Submarine-repeater pentode, P.O. type 6P10, made by the Post Office, England, 1950-1970.
Valves used in submarine telephone cables need to be very long lasting, due to the effort and cost involved in retrieving a cable from the seabed in order to replace one. The 6P10 valve was used in shallow water submarine communications systems. It had a platinum core, as tests at the Post Office Research Centre had shown that this was more effective at preventing resistance increase. 6P10 valves were used in submerged repeaters laid between Holyhead and Dublin, and Lowestoft and Hague in 1953-1954.