Tiny valves, an inch long and thinner than an ordinary pencil, are the latest contribution of Mullard Ltd. to hearing aid design. Battery economy, making possible even smaller hearing aid appliances, has been achieved by reducing filament current to two-thirds of that of previous types.
The new valves are physically smaller and consume less power than previous types. Hearing aid valves at present available have filaments taking 15 mA. The new valves, designated the Mullard DF64 and DL64, take only 10 mA filament current, thus reducing LT drain by 33 per cent. This marks a significant advance in LT battery economy, the most pressing problem of hearing aid design. In practice the LT current will now be 20mA for a three-stage amplifier using types DF64 and DL64, compared with 30 mA using the 15 mA types. The nominal value of the LT voltage, 1.25 V, permits the use of any of the types of LT battery at present available, including mercury cells.
The potential reduction in battery size using the new Mullard valves is not confined to the LT cell. These new sub-miniature valves have been specially designed to give optimum performance with HT supplies of 15 V, as opposed to 22.5 V. in the case of previous types. Details of size and performance are given below.
A flat wire-in type sub-miniature voltage amplifying pentode on the B5A base with maximum dimensions, excluding leads, of 25.4 mm × 7.25 mm × 5.6 mm (1 in × 0.28 in × 0.22 in). In a typical circuit with 15 V HT it has a gain of 28 db for a total cathode current of 6.4 micro-amps. Filament 0.62V, 10mA.
An output pentode of similar form to type DF64 but with slightly increased length (31.7 mm maximum, about 1.2 in). With 15 V HT the output is 950 microwatts at 10 per cent total harmonic distortion for 0.85 V grid input and a cathode current of 184 micro-amps. Type DL64 has been specially designed to give a good compromise between sensitivity, power output, and HT drain. The filament rating is 1.25 V, 10 mA.