The Harris MW-50A AM Transmitter
Information provided by Curt Lutz

This transmitter employs Pulse Duration Modulation. One of the 4CX-1500 tubes serves as a driver for the 4CX-35000 Modulator tube. The other 4CX-1500 tube is the driver for the RF Output 4CX-35000 tube. The rest of this transmitter is totally solid state -- in fact, later versions of this MW-50 Transmitter replaced those two 4CX-1500 tubes with solid-state amplifiers, leaving only the two 4CX-35000 tubes.

The modulation scheme is a Harris patented system, using a 70-kHz oscillator to generate pulses (square waves), which are varied in width (or duration) by the audio input signal. These 70 kHz square waves are used to turn the modulator tube on and off at a 70 kHz rate, but with varying pulse widths. Since the final RF Amplifier and the modulator tube are in series, the final amplifier is actually turned on and off at that 70 kHz rate -- with the varying pulse width amplitude modulating the RF output of the transmitter. This type of modulator, since it is actually an electronic switch, is called "Class D" operation. Most transmitters of this vintage used conventional high-level modulation, and the modulator section often used more power from the AC mains than the RF section of the transmitter (after all, the RF output amplifier was operating as a class C amplifier at around 60 to 70% efficiency, while the audio modulator section would have been operating in class AB1 or AB2, perhaps at only about 30 to 35% efficiency). Since the two 4CX-35000 tubes are connected in series, and the modulator tube is operating in class D (switching mode), that high power modulator is even more efficient than the Class C output tube.

The output of the RF amplifier tube will contain the 70 kHz signal, so there is a special filter circuit in the output network of the transmitter to remove virtually all of that 70 kHz signal (otherwise this type of transmitter would create some sidebands about + & - 70 kHz from the carrier frequency), which would be extremely illegal, and would create a mess over most of a bandwidth of + or - about 75 to 80 kHz when the transmitter is modulated.

The newest Harris AM Transmitters of 10 KW and higher (up to 1 megawatt and more) are totally solid state and use a completely different modulation system called Digital AM Modulation. This scheme is another Harris Patented system, using a large quantity of plug-in modules, each one generates RF at a different modulation level; in order for this to function, any incoming audio (if analog) is broken down into digitized data, then used to drive the various modules at the varying levels needed to have an amplitude modulated output. It sounds pretty simple, in fact, each module has a torroid (coil) which is the load for that module; all those torroids are lined up and an iron pipe is run through these torroids so that the combined output of all the modules is coupled into this pipe. One end of the pipe is at ground, the other end is the RF output of the transmitter. There is an output network to match impedance of the pipe, which is probably only a few Ohms, to the required output impedance which is usually 50 Ohms, although sometimes high power transmitters are set up to provide 75 Ohms, once in a while even 300 Ohms, to drive an open wire type transmission line system. This type of AM transmitter has an overall efficiency (power line in versus RF out) of around 85 to 90%. That is better than most high power FM Transmitters, which can also be very efficient, as they do not waste any significant power in the modulation scheme because they do not have to vary the amplitude of the transmitters output signal.