The PM5X general purpose and audio output valve is part of the 1927 six volt range from Mullard. The details on the valve emphasise that there is practically no glow from the filament. The PM filament was an oxide coated azide type.
The balloon envelope is 43 mm in diameter and, excluding the B4 base pins, is 88 mm tall.
References: 1931 datasheet & 1043. Type PM5X was first introduced in 1927.
Absolute Maximum Operating Conditions¶
Thanks to Andy Cowley for supplying the above PDF datasheet.
Updated May 26, 2018. shape:balloon construction:pinch type:triode age:1920.1930 base:b4 heater:6.0v pins:4 pin:1.a pin:2.g1 pin:3.f pin:4.f