Mullard RO/2.5kw

An early Mullard silica transmitting valve.These were very difficult to manufacture and Mullard versions had delicate and thermally vulnerable lead plug seals on the electrode entries.The first batches made failed under test because hydrogen from the gas torch used to seal them diffused into the envelope. This problem was solved by using an electric arc to melt the seal. They were the only alternative to external anodes for high power in a reasonable size. This bright emitter drew 14.5 A of filament current at 16 V. The anode voltage was 6 kV and dissipation 2.5kW.It may be a Royal Navy type N.T.22 or N.T. 23. These were fabulously expensive as each was entirely hand built and many failed under test.

This was photographed at the Porthcurno Telegraph Museum . A visit is strongly recommended for those with an interest in old technology. Among the exhibits are two working spark transmitters, one a replica of the Titanic radio room, as well as masses of submarine cable telegraph equipment.