170 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1957 



The cathode sleeve, which is necked down at one end as shown in Fig. 3, 

 is sUpped over the heater assembly and welded to the central molyb- 

 denum rod which becomes the cathode lead. By this means a uniform 

 temperature from end to end of the cathode is obtained. Under normal 

 operating conditions the heater temperature is approximately 1100°C, 

 which is very considerably under the temperature found in other tubes. 

 Connection of the heater to the leads from the stem presented a serious 

 design problem. Crystallization of tungsten during and after welding 

 and mechanical strains developed by thermal expansion frequently are 

 the causes of heater breakage. This problem was successfully overcome 

 by the means illustrated in Fig. 4. Short sections of nickel tubing are 

 slipped over the cleaned ends of the heater coil and matching pieces of 

 nickel wire are inserted as cores. These parts are held together by tack 

 welds at the midpoints of the tubing. The heater stem leads are bent, 

 flattened and formed to receive the ends of the heater, which are then 

 fastened by welds as indicated in the drawing. 



Fig. 4 — Heater tabbing arrangement of the 175HQ. 



