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This large square four pin base originated in the UK in about 1925. It was originally used on medium power transmitting valves for the UK military. Later use was on the audio valves such as the DA100.
The L4 base is 56 mm square and 25 mm thick. The locating circle at the base of the moulding is 43 mm in diameter and 11 mm thick.
The pin-outs below refer to the data researched by Keith R Thrower OBE.
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