176 THE BELL SYSTEM TECHNICAL JOT'RNAL, JANUARY 1957 



watt (61o°C) group shows serious instabilities in its performance. In some 

 of the tubes the cathode temperature has not been sufficiently high to 

 provide the required emission levels. 



The design of the repeaters in the Newfoundland-Scotland section of 

 the cable is such that reasonably satisfactory cable performance would 

 be experienced if the transconductance in each tube dropped to 65 per 

 cent of its original value. The life test performance data presented in 

 Figs. 7 and 8, and other tests not shown, indicate that operation of the 

 175HQ tubes in the transatlantic cable at approximately 4.0 watts will 

 assure satisfactory thermionic performance for well oxev 20 years. 



Mention was made that cleanliness in the assembly of the mounts was 

 a factor which affected thermionic activity. Interesting evidence sup- 

 porting this view was obtained during the fabrication of tubes for the 

 Key West-Havana cable. The cjuality control type of chart reproduced 

 in Fig. 9 shows the average change in transconductance between two set 

 values of heater current for the first 5 tubes in each group of approxi- 



11.6 



11.2 



I- 

 z 



LU 



O10.8 



DC 



LU 



Q-10.4 



^10.0 

 O 

 < 9.6 



LU 



^ 9.2 



LU 



8.4 



10 



15 



20 25 30 ' "35 40 4 5 



FABRICATION LOT NUMBER 



Fig. 9 — Control chart showing the effect of air cleaning on the cathode ac- 

 tivity level. The per cent change in transconductance for normal heater current 

 and a value 20 per cent lower is used as the measure of cathode performance. 



