Plate AM modulators

In plate modulation systems the voltage delivered to the stage is changed. As the power output available is a function of the supply voltage, the output power is modulated. This can be done using a transformer to alter the anode (plate) voltage. The advantage of the transformer method is that the audio power can be supplied to the RF stage and converted into RF power. With anode modulation using a transformer, the tetrode is supplied with an anode supply (and screen grid supply) which is modulated via the transformer. The resistor R1 sets the grid bias, both the input and outputs are tuned LC circuits which are tapped into by inductive coupling. In series modulated amplitude modulation, the tetrode is supplied with an anode supply (and screen grid supply) which is modulated by the modulator valve. The resistor VR1 sets the grid bias for the modulator valve, both the RF input (tuned grid) and outputs are tuned LC circuits which are tapped into by inductive coupling.

When the valve at the top conducts more than the potential difference between the anode and cathode of the lower valve (RF valve) will increase. The two valves can be thought of as two resistors in a potentiometer.

1. Cheney, M., Uth, R., & Glenn, J. (1999). Tesla, master of lightning. New York: Barnes & Noble Books. Page 71.

2. Ronald Kitchen, RF Radiation Safety Handbook, Butterworth Heinemann 1993, ISBN 0 7506 1712 8 Chapter 10

3. Sleeper, M. B. (1922). Design data for radio transmitters and receivers. Everyday engineering series, [no.] 6. New York: Norman W. Henley Pub.

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