Saturday, August 29, 2009

Ron Henselman: And You Thought You Had a Big Stereo ?!

One might have a difficult time trying to imagine what it was like being around a transmitter the size of the one used at KAAY, and that includes me. I've never had my fingers in anything larger than five kilowatts.

Clyde Clifford stated the KAAY final RF amplifier section consumed 80,000 Watts when no audio (modulation) was applied. The transmitter used the best sounding form of modulation available during the time period in which the transmitter was built: plate modulation. If one wishes to modulate that transmitter to the full legal limit, then 40,000 Watts of audio is required. (And I thought my 200 Watt stereo could shake the walls!) If one were to use more audio, the transmitter would splatter over into the adjacent channels.

If the final RF amplifier consumed 80,000 Watts from the power supply and delivered 50,000 Watts RF to the antenna, then where did the other 30,000 Watts go? The answer is it was wasted as heat. This is also true for the modulator/audio section. All of the other stages in the transmitter contributed to the total amount of heat generated by the transmitter. (And I thought my little electric space heater did a great job heating up a room!)

The type of tubes used in a power amplifier section of a transmitter such as the BTA-50F don't even resemble the vacuum tubes most of us have seen in consumer electronics. Many large tubes are built with the anode (plate) area exposed, and they have fins which air is forced through for better cooling efficiency.

Many of us have our fantasies about playing with these giant transmitters, but very few of us ever get close to one. My fantasy was to be close to the modulation transformer in the old border blaster, XERF (250 KW), when they played the song "Have I the Right" by The Honeycombs. The big bass drum of Honey Lantree was combined with the rest of the group stomping on a staircase to increase the booming bass sound. I can imagine that the iron laminations in that transformer vibrated and shook most of the transmitter building! I've built a few transmitters that had 500 Watts of audio, and it was a weird feeling to feel the floor shake when I spoke into the microphone --- just think about 50,000 Watts! I'm 62 now, and I've come to one conclusion: some of us technical people never grow up.

How many readers have actually had the honor of repairing one of those 50 kilowatt monsters? Many of us had the required license to work on these units, but the chief engineer would never let us touch the insides. Sometimes we were there only to fulfill an FCC legal requirement....

---Ron Henselman

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