Friday, December 10, 2010

Hollis Duncan Comments, Re: The KAAY Harris MW-50B

Ah, the filthy. maladjusted KAAY Harris MW-50B. This sure brings back memories. And at least a few nightmares.

The Harris MW-50B was reliable and never failed on my watch. This alone is remarkable because the MW-50B was the filthiest transmitter that I ever saw and had been completely misadjusted by my predecessor (an unlicensed engineer). Nonetheless, it just kept chugging along.

When I arrived in 1982, KAAY was sounding pretty rough. The MW-50B transmitter was filled with soot following a power transformer fire. That story has been told elsewhere on your site. Because I only had access to the transmitter between 1 am and 4:30 am on Monday morning, I spent the first four weeks cleaning the transmitter. It took many buckets of distilled water and ammonia (used because it doesn't conduct electricity and its absolutely wonderful smell) and I had to get a fresh bucket after a couple of wipes with a wet sponge. It was (finally) reasonably clean but all of those white wires will never be anything but a dull grey.

While I was cleaning this poor beast, the audio monitor was tuned to the studio signal. I kept hearing white noise rising and falling in the audio feed and soon realized that the KAAY studio-to-transmitter microwave link had a noticeable fading problem. That would lead us to the story of the infamous Tower Building KAAY-KLPQ microwave relay site. But that's a story for another time.

Here's my analysis of one of the secrets of the classic KAAY Sound.

The KAAY RCA BTA-50F was a high-level (plate) modulated transmitter, meaning that the audio and RF are combined at the output terminal of the final RF tubes. As noted, the biggest problem with a plate-modulated transmitter is the physical size and weight of the transformers. Plate-modulated transmitters can be very stable and can sound very good. The KAAY RCA was probably as good a plate-modulated transmitter as was ever produced. The MW-50B couldn't match it for several reasons.

The biggest difference between the RCA BTA-50F and the Harris MW-50B was that the Harris was installed, while the RCA was engineered as a part of a complete RF system. To explain, a little KTHS engineering history is in order.

Consulting Engineer A. Earl Cullum of Dallas designed and constructed the KTHS-KAAY array at Wrightsville, Arkansas. Mr. Cullum had been the KTHS Consulting Engineer since its days on the Arlington Hotel. Mr. Cullum was the top AM Radio Consulting Engineer in the United States and he was in active from the 1930s well into the 1970s and was still active when I met him in 1977.

Mr. Cullum designed the KTHS-KAAY antenna system to provide a proper RF impedance match both at the carrier frequency (1090) and at the sideband frequencies (1080 and 1100, or 10 kHz either side of the carrier frequency). If the antenna system does not present a good match at both of these sideband frequencies, then the station will sound crummy and no amount of audio processing will help.

To complicate matters further, this sideband impedance match must occur at the plate of the final RF tubes in the transmitter. Mr. Cullum knew this and engineered the KTHS-KAAY array to provide a good match to the output tubes of the RCA transmitter during both daytime and nighttime operation. And that is one of the underlying secrets to the classic KAAY sound.

To give an example of how finely-tuned the KTHS-KAAY RF system was, I removed the Common Point RF ammeter for calibration and the removal of that small inductance put the KAAY nighttime antenna system out of adjustment. If I hadn't seen it I wouldn't have believed it.

As an aside, Mr. Cullum engineered a virtually-identical system at KOMO-1000 in Seattle, complete with an RCA BTA-50F and a 3-tower nighttime array.

Unfortunately, Mr. Cullum's engineering did not produce a good sideband match for the KAAY Harris MW-50B. The MW-50B RF output was fed through 50 feet (or so) of rigid coaxial cable to a switch near the output of the RCA transmitter. This additional length of coax, when added to the MW-50B output network, provided a poor match at the sideband frequencies and made it impossible for the MW-50B to achieve full effective modulation.

To make matters worse, my immediate predecessor (an unlicensed engineer) had adjusted virtually every available control on the MW-50B. It took me another month of Monday mornings to restore the MW-50B to its factory settings.

Although I had tuned the MW-50B with an intermodulation distortion analyzer, Veteran Multimedia Engineer Terry Baum came to town and and we used his magic audio spectrum analyzer to tune the MW-50B for lowest distortion. We discovered that every control on the transmitter affected the distortion level. By the time that we were through, the MW-50B sounded very clean, although still not particularly loud.

That was the best that I ever was able to get KAAY to sound. Because my successor decide that his first act of business should be to retune the transmitter, that sound lasted about two weeks, and KAAY never sounded that good again.

All of my work at KAAY was a complete waste of time. By then (as it soon became apparent), Multimedia had decided to sell KAAY and had given it (and KEEL-710 in Shreveport) over to a corporate lackey to play with. When the aforementioned lackey ordered me to fire Felix McDonald and Tom Rusk and I refused, I realized that my tenure at KAAY would not be long. But in terms of an experience, KAAY certainly was one.

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