AC Mains Superhet
This McMichael was made in 1934. Despite this fairly early date it embodies some advanced design concepts, described as "novel" by "The Service Engineer" journal of June in that year. The triode-pentode frequency changer and new design of iron-dust cored IF transformer coils were admired by the reviewer.
The valve line-up is:
AC/TP Frequency changer
AC/SGVM IF amplifier (IF = 406 kHz)
AC/HL/DD Detector and AF amplifier
The IF transformers in this set are new high-Q types, and the IF frequency has been set at 406 kHz instead of the hitherto traditional 110 - 125 kHz. This means that a single stage of aerial tuning suffices for adequate image rejection and second channel interference. Oscillator tracking is by capacitative padders.
However, the AC/TP frequency changer valve does not incorporate an internal injection grid for electronic coupling of the oscillator and broadcast signals. Coupling coils in the cathode circuit are used in an additive mixing arrangement, relying on the non-linearity of the pentode section the produce the IF component.
The double-diode detector powers AVC (undelayed). A mains energised speaker is used, and the field coil in the HT positive supply provides smoothing. Automatic bias in the cathode circuits of each AF valve means that the power supply design is straightforward.
Power supplies are derived from a Westinghouse metal rectifier used in a voltage doubling circuit.
The selling price in 1934 was 14 Guineas.
The circuit is futher discussed in the Technical note on this site (page 10).
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