>What the DX50 has is several racks of plug-in modules, each capable of 250
watts output."
I installed a Harris DX-50 as my last project before exiting the broadcast
engineering profession in 1995. The DX-50 was designed by Hilmer Swanson, a
pioneer in solid-state broadcast transmitter design. The transmitter is
still operational here in Jacksonville on 690 kHz and is located at the site
of the legendary WAPE-AM. I ordered the DX-50 to replace a
Doherty-modulated Continental 317-C2. The 317 had seen many years of abuse
under a prior ownership and when we switched to the 50KW daytime site each
morning, we could no longer depend on it to come up reliably without a
myriad of problems.
One provision I included in the Harris contract stipulated that the DX-50
had to pass a 50 Hz square wave at 100% modulation for one minute. Even for
the DX-50 with no "classic iron," that's a rough test. As I recall, total
PA current approached 300 amps.
The DX-50 is capable of 150% positive modulation peaks but it never reached
the same glory as the homebrew water-cooled "Brennan" transmitter that
occupied the site from 1958 through 1993. It was designed by Bill Brennan
and constructed by his brother Cyril who recently became SK.
The Brennan used Machlett Labs peak and carrier tubes in a modified
Welden-Doherty configuration. The Brennan could, and did, do 200% at 50KW
carrier all day long in an era when "super-modulation" ruled the Top-40
airwaves. Of course, with that much positive peak modulation, distortion
becomes grungy when passed through a diode envelope detector, but audio is
beautiful when listening to more complex full-wave and synchronous
detectors. When I mention Machlett, I don't mean today's Asian knock-off.
I mean the real Machlett that created tubes at power levels even Eimac was
too timid to produce.
690 kHz in Jacksonville is located adjacent to the brackish waters of the
St. Johns River. Antenna height is 3/8 wave and the DX-50's daytime
reception at high noon is solid along the coast from Charleston, SC to South
Florida, and east to the Bahamas.
Paul, W9AC
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