12 BELL SYSTEM TECHNICAL JOURNAL 



The filament grid assembly consists of two lavite discs D and E, 

 spaced b" apart by a seamless steel tube. The grid F is made in the 

 form of a helix, and is held in position by allowing the ends of the lon- 

 gitudinal wires, to which the turns of the helix are welded, to pass 

 through holes in the lavite blocks D and E. The filament G is 

 mounted between hooks fastened to the lavite blocks and is kept 

 taut by the springs H. The grid lead is shown at J, and the filament 

 leads at K K. In this tube platinum seals are used for the lead wires. 

 The use of the springs H make it necessary to supply the filament 

 with current from the opposite end of the assembly and this is done 

 by passing the current through the steel support tube and returning 

 it through a lead passing through this tube and insulated from it by 

 a quartz tube. 



The whole assembly is carried by two supports B B. These sup- 

 ports are welded to a corrugated nickel collar A which grips the glass 

 stem C. 



The pumping of these tubes at first presented considerable difficulty, 

 chiefly on account of the large amount of occluded gas contained by 

 the metal parts. This caused the time of pumping of the tube to be 

 very long and a dangerous warping of the internal structure developed 

 owing to the fact that during exhaust the tube elements are maintained 

 at a much higher temperature than they are subjected to during nor- 

 mal operation. The trouble was overcome by heating the various 

 parts of the tube to as high a temperature as possible in a vacuum 

 furnace, prior to the final assembly, and thus getting rid of a large 

 amount of the occluded gases. The anode was preheated before 

 the glass seal was made and the whole filament grid assembly was pre- 

 heated just before it was mounted on the glass stem. The preheating 

 of the parts brought about an enormous reduction in the time re- 

 quired for pumping and gave a much more uniform product. 



Although successful from the standpoint of operation, this tube 

 had several undesirable features that it was thought well to eliminate. 

 In the first place the welding of the end into the tube was not particu- 

 larly desirable, and in general any troubles that occurred due to leaks 

 in the metal could be traced to this point. Further, in the assembly 

 of the tube there were a very large number of welds to be made 

 which constituted points of weakness at the high temperature neces- 

 sary for the evacuation of the tubes. It was, therefore, decided to 

 go to a type of tube in which the anode would be drawn in one piece 

 and in which as many welds as possible would be eliminated in the 

 assembly of the internal elements. At the same time it was considered 

 desirable to go to a somewhat larger type of structure in which high 



