One point to bear in mind is that the secondary of the modulation
transformer has a lot of DC in it, which means that you need more iron
(and probably an air gap) to avoid the core being saturated by the Dc feed
to the PA. This is apparently why modulation reactors are used - the
transformer is designed for no DC, and feeds through a blocking capacitor
to the plate supply and ground, there being the 'modulation reactor' in
series with the plate supply to avoid the AF being shunted. The etchniques
is apparently fairly common in the AM broadcast area for powers of 1 to 5
kW.
Maybe much nicer is the old original method of series modulation, with a
cathode follower providing the DC feed to the PA. Needs a lot of HV,
though - about 5kV for a pair of 813s! There's a lot of dissipation in the
modulator, too, as its effectively running Class A..
If the screens are fed via a dropping resistor, it can help to put a
capacitor across the dropping resistor to increase the percentage of
screen modulation. This is very helpful in cases of downward modulation -
where the power doesn't get up to twice the carrier envelope voltage on
positive peaks, but tehre's enough grid drive. My father 'discovered' this
in the mid 1950's, and wrote it up in the RSGB Bulletin. Later, I found
that RCA were talking about and using this technique in 1936, but it
seemed to have got lost.
73
Peter SM/G3RZP
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