180 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1957 



of one tube type for telephone systems and another for telegraph, it was 

 decided that the replacement for the SP61 should have a 2 watt cathode 

 with a 300 ma heater. 



During the years 1944 and 1945 a very successful miniature high slope 

 pentode, the CV138, was produced for the armed services. The electrical 

 characteristics of this tube were superior to those of the SP61 and, in 

 addition, it used a 2 watt cathode. It was therefore decided to base the 

 replacement tubes, electrically, on the CV138, whilst, at the same time, 

 retaining freedom to amend the mechanical features in any way which 

 might seem to favor the specific requirements of submerged repeater 

 usage, in particular, maintenance of the level of transconductance un- 

 changed for long periods. Consequently three major mechanical changes 

 were made at the outset of the project. The miniature bulb of the CV138 

 was replaced by one of normal size (approximately 1 inch diameter and 

 2| inches long). This was done to reduce the glass temperature and so 

 reduce gas evolution. At the same time a normal press and drop seal 

 were substituted for the button base and ring seal of the CV138, as it 

 was felt that, wuth the techniciues available, the former would be more 

 reliable than the latter. With the use of a normal press there immediately 

 followed a top cap control grid connection, so producing a double-ended 

 tube in place of the single-ended CV138. 



These three modifications and a number of major changes to improve 

 welding and assembly techniques led to the G.P.O. type known as the 

 6P10 [A pentode (P) with a 6.3-volt heater (6) of design mark 10]. The 

 6P10 replaced the SP61 in the 18 shallow water repeaters laid in various 

 cables after 1951. There are, therefore, 54 tubes type 6P10 in service 

 on the sea bed with periods of continuous loading ranging from two to 

 four years. There has been one failure due to a fractured cathode tape, 

 and one other repeater was withdrawn from service to investigate a high- 

 frequency oscillation associated with a tube. The oscillation cleared, 

 however, before the cause could be identified. 



The first eighteen 6P10 type tubes used in repeaters had conventional 

 nickel cathode cores. Appreciation of the problem of interface resistance 

 led to the use of platinum as a core material for the following 36 tubes. 

 The steps leading to this radical change of technique will be described 

 later. 



Development of the 6P12 for Long Haul Systems 



Although .submerged repeater development started naturally in Brit- 

 ain with shallow-water systems, it was inevitable that attention should 



