Monday, November 22, 2010

Harris Or Continental Transmitter For KAAY? Dave M. Elaborates...

I remember the long running technical discussions and internal debates that ultimately led to the decision to buy the Harris transmitter. The Harris 50kw transmitter model of the time used a hybrid solid state / vacuum tube technology, and it was generally felt that the Harris might not measure up to the RCA in terms of robustness and general audio quality. The Harris used a brand new method of modulating, something called Pulse Duration Modulation.

Continental also manufactured a line of high power shortwave transmitters that used a more "traditional" vacuum tube technology and modulation schemes. It was a difficult but necessary debate deciding how to invest a lot of money that would ultimately have to operate every hour of every day, flawlessly, for 20+ years. We debated this for months: Continental?, or Harris?. The decision was backdropped by the impressive performance history of the RCA transmitter - which had at that time been on the air for about 30 years.

Jim Loupas once told me that there were 13 of the RCA BTA-50F model variants built (I think the number is about correct), and the main reason they were being replaced with newer technology was the operating cost, and the fact that eventually, everything wears out no matter how well built it is. Some parts, including the 6600 pound modulation transformer could not be replaced if it ever failed. And the old RCA was also power-hungry when compared to newer transmitters. Translated: a new transmitter would bring lower overall operational cost.

When I finally left in January 1980, the decision had been all but made, and the wheels were turning to acquire the Harris. A spot had been made in the transmitter building that would allow the installation of the new Harris transmitter while the RCA remained on the air. I was gone by the time the Harris went on the air, so I can't speak to its performance or reliability - something that Hollis Duncan would be able to address.

In a couple of pictures on the website listed below you can see part of the Harris transmitter's front panel - it is the blue colored panel in the photos. (There are some neat pictures of Felix McDonald here in front of the Harris and RCA transmitters. bs)

Here's a tech note penned by Curt Lutz, a Harris sales engineer and acquaintance, writing about how the new Harris transmitter worked - this might be a bit boring to the blog readers, but reading this points to a significantly new way of doing things that departed from the traditional "high level" RCA transmitter technology.

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 toroid (coil) which is the load for that module; all those toroids are lined up and an iron pipe is run through these toroids 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 transmitter's output signal.

Thanks to Dave for this interesting information on the "heart" of KAAY.  We all grew up, loving that old RCA, didn't we?  Hopefully, we haven't bored you, dear reader and visitor.

Remember The Last Day?  David B. Treadway called to the transmitter site & asked Felix McDonald if the old RCA could be employed one last time.  Felix told him to wait a few minutes to let it warm up.  When the old RCA came on-line, everyone could tell the difference!

In later years, we have noticed that the transmitter power hasn't been like it used to; Jerry Sims/"Sonny Martin II" mentioned that when he came to Mobile, AL on business, he couldn't even hear the station.  I still occasionally tune there...about a week ago, they had a fair signal, but nothing like they used to.  Sometimes, the new technology isn't as good, even though it IS more efficient....

1 comment:

  1. During my tenure the Harris MW-50B performed very well and KAAY was never off the air because of a transmitter problem. The RCA BTA-50F1 was still attached to the antenna switch and there was no other standby.

    I attended MW-50 school in beautiful Quincy, IL, courtesy of KAAY. PDM is a nice system. So is the Doherty system that Continental uses and the Ampliphase system that RCA sold for many years. All of these are schemes to generate an AM signal at lower cost and all work just fine into a dummy load but not always so well when connected to a real antenna.