The PenA4 design is a 1937 audio pentode power output valve.
For the heater power the emission is not high and the audio output achieved is small. It is specified for single ended use, as would be found in radio receivers.
The Pen/4VA was a first-generation output pentode in that, like the AC/Pen, MPT4, and others of its generation, it had relatively low gain and required a substantial grid input signal.
Second-generation output pentodes had much higher gain but required larger cathode area and therefore more heater current. Mullard's first second-generation output pentode was the Pen4VB. Released shortly after the (late-to-market) Type Pen/4VA, the Pen4VB had teething troubles and was rejected by the market in favour of more successful equivalents such as the Mazda AC2/Pen. When Mullard's got their act sorted out they relaunched the Pen4VB but renamed it the PenA4. It was reasonably successful. See also the Ever Ready A70D also made by Mullard.
The sample shown in the photo is of post-war manufacture for replacement purposes.
The classic envelope is 41 mm in diameter and, excluding the B7 base pins, is 116 mm tall.
References: Data-sheet, private correspondence & 1043 Type PenA4 was first introduced in 1937. See also 1937 adverts.
Absolute Maximum Operating Conditions¶
Updated January 04, 2022.