[BC] AM Plate Modulation Concept

[Tom Dimeo]

I hear a ham running AM on the seventy-five meter ham band 
and from his description of his modulation system it is 
really confusing to me and I am wondering about what is
going on.

He has a Behringer five hundred watt solid state amplifier he 
uses as a modulator.  He feeds the output of the amplifier 
into an audio output transformer connected in reverse, the 
amplifier feeding the eight OHM winding and the high 
impedance winding connected into the RF final amplifier, a 
pair of 6146s tubes.  He says that the negative half cycle 
does not cut off the carrier but he is able to get upward of 
two hundred percent modulation in the positive direction.

I suspect the reason he doesn't go over a hundred percent 
modulation in the negative direction is because he isn't 
getting enough step up voltage in the transformer to produce 
voltage equal or higher than the DC voltage on the 6146s.  I 
think he says he has about six hundred and seventy volts DC 
on them.  As I understand, it would take six hundred and 
seventy volts of negative cycle audio to cut off the carrier.

The confusing part to me is how he gets such a high 
percentage of positive modulation without producing a 
positive half cycle of audio greater than the six hundred 
seventy volts.  How exactly does audio power correlate to 
percentage of modulation when there is an impedance mismatch 
that apparently doesn't produce enough audio voltage equal or 
higher than the DC voltage at the plates of the final RF 
amplifier?  Could it be that the audio power is increasing 
the input power to the final RF amplifier but not actually 
modulating the carrier at two hundred percent as he thinks it 
is doing?

I think I heard him say he is not modulating the screens of 
the 6146s so I don't know what effect there would be by just 
modulating the plates.

I kind of remember something about in broadcast AM 
transmitters back in the forties and fifties they 
deliberately had a mismatch between the modulator tubes and 
the final RF amplifier to prevent over modulation.  Is this 
the concept that is occurring in this ham's transmitter?

Thanks,

Tom