Question 6A383

When a transmitter is adjusted for maximum power for continuous wave (CW or telegraphy), its final amplifier tubes are typically operating near their maximum voltage and current limits. For Amplitude Modulation (AM), the instantaneous peak voltage during 100% modulation can reach twice the unmodulated carrier voltage. To prevent exceeding the tubes' maximum voltage and current ratings during these modulation peaks, the carrier plate voltage must be significantly reduced (A). If the plate voltage were not reduced, the tubes would be subjected to voltages twice their normal operating level and currents four times their normal carrier level at modulation peaks, leading to damage. Furthermore, a 100% amplitude-modulated signal carries 1.5 times the average power of the unmodulated carrier (P_total = P_carrier + P_sidebands = P_carrier + 0.5 * P_carrier = 1.5 * P_carrier). If the plate voltage were not reduced, the tubes would have to dissipate this increased average power (and much higher peak power), potentially exceeding their thermal limits (C). Therefore, both reducing the plate voltage and acknowledging the increased power handling requirements are essential for proper AM operation.