GAS-DISCHARGE TRANSMIT-RECEIVE SWITCH 87 



The lack of water vapor in a tube which has been operated for some 

 hundreds of hours may manifest itself by a failure in either protection or 

 recover)' time. The operating frequency determines which failure becomes 

 important first; at long wavelength it is likely to be recovery, while at shorter 

 wavelength the spike protection is likely to fail first. 



The life of a TR tube operated without igniter is ver>' much longer. This 

 may be understood from the picture given above under "recovery," of the 

 state of affairs existing in the radio-frequency discharge. Electrons do not 

 completely traverse the gap, but oscillate about some mean position, while 

 the positive ions hardly move at all. Thus there is little more interaction 

 between the metal electrodes and the gas molecules with the R.F. discharge 

 on than with it ofif. A few 721A's have been operated without igniter for 

 as long as 5000 hours with no measurable change in either protection or re- 

 covery. This experiment was done at a transmitter power level of 250 kw. 

 peak power. The best life that can be expected with the igniter operating 

 at 100 microamperes is 500 hours, at which time the recovery time is badly 

 deteriorated. In order to maximize the life of the tube, the initial gas filling 

 consists of a minimum amount of hydrogen and as much water vapor as 

 may be introduced without causing excessive leakage power (see Figs. 20 

 and 23). 



Manufacturing and Testing 



Some interesting problems occur in the manufacture of copper-disc seal 

 TR tubes which are quite different from those encountered in the construc- 

 tion of more conventional tubes. The copper-disc seals are usually made by 

 high-frequency induction heating. Close control of the spacing between 

 discs must be maintained during the bulb-making operation in which the 

 discs are fused to the glass parts of the tube. One way of accomplishing 

 this is shown in Fig. 34, which depicts a machine setup for making the 724B 

 TR tube. The parts are held by lavite forms which support and locate them 

 during the bulb-making process. The seal is made possible by a correct 

 choice of copper thickness. The copper disc is stressed due to forces set up 

 by the different expansion coefficients of the glass and the copper, and if too 

 thick will pull the seal apart. If too thin, the copper itself will tear. Never- 

 theless, a properly designed copper-disc seal is ver}' strong; the copper-disc 

 seal TR tubes will pass the JANl-a* mechanical and thermal shock tests for 

 glass tubes without any difficulty. 



The electrical pretuning operation, referred to earlier, comes right in the 

 middle of the manufacturing process. Before the igniter is sealed in, the 

 bulb is placed in a special pretuning cavity. The setup includes an oscillator 



* Joint Army-Navy Specification for elev.'tron tubes. 



