. The Bell System technical journal . renot supported by the seal. It might be questioned why the modern alloysand fiat stem structure have not been used. It is to be remembered thatone gas leak along a stem lead would disable the system, and experiencebuilt up with the older materials provides greater assurance of satis-factory seals. The structure of the heater and cathode assembly is uniciue, as maybe seen in Fig. 3. A heater insulator of aluminvmi oxide is extruded with7 holes arranged as shown. This insulator is supported by a inchmolybdenum rod inserted in the center hole. The heat
. The Bell System technical journal . renot supported by the seal. It might be questioned why the modern alloysand fiat stem structure have not been used. It is to be remembered thatone gas leak along a stem lead would disable the system, and experiencebuilt up with the older materials provides greater assurance of satis-factory seals. The structure of the heater and cathode assembly is uniciue, as maybe seen in Fig. 3. A heater insulator of aluminvmi oxide is extruded with7 holes arranged as shown. This insulator is supported by a inchmolybdenum rod inserted in the center hole. The heater consisting ofabout 36 inches of inch tungsten is wound into a helix having anoutside diameter of inch. After dip coating by well known tech-niques the heater is threaded through the 6 outer holes in the suspension of aluminum oxide is then injected into the holes in theinsulator so that on final firing the heater becomes completely embedded. CATHODE SLEEVE, MOLYBDENUM. CATHODE/ CORE ROD ^ CERAMICINSULATOR \. , TUNGSTENI /HEATER I / I /I i I Fig. 3 — Heater, heater insulator and cathode assembly of the 175HQ tube, 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 uniformtemperature from end to end of the cathode is obtained. Under normaloperating conditions the heater temperature is approximately 1100°C,which is very considerably under the temperature found in other of the heater to the leads from the stem presented a seriousdesign problem. Crystallization of tungsten during and after weldingand mechanical strains developed by thermal expansion frequently arethe causes of heater breakage. This problem was successfully overcomeby the means illustrated in Fig. 4. Short sections of nickel tubing areslipped over the cleaned ends of the heater
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