The Pen45 audio output valve is quoted in the literature as giving a push pull output of 11.5 Watts when operated with a load of 7500 Ohms in class AB1. The details on the box add that the anode dissipation is 10W, suggesting a higher maximum output power.
The details from the manufacturer are: 'An indirectly heated pentode for use with an AC mains supply. A valve of high sensitivity and large output power'. The price was 16/- (16 shillings) plus purchase tax.
The makers, Edison Swan, used the name Ediswan and later Mazda when part of the AEI group of companies.
This little valve, designed in 1938, was Mazda's post-war answer to the ubiquitous American type 6V6G. It was intended as a generously rated general-purpose output valve for domestic radios, radiograms, etc. Pen45 was a very good valve and a stalwart workhorse which seldom went wrong and therefore survives in fair numbers.
It was the standard output valve in Murphy AC radios for nearly 10 years after the War. Despite its 'Pen' type number we believe it was in fact a beam tetrode (like the 6V6) rather than a true pentode.
The snag, of course, is that being fitted with a Mazda Octal base cap limited its market. Painted thus into a very parochial corner, this splendid little valve had nowhere to go.
The classic envelope is 42 mm in diameter and, excluding the MO base pins, is 91 mm tall.
References: Datasheet, 1040 & 1043. Type Pen45 was first introduced in 1938.