Today's blog entry deals with the Mullard miniature battery valves that have use in the AF side of a receiver or battery amplifier.
For detector and AF voltage amplification applications, the DAF91 diode-pentode may be used. The pentode section is a short-grid-base valve specifically designed for audio amplification whereas the diode section, located at the negative end of the filament, is designed for speech detection. Using an HT of 90V a stage gain of 100x may be achielved and the performance is no less impressive with a voltage drop as HT at 45V which will provide a stage gain of 50x.
For output stage duties, there are two choices, types DL92 and DL94. Both of these devices have centre tapped filament which can be operated either in series or parallell. LT drain is 50mA at 2.8V for series working and 100mA at 1.4V for paralllel working. Type DL94 is designed to operate with equal voltages on both anode and screen and with a 90V potential, will provide a 270mW output at 7% THD with a drive voltage of 3.2V. Type DL92 can operate with unequal voltage on anode and screen and like it's bigger brother, is capable of an output of 160mW under similar working conditions. Although some of you will be thinking this ain't exactly hi-fi, these valves were used in a number of commercial battery operated gramophone amplifiers which were well received at the time.