KCBS Transmitter Site

Inside the KCBS Transmitter Building

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KCBS AM Harris MW-50 TransmitterKCBS AM Harris MW-50 4CX35000C Transmitting Tube

Here is one of the transmitters being replaced. This is the MW50. Until recently, it was our backup transmitter in case we need to work on the DX50. The MW50 was purchased sometime in 1972 and was installed in 1974. It was a breakthrough at the time for AM transmitters. It used PDM, Pulse Duration Modulation, to modulate the transmitter instead of a big 25,000 watt audio amplifier as previous 50,000 watt transmitters used. As a consequence, the transmitter was much more efficient and used less AC input power to make 50KW. It also had lower distortion than other 50KW rigs made at the time. The picture on the right shows one of the output tubes used in the MW50. It is a 4CX35000C. There were two in the transmitter. One was the RF power amplifier and one was the PDM tube. These are pretty big tubes. They stand about 18 inches tall and weigh in at about 50lbs.

On a personal note. In 1973 when I was a senior in high school, some friends and I dropped in at the KCBS transmitter site. The old General Electric BT25A transmitter was still on the air and the engineer showed us the new Harris MW50 sitting in a corner awaiting install. Little did I know that 34 years later I would be overseeing the removal of that transmitter.

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Updated 01-14-08

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