72 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1951 



begun on a tube suitable for this use as far back as 1933. Thus, when a deci- 

 sion was made to lay the new cables, a long background of experience was 

 drawn on in the manufacture of the tubes. Early models of the type had been 

 operated on continuous life test for as long as ten years. Designed primarily 

 for long life, the tube is a suppressor grid pentode with an indirectly heated 

 cathode. Of rugged design to withstand the shocks of cable laying, the spac- 

 ings between electrodes are relatively large. Unusual care was taken in 

 manufacture to insure solid welds and to avoid the presence of loose particles. 

 During various stages of assembly, rigorous inspections were made on all 

 tubes by engineering personnel. Selection of tubes for use in the cable was 

 based on a thorough examination of all details in the history of each tube. 



Fig. 4 — Repeater network assembly. 



as well as the history of the group in which it was manufactured. All tubes 

 which were candidates for the cable were aged several thousand hours before 

 preliminary selection was made. In addition, other tubes from the same pro- 

 duction group were further life tested several more thousand hours to estab- 

 lish the quality of the group. One early decision was to power repeaters by 

 direct current fed from land over the cable conductor. The tube heaters con- 

 nected in series would furnish plate and grid potentials. This was an impor- 

 tant factor in setting the nominal power requirements for the tube, which 

 are about J ampere at 20 volts for heater supply and plate potentials of 40 

 to 60 volts. 



While the electron tube is usually the most vulnerable element in electrical 

 circuits from the standpoint of life, attention must be given to other ele- 



