Even though the National Grid came on stream in 1947, the public electricity supply in the UK was a real hotch - potch of AC/DC offered at differing voltages - indeed my own parents came cropper of this in their flat at Arnold Place, Whetley Hill,  Bradford as one plug was DC yet another AC resulting in a prized radio going BANG - what a mess!  

Little wonder then that 20% of all radios fittted with Mullard valves in the early half of the 1950s were fitted with battery valves.   The Mullard battery valve series comprised of five types of valve: -

DL92 - a heptode frequency changer.

DF91 -  a vari-mu HF pentode to act as RF or IF amplifier.

DAF91 -  a short-grid base pentode t act as AF amplifier combined with a diode detector.

DL92 - an output pentode for portable battery powered receivers.

DL94 - an putput pentode for domestic battery receivers.

All of these valves were designed to be B7G based with a footprint of only 19 x 50mm meaning they were very portable device orientated.  The filament current for all types was 50mA @ 1.4V.

These devices were a masterpiece of miniaturisation, allbeit developed from the American 1S & 1T series, these devices meant a freedom from accumulator and mains supplies that "kids of today" take for granted 60 odd year later with their new fangled i-Pod -  awww, the ittle darlings..

Eeeeeh, progress tha knows!

A little more on these enigmatic devices in a future blog..................