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BELL SYSTEM TECHNICAL JOURNAL 



of appropriate frequency range, a wavemeter, and some device for indicating 

 resonance. The part inside the glass of one of the copper discs is bent, by- 

 gentle tapping, until the resonant frequency of the bulb in the special cavity 

 is within the required tolerance (which may be as small as 0.25%) of the 

 pretuning frequency. 



No heat treatment of any kind is used in the pumping of TR tubes. It 

 is obviously unnecessary to subject the tubes to the usual baking, the princi- 

 pal purpose of which is to remove water vapor tilm from the tube parts. On 



iMg. 35 — Pump Station for the 724B Vacuum Tube 



the contrar}', it is difficult to control the water vapor pressure in tubes which 

 have been baked as the parts absorb a surprising quantity of water. The 

 tubes are filled to fairly high pressure, so a diffusion pump is not necessary. 

 Fig. 35 shows a pump station used in production of the 724B. 



The test procedure for TR tubes must verify that each individual tube 

 will fulfill its fourfold function of protecting the crystal, of recovering rapidly, 

 and of introducing neither excessive high-level loss nor excessive low-level 

 loss. The tuning of each tube must be verified, and it must pass mechanical 

 and dimensional tests. Fortunately a tube which is otherwise sound will 

 never introduce excessive high-level loss, so no specific test is required. 



The protection test may be made either with a c-w- oscillator of suffi- 



