834 BELL SYSTEM TECHNICAL JOUkNAL 



tube since in a laboratory test equipment a pulse power of 1.5 to 2 megawatts 

 was needed. The laboratory number 426XQ tube sho\vn in Fig. 10 was the 

 result. Four of these tubes in parallel were capable of providing pulses of 

 about 1.75 megawatts. In the 426XQ tube, the bulb used on the 701 A 

 was employed and the plate supported entirely from its terminal in this 

 bulb. The same four cathodes were used, but were spaced farther apart 

 than in the 715-type tube. Two separate control-grids and two screen- 

 grids were used, each pair encompassing two cathodes. This allowed a 

 reduction of dissipation per grid compared to the 715-type, otherwise simi- 

 lar techniques were employed. The characteristics are shown in Fig. 11. 

 The tentative ratings applied to the 426XQ are given in Table I. The 

 allowable peak plate current was increased for this tube because the tech- 

 nique of processing had improved so that a higher level of cathode activity 

 was consistently realized. Also the greater spacing between cathodes and 

 use of two sets of grids resulted in better grid cooling. The tube was not 

 used in any radar equipment, because by the time it was available the trend 

 in radar equipment was toward small, compact apparatus in which spark 

 gap and transmission line modulators^ found considerable application. 

 The 426XQ proved very satisfactory in laboratory test equipment. One 

 set of these tubes operated for somewhat more than 2000 hours. 



The Chief Problems 



The difficulties experienced with this series of oxide-cathode pulse modu- 

 lator tubes can be divided into three general classifications, namely: spark- 

 ing, cathode emission, and grid emission. 



The sparking in these tubes can roughly be divided into two types, 

 which may be called inter-electrode sparking and cathode sparking. Inter- 

 electrode sparking is a discharge between two electrodes of the tube caused 

 by the momentary breakdown of the insulation between them or by a gas 

 discharge. If the breakdown of insulation is caused by light deposited films, 

 the resultant discharge usually causes removal of the film and cures the 

 trouble automatically, provided no other damage is done to the tube, (ias 

 discharges from isolated pockets may be initiated by the high fields or by 

 bombardment by stray electrons. If these pockets are not numerous they 

 are usually dissipated after a few minutes of tube operation such that fur- 

 ther sparking is very intermittent and probably not of sulficient intensity 

 to interfere with operation. The gas so released is ordinarily taken up by 

 the getter in the tube so that operation is not subsequently impaired. 



Cathode sparking may be caused by positive ion bombardment of the 

 cathode or by poor adherence of cathode material when subject to electro- 



