Cossor Melody Maker 336

 Another old radio that I forgot to include in my radio museum. This one was made by Cossor around 1931 and is the AC mains powered version of the similar Model 335. Two or three years later mains powered sets were the norm and new superhet designs simplified operation.
 

 This set has two-knob tuning because the manufacturer wasn't able to match the aerial and RF amplifier coils sufficiently well to gang their tuning. To operate this set one must first tune to the desired station on the right hand dial keeping the second (aerial tuning) dial at roughly the same reading then once tuned the aerial tuner is adjusted for best volume.

See an Osram set of slightly earlier vintage which has single-knob tuning.
 

 Rear view showing the chassis and valves, from left to right: MU14 (originally a 442BU) rectifier, MVSG RF amplifier, 41MH RF amplifier/detector and 41MP triode audio output. The metallised finish on the valves was vital to prevent instability. In many sets of this age the coating has deteriorated, but in this example remains perfectly intact.
 

 Here's the mains transformer and rectifier valve. These mains powered receivers were much more expensive than their battery equivalents and freed their owners from having to keep recharging the filament accumulator and the frequent purchase of an HT battery however, many people just couldn't afford the extra cash so stuck with their battery model until prices fell.
 

 The circuit diagram shows the need for an external louspeaker, something about which many buyers would have had some complaints, although most mains powered receivers from 1927 to 1931 did not incorporate a loudspeaker. Some did however, and a hidden saving allowing them to be competitive and no doubt stopping production of speakless sets was the HT choke. As you can see this set has a 30H choke which is quite expensive. The first mains powered sets to incorate a loudspeaker used a "mains-energised" type. This used the HT choke to not only smooth the high voltage supply but also provide the magnetic field to work the voice coil.
 

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