A new use for the metal rectifier

 In an effort to sell more metal rectifiers Westinghouse thought up this scheme....

Loudspeakers were still in their infancy and relatively expensive.

To cut their cost, permanent magnets were replaced by large coils through which a suitable current would produce the desired degree of magnetic field to produce a substantial audio output when the voice coil was driven by the output valve. Presumably the power rating of the coil (the square of the field volts divided by the coil resistance) was closely related to the audio power output.

In radio sets manufactured with a mains energised speaker, the energising current was provided "free" by inserting the coil in series with the HT to the receiver. The large coil also added the bonus of doubling up as a low frequency choke for smoothing the HT supply, when given a suitably high inductance of say a few Henries, the smoothing capacitors could be made smaller also. So, by these means, the mains energised speaker provided a cheap way of providing a decent output from a ready built radio.

Westinghouse were aiming their 1932 campaign at two markets...

The homebrew market where constructors would buy a metal rectifier rather than a rectifier valve to power their new radio, and secondly, where people wishing to buy a loudspeaker for their set would purchase one of the cheaper new mains energised types but required a way of energising it, in the case perhaps where their radio was battery operated.

If you study the figures in the table below it is apparent that there was a significant loss in the rectifier, as typically, to power a six volt coil required a nine volt power supply. In the case of modern silicon rectifiers this power loss is so low as to be insignificant, however when one compares a metal rectifier to a rectifier valve one did not have to supply wasteful heater current.

 I must admit that I have never handled a radio where the speaker is energised from a circuit other than the sets normal HT line.

Surprisingly only a few radios used a metal rectifier for their HT. Clearly economics played its part as manufacturers would have used a metal rectifier if it could reduce their price and made their set more competitive, bearing in mind also that cash was stumped up to Marconi "per valveholder" and this included the rectifier valve. I have an old Lotus radio with a metal HT rectifier and its pretty big.. maybe the size and mounting problems of these things also made it a difficult proposition?


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