Now then, everyone tends to think of Hivac when they think of miniature valves for hearing aids but Mullard made them too. Hearing aid valves were specialised in that they needed a small envelope size, must operate at low filament current and be non microphonic.
Due to the National Health hearing aid being introduced in 1948, Mullard developed a series of 10mm subminiature valves to power it ......we have two of these smashing hearing aids available for sale elsewhere on the site - anyhow, I digress.........
These 10mm specials were swiftly superceded by the DF66, DL66 and DL68 which show a 15mA as against the 35mA used by the previous types. And this was done by the use of 8 micron (3/10000 inch) filament wire - now that is small as you can see in the photo below, on the left, we have a spider's web with sticky blobs on it to trap insects, on the right, we have the hearing aid tungsten wire, pretty neat huh?
The next picture just shows how small the 1950s hearing aid valves from Mullard actually were, their size approximated to 1/40th the size of a typical radio valve: -
My last picture shows, progressing from left to right, how hearing aid valves from Mullard shrunk from the 1936 original to the 1947 10mm and finally the 1952 D series: -
This development work on hearing aid valves served Mullard well as variants of these were used in the Ferranti radars of second generation RAF fighter jets such as the Lightning and the Buccaneer.