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Picture: Marconi 42

Model 42

The Marconiphone 42 is a high quality TRF two waveband receiver released in 1931. It uses a permanent magnet moving coil loudspeaker, and there are three tuned circuits.

The model was an exhibit at the V&A "Wireless
Show - 1977"
Picture: Marconi 42 The valve line-up is:

MS4B RF amplifier
MH4 Detector
MPT4 Output
U10 Mains rectifier

Band-pass tuning is used in the aerial circuit. The screen-grid valve V1 is coupled to the detector tuned circuit by means of an anode choke.

Reaction is applied to the leaky grid detector V2 by feedback coils; a potentiometer in series with these coils determines the amount of RF feedback. Operation of this coil also varies the voltage applied to the screen grid of V1, so that the gain of V1 and the amount of reaction are controlled in step.

The detector has an anode choke, and is coupled to the output pentode with a parallel fed audio transformer.

A full wave rectifier with resistance-capacity smoothing supplies the HT. The smoothing capacitors are paper types built into the capacitor block, which has spare units built in. The resistors are nearly all glass encapsulated types.

Provision is made for a gramophone pick-up connection, and the volume is controlled by a separate potentiometer ganged to the reaction control. A special contact on the band-change / gram function switch mutes the speaker as the switch is operated to prevent loud clicks.

Note that the loudspeaker shown in the photograph above is a replacement unit.

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