and a top width of about 1.75 n seconds; repetition rate was 1600 

 per second. When the tubes were operated under these conditions there 

 was customarily some sparking within the tube in the first few minutes and 

 then it apparently "aged in," and operated satisfactorily. At the rated 

 heater voltage, the cathodes operated at about 800°C (brightness). This 

 temperature would normally provide a cathode life of more than 1000 hours. 

 Life tests in the laboratory indicated satisfactory performance for about 

 2000 hours. Reports from the Navy were difficult to obtain but those 

 which were obtained indicated similar results. End of life was caused by 

 both loss of cathode emission and by primary grid emission. Mechanically 

 the tube proved to be reasonably rugged for normal service. However, 

 shocks sustained in shipment of tubes caused mechanical misalignment in 



Table I 



Table of Ratings of Oxide -Cathode 



Pulse Modulator Tubes 



some cases, indicating a need for a more rugged structure for use in the 

 armed services. 



The 715A Tube 



The advent of airborne radar made the development of high power light- 

 weight transmitters an urgent requirement. In this case long life was some- 

 what subordinate to lightweight and small dimensions. Ruggedness was 

 also a requirement. The electronic properties of the 701A tube were well 

 suited for airborne radar but the large bulk was an extreme disadvantage. 

 Work was begun on a tube using the same cathodes as the 701A but having 

 a simpler and more rugged mechanical structure. Out of this evolved the 

 715A tube. In this tube the cathodes were placed side by side and en- 

 veloped by a single control-grid, screen-grid and plate. In order to provide 

 the necessary ruggedness and to keep the grids cool, heavier grid wires 

 were used and they were wound on very heavy supports of high heat- 



