ELECTRON TUBES FOR A TRANSATLANTIC TELEPHONE CABLE 179 

 TUBES FOR THE NOVA SCOTIA-NEWFOUNDLAND CABLE 



Trend of Tube Development in British Submarine Repeater Systems 

 Early Use of Commercial Receiving Tubes 



The development of submerged telephone repeaters in Britain has 

 taken a somewhat different course from that followed in the United 

 States. Off North America, deep seas are encountered as soon as the 

 continental shelf has been passed. Consequently emphasis has been 

 placed from the beginning on the design of repeaters for ocean depths. 

 In Britain, separated from many countries by only shallow seas, it was 

 natural for development to start with a repeater specially designed for 

 shallow water. Such a repeater was laid in an Anglo-Irish cable in 1944. 



The tubes used in the amplifier of this repeater were normal high 

 transconductance commercial pentodes type SP61. These tubes were 

 known, from life test results, to last at least for two years under condi- 

 tions of continuous loading. Their performance in the first and subse- 

 quent early repeaters exceeded all expectations. So far one tube has 

 failed, and this from envelope fracture, after a period of four years 

 service. There remain 23 of this type on the sea bed with a service Hfe 

 of five or six years, and 3 tubes which have survived ten years. 



All these SP61 tubes were part of a single batch made in 1942 and 

 their performance set a high standard. It was, however, found that sub- 

 sequent batches did not attain the same standard set by the 1942 batch. 

 In 1946, therefore, the British Post Office was faced with the fact that 

 future development of the shallow water system of submerged repeaters 

 was dependent on the production of a tube type which could take the 

 place of the 1942 batch of SP61 tubes. This situation led to the formation 

 of a team at Dollis Hill whose terms of reference were, specifically, to 

 produce the replacement tube and, generally, to study the problems 

 presented by the use of tubes in submerged repeaters. Apart from 

 changes in the specific requirements, these terms of reference have re- 

 mained unchanged from that day to this. 



Replacement by the G.P.O. 6P10 Type 



Coincident with the rapid exhausting of stocks of satisfactory SP61 

 tubes for submerged telephone systems, there arose the need for a tube 

 type for a submerged telegraph repeater. This latter requirement was 

 complicated by the fact that the telegraph cable was subject to severe 

 overall voltage restrictions which precluded the 630 ma heater current 

 required for the 4 watt cathode of the SP61. In order to avoid production 



