With the exception of an open filaments, valves are seldom defective, even after 70 years. More likely the valve is missing or broken. On vintage radios before 1930, to find exactly the right valve from the original manufacturer is not always possible. Each manufacturer used its own marking's, if the truth were known, most valves were virtually equivalent to those of there competitor's.
Cross referencing valves by British manufacturers such as:
Marconi, Mullard, Mazda, Cossor, Osram etc
Cross referencing valves by Eupopean manufacturers such as:
Cossor, Dario, Fotos Grammont, Gecovalve, Mazda, Mullard, Philips, Telefunken, Tungsram, Valvo, Vatea, Visseaux, Zenith
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