[AMRadio] Re:Reactor Wanted


ne1s at neandertech.com ne1s at neandertech.com
Mon Nov 12 14:33:26 EST 2007


> -----Original Message-----

> Interesting indeed.
>
> That approach flies in the face of decades of design convention; as a
rule, AM
> rigs either use the same supply for mods and final, or as the rig gets
bigger,
> the mod has it's own supply which is usually a lower voltage than the PA
plate
> supply.
>

Greetings, Mr. T!

Here are a few random thoughts of mine on this subject. First, I can think
of the following arguments FOR using a common power supply. First, cost &
space savings. Second, with a choke input supply, the class C final
maintains a minimum load on the supply, reducing the size of the choke
required to maintain critical inductance. Otherwise, the separate
modulator PS would need to be capacitor input, or would have to waste a
huge mount of power in a bleeder resistor to maintain the required minimum
load on the filter.

I think the historic reason for a lower-voltage modulator PS in the case
when the supplies WERE separate originates from the conventional wisdom
that the audio power required to 100% plate-modulate a class-C final is
50% of the class-C plate input power. But this (1) ignores losses, (2)
ignores the "headroom" required for clean modulation, (3) ignores waveform
asymettry, and (4) perhaps most importantly, assumes a modulation
transformer of the correct turns/impedance ratio can be had. These days,
we're lucky find whatever mod iron we can in the power class we need, and
build around it. It is often the case we need to run a higher voltage on
the modulators than on the final to achieve enough audio voltage at the
mod tranny secondary to properly modulate the final, with the iron we are
able to scrounge.

73,
-Larry/NE1S


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