ELECTRON TUBES FOR A TRANSATLANTIC TELEPHONE CABLE 185 



22 

 20 



if) 18 



UJ 



a. 



UJ 



D. 16 



5 

 < 



14 



12 



z 10 



cr 

 a. 



3 « 



LU 



8 e 



z 

 < 



20 



40 



60 80 100 



ANODE VOLTAGE 



120 



140 



160 



Fig. 12 — Typical anode voltage-anode current characteristics for a type 6P12 

 tube (No. 457/6). 



transatlantic telephone (T.A.T.) amplifiers. The screen voltage and 

 anode current of the first two stages of the British T.A.T. amplifier were 

 chosen to be 40 volts and 3 ma respectively. 



Fig. 12 shows the normal anode voltage-anode current characteristics 

 for conditions corresponding to the output stage of the amplifier (static 

 operating point, anode voltage = 90, screen voltage = 60, anode cur- 

 rent = 6 ma) . A final electrical characteristic worthy of comment is the 

 level of reverse grid current. For all specimens tested at the time of se- 

 lection, after about 4,000 hours life test, the level is very low, about 100 

 micromicroamperes per milliampere of anode current. 



Lije Performance 



The life performance of the 6P12 is still a matter for conjecture. The 

 only concrete evidence available is the behaviour of a group of 92 tubes 

 which w^ere placed on hfe test some three years ago. The change of the 

 average transconductance of this group (with anode current constant at 

 6 ma) is shown in Fig. 13. It may be clearly appreciated that there is no 

 definite trend over the past year which permits any firm prediction of 

 life expectancy. Examination of other tube characteristics is equally un- 

 productive from the point of view of prediction of failure. 



