[Amps] Plate modulator xmfr ?

peter.chadwick at Zarlink.Com peter.chadwick at Zarlink.Com
Mon Oct 11 04:40:50 EDT 2004


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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