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.