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THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1957 



that at the end of 17 years the average transeonductance is 80 per cent 

 of its original value, and the poorest tube has dropped to 69 per cent. 

 There is reason to believe that test set difficulties may very well account 

 for a large part of the variation shown in the first three years. 



The cathode coatings used in all experimental and final tubes for the 

 Newfoimdland-Scotland link of the transatlantic cable are the con- 

 ventional double carbonate coatings. The cathode base material is an 

 International Nickel "220" nickel. The particular melt used for the 

 transatlantic cable is known as melt 84. A typical analysis for melt 84 

 nickel cathodes is given in Table II. 



Table II — Typical Analysis of Inco 220 Nickel Cathode Melt 84 

 (Analysis made prior to hydrogen firing) 



The relatively high carbon content (0.058 per cent) of melt 84 cathode 

 nickel is capable of producing excessive reduction of barium in the 

 cathode coating.-- ^ A treatment in wet hydrogen, prior to coating, at 

 925°C for 15 minutes reduces the carbon in the cathode sleeve to about 

 0.013 per cent. 



Melt 84 was as close as was obtainable in composition to melts 60 and 

 63 previously used for the Key West-Havana tubes. The results of up 

 to five years of life testing were thus available on materials of very similar 

 composition. 



One common cause of tube deterioration mth life is the result of 

 formation of an interface layer on the surface of the cathode sleeve. It 

 is known that the rate of development of this layer depends in a complex 

 way on the chemical composition of the nickel cathode core material. 

 The effect of such a layer is to introduce a resistance in series with the 

 cathode. This results in negative feedback and reduces the effective 

 transeonductance. Since the effect of a given feedback resistance in this 

 location is proportional to transeonductance, the relatively low value for 

 the 175HQ tube tends to minimize this feedback effect. In addition, the 

 low cathode temperature tends to reduce the rate of formation of inter- 

 face resistance, and the relatively large cathode area tends to further 

 minimize the effects. The final decision to use melt 84 was based on ac- 



