ELECTRON TUBES FOR A TRANSATLANTIC TELEPHONE CABLE 181 



ultimately turn towards trans-ocean cables. The tube requirements for 

 such long haul systems differ from those for short haul shallow-water 

 schemes in that operation at a lower anode voltage is essential. A new 

 tube to replace the 6P10 was therefore unavoidable. 



By the time emphasis started to shift in Britain from shallow-water 

 to deep-sea systems some considerable experience had been gained at 

 Dollis Hill on the production techniques required to fit a 6P10 type tube 

 having a platinum cathode core for submerged repeater usage. When it 

 became apparent that a new tube had to be designed for the first long 

 haul system, it was resolved to retain as much as possible of the 6P10 

 structure in order to take full advantage of familiar techniques. The 

 ()P10 was therefore redesigned for 60-volt operation simply by a major 

 adjustment to the screen grid position and minor alterations elsewhere. 

 The new tube became known as the 6P12 and was used in seven repeaters 

 installed in the Aberdeen-Bergen cable. This scheme was regarded as a 

 proving trial for the Newfoundland-Nova Scotia section of the transat- 

 lantic project. 



It has always been appreciated that the use of high transconductance 

 tubes using closely-spaced electrodes will involve a higher liability to 

 mechanical failure by internal short circuits. Practical experience in 

 shallow-water schemes, where repeater recovery is a comparatively cheap 

 and simple operation, has shown however that such risks seem to be out- 

 weighed by the economic advantage accruing from a tube capable of 

 wider frequency coverage. In point of fact a failure by internal short 

 circuit has not yet materialized on any shallow-water system. 



This background of experience explains the British choice of a high 

 transconductance tube for deep-sea systems, but the greater liability to 

 mechanical failure is acknowledged by use of parallel amplifiers. Con- 

 fidence in this policy has been increased by the successful operation oi' 

 the Aberdeen-Bergen system. 



Problems of Development of the 6P12 Tube 



The main preoccupation of the thermionics group at Dollis Hill since 

 1946 has been a study of the electrical life processes of high transcon- 

 ductance receiving tubes. This effort has led to a conviction that all 

 changes of electrical performance have their origin in chemical or elec- 

 tro-chemical actions occurring in the tube on a micro- or milli-micro scale 

 of magnitude. The form of change of most importance to the repeater 

 engineer is decay of transconductance and this will be considered in brief 

 detail as typical of the development effort put into the 6P12 tube. 



Transconductance decay in common tubes results from two separate 



