256 BELL SYSTEM TECHNICAL JPU_RNAL 



backs the use of metal tubes has been discarded in favor of glass tubes except 

 in the case of a few special designs. 



The procedure of mounting a crystal unit on a stem and sealing it in 

 glass is much the same as for a radio tube. Figure 13.2 shows crystal units" 

 mounted on stems ready for sealing and also shows units sealed in glass and 

 based. The extensions of the straights through the bottom micas are welded 

 to the formed wires emerging from the glass seals. In the glass-sealing 

 operation care must be taken not to heat up the assembly to the point where 

 the solder attachments will be melted or even softened enough to permit the 

 crystal to change position. To accomplish this it is necessary to use hot, 

 sharp-pointed fires localized to the region where the seal will be made. The 

 use of oxygen-gas flames is virtually essential to accomplish the seal quickly. 

 Having the fires strike the bulb at tangency is also desirable. The ordinary 

 type of glass-seaUng head for use with gas-air fires is not well adapted to 

 this work since the rotating pillars require the fires to be held too far away 

 from the work thereby necessitating larger flames and consequently more 

 heating up of the crystal unit assembly. The screening effect of the pillars 

 as they revolve also slows up the work of the fires thus increasing the over-all 

 heating of the assembly. A special glass-sealing machine developed for 

 seaUng crystal units is shown in Fig. 13 . 12. Immediately following the seal- 

 ing operation the glass units should be placed in a suitable annealing box 

 or leer where they can cool off very slowly. A large wooden block equipped 

 with holes to admit the individual bulbs is convenient. The holes ma}^ be 

 covered with a cloth to prevent air circulation. 



After the units have cooled they are placed on a vacuum pumping station 

 and evacuated. During the first half hour of pumping they are enclosed 

 in a heated oven in which the ambient temperature is maintained at about 

 240°F. This drives off any traces of moisture that might have entered the 

 tube prior to sealing. Following the heating interval, the tubes are pumped 

 for another half-hour during which time they will have cooled down to room 

 temperature. At this point the pressure in the tubes should be at the 

 minimum of which the pump is capable of attaining. This value should be 

 at most 20 microns and preferably less. However, with a six or eight-tube 

 station better than 15-20 microns is not likely to be attained unless a Hquid 

 nitrogen trap is employed in the system for eliminating moisture. 



After the pumping period, vacuum-type units are sealed off, with pump 

 running, by melting the glass tubulation with a fine-pointed oxygen-gas 

 flame as close to the stem as possible. If air is to be admitted, the pump is 

 closed off from the system and dry air admitted to the tubes after which 

 they are sealed off. After testing the crystal unit to see that it meets its 

 requirements, the unit is equipped with a base in the same manner as fol- 

 lowed for radio vacuum tubes. 



