Cossor Model 396

 

 Here's a nice example of a late 1930s wireless set. It was given to me by Susan Bowditch and delivered by her cousin Tessa Hunt. The set belonged to Susan's family and has sitting in much the same place since 1938. It's still fitted with an old 5 Amp mains plug so it's probably not been turned on for 50 years.

 
 The dial shows those exotic-sounding transmitters that were around before WW2

 
 Two separate chassis, probably so several different models with different output stages could be supplied

 
 The RF chassis with its valves in remarkable condition for their age. Possibly because the set's been in a stable environment as most examples of these valves lose their metallising. There's a fourth valve hidden behind the can on the right.

 
 The audio ouput valve, a PX4 and the mains rectifier. The former has a filament rather than a heated cathode and will have a balanced AC supply to its heater to minimise hum.

 

 As you can see the valves were originally: MVS/PEN, 41STH, MVS/PEN.B, DDT, 4XP and 43IU so the output valve has been changed from a Cossor 4XP to an equivalent Mullard PX4, either capable of over 4 watts output. The output valve is a little odd because it has a B4 base and a directly heated filament which of course requires to be grounded to act as a cathode but with that connection being arranged so that the AC voltage across the filament does not introduce hum on the audio. It also requires to be negatively biased to determine the anode current.

Interesting is the spec for the dial lamp which is 6.3 volts but run from 4 volts presumably for longer life and more subdued illumination. I'm not too happy about the external loudspeaker rating. This implies that the speaker should have a built in transformer which means the wires will carry at least 250 volts DC. The warning will be to preserve the life of the output valve as its screen would light up like a lamp if the external speaker is plugged fully home with a break in the wiring! Using headphones, naturally of the high impedance variety would be frowned on nowadays because their connections might give the user a nasty shock.

 Here's the circuit diagram for the Cossor 396. Note the bias arrangement and "humdinger" associated with the output valve.
 

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