Marconi Model 858

 

 This rather heavy table model radio was unveiled in July 1938 and is typical of those being sold around that time. It's particularly aimed at those listeners wishing to explore shortwave bands although not providing separate bandspread ranges such as the later Murphy A92.

The metal speaker grille is much more resilient than cloth. I think all the sets in my collection with cloth have some damage because the stuff is degraded by light.

The tuning arrangements include a small logging dial so you can return to a previous dial setting and an indented knob for rapid tuning across the dial. It also includes the luxury of a magic eye tuning indicator so you can adjust the tuning more accurately.

 
 Unlike some sets it has a three-gang tuning condenser which allows for tuning the RF amplifier as is the case in communications receivers. This is very important as it reduces image reception, a significant problem for set designers when using a low intermediate frequency... in this case the standard 465Kc/s. The chassis isn't painted like those from Murphy, but then again, without removing the back you wouldn't really notice this economy.

 
 Above a decent sized loudspeaker which is of the "mains energised" type which doubles as an HT choke. Below a green label shows the valve types fitted in the set. Of course Marconi valves were chosen, not especially for the reasons stated, but because they're made by the same manufacturer as the set and therefore a lot cheaper to source! All have International Octal bases.

 

 The circuit diagram which you can see in the Trader Sheet (below) is interesting as it shows the HT smoothing choke in the supply negative lead rather than the positive lead and the connections provide various bias supplies for biasing back some of the valves. This means that the KT63 output valve has its cathode connected to the chassis rather than via the resistor-condenser arrangement normally provided for auto-bias. A small choke is provided in the speaker wiring to eliminate, or at least to reduce hum.

This Marconiphone model is electrically identical to the HMV 656 although the latter has a horizontal rather than vertical shape achieved by mounting the speaker to the right of the dial. There's also some rather super versions using motor controlled tuning. These are the models 853 & 657 table models and the models 871 & 668 radiograms. I have a Murphy A72 which has provision for adding motor driven tuning also. I wonder how many motor-driven receivers were actually sold?

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Click to see the Trader Sheet

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