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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 filament drew 14.5 Amps at 16 V. The anode voltage was 6 kV and dissipation 2.5 kW. It might 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 (now the Museum of Global Communications).
This page last modified by AJC on 30-May-2010. Last updated 22-July-2021.