THIS WEEK THAT WAS – 6 February 1948

Sub-miniature valves for hearing aids

An interesting advance in valve manufacturing techniques is to be used for the state sponsored hearing aid Medresco. The Ministry of Supply has ordered 400,000 of the new sub-miniature thermionic valves from Mullard Wireless Service Company.

The Medresco unit is a small amplifier in a wedge-shaped case measuring 21/2in wide at the top, tapering to 11/2in at the bottom.

The compact case houses an amplifier using two of the sub-miniature voltage amplifying pentodes, DF70 and either a DL71 or DL72 output pendode. It is claimed that these valves have greatly reduced power consumption compared with the American types, CK505AX, CK502AX and CK506A.

The Mullard valves are only 10mm in diameter, the DF70 pentode is 30mm long, while each of the output pentodes DL71 and DL72 is 38mm.

The electrode structure is built up on a flat glass disc, similar to that employed in the all-glass technique used by the Mullard laboratories. This method of construction ensures a high degree of rigidity and freedom from microphony. The lead-out wires are tinned to facilitate soldered connection into the circuit.

Filament current ratings of the Mullard sub-miniatures are low. A three-stage amplifier in the hearing aid consumes a total filament current of only 50mA, compared with the 75mA for a circuit using American sub-miniatures. This represents a saving of over 30%.