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The PM3A was introduced in 1927 as a resistance capacity coupled amplifier triode with a μ of 35.

The identification details are etched onto the side of the envelope together with a code 7C68. The main details are also carried on a paper strip fixed above the Bakelite base. Looking up under the mirror of the Magnesium gettering reveals a horizontal box anode. This is supported from the pinch by stout wires that form a wide U. The anode is stippled.

The Balloon envelope is 43 mm in diameter, and excluding the B4 base pins, is 90 mm tall.

References: 1931 data-sheet & ¹⁰⁴³. Type PM3A was first introduced in 1927. See also 1927 adverts.

Absolute Maximum Operating Conditions^¶

Thanks to Andy Cowley for supplying the above PDF datasheet.