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The PM254 triode design dates from 1927 and was improved in 1929. It is an audio output valve for battery wireless receivers and described at the time as a 'super-power valve'. The μ was 4.2 and rated output was 380 mW into a 4,500 Ohm load. High impedance headphones are of the order of 4,000 Ohms. The envelope has etched into it a BBC logo, as well as the 254 indicating it is from the original issue. The Bakelite base has a location groove filled with white paint.

The balloon envelope is 43 mm in diameter and, excluding the B4 base pins, is 93 mm tall.

References: 1931 data-sheet & ¹⁰⁴³. Type PM254 was first introduced in 1927. See also 1927 adverts.

Absolute Maximum Operating Conditions^¶

Thanks to Andy Cowley for supplying the above PDF datasheet.