ELECTRON TUBES FOR A TRANSATLANTIC TELEPHONE CABLE 1G9 



convenience in wiring in the circuit and to hold down the control-grid to 

 anode capacitance, the grid lead has been brought through the opposite 

 end of the tube from the other leads. 



A number of somewhat unusual constructional features appear in the 

 tube. The stem on which the internal structure is supported consists of 

 a molded glass dish into which seven two-piece beaded dumet leads are 

 sealed. The parts used in a stem, and also a finished stem, are shown in 

 Fig. 2. It is usual to embed the weld or "knot" between the dumet and 

 nickel portions of the lead in the glass seal to provide more structural 

 stiffness. This has not been done in this stem because it was believed that 

 a fracture of a lead at the weld could be detected more easily if it were 

 not supported by the seal. It might be questioned why the modern alloys 

 and fiat stem structure have not been used. It is to be remembered that 

 one gas leak along a stem lead would disable the system, and experience 

 built up with the older materials provides greater assurance of satis- 

 factory seals. 



The structure of the heater and cathode assembly is uniciue, as may 

 be seen in Fig. 3. A heater insulator of aluminvmi oxide is extruded with 

 7 holes arranged as shown. This insulator is supported by a 0.025 inch 

 molybdenum rod inserted in the center hole. The heater consisting of 

 about 36 inches of 0.003 inch tungsten is wound into a helix having an 

 outside diameter of 0.013 inch. After dip coating by well known tech- 

 niques the heater is threaded through the 6 outer holes in the insulator. 

 A suspension of aluminum oxide is then injected into the holes in the 

 insulator so that on final firing the heater becomes completely embedded. 



CATHODE SLEEVE, 



MOLYBDENUM 

 . CATHODE 

 / CORE ROD 



^ CERAMIC 

 INSULATOR \ 



, TUNGSTEN 

 I /HEATER 



I / 



I / 

 I 



i I 



Fig. 3 — Heater, heater insulator and cathode assembly of the 175HQ tube, 



