The Mullard DL66 is a sub-miniature audio output pentode designed for use in hearing aids. The filament operates from a single dry call and the anode requirement is 22 Volts. At an anode voltage of 22.5 Volts the output is 2.7 mW. If operated at the maximum of 45 Volts HT the output rises to 16 mW.
The gaps in the side wall of the anode reveal the suppressor grid support. The dark stain on the glass is where a rubber locating ring was sited.
A UK made valve. The filament tension spring can be seen on the right. The red paint spot indicates the anode wire.
Thanks to Ian Summers for donating this valve.
The thin glass tube envelope is 5.5 mm x 7 mm, and excluding the base wires is 33 mm tall.