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KPRC Houston TX & KROF Abbeville LA circa 1949

k5jri radio

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While cleaning out and reorganizing old files at KROF Radio (1KW, 960 KHz AM), Abbeville LA, I ran across a letter from KPRC Radio (5KW, 950 KHz AM), Houston TX dated sometime in 1949. I don't remember the name or title of the sender, but it was likely the Chief Engineer or the General Manager of KPRC Radio. In the letter KPRC stated they were receiving adjacent channel interference from KROF, which is more than 200 air miles away from the Houston area. The letter goes on to say that, in their technical staff's opinion, the problem was likely due to improper adjustment of the high-efficiency Doherty amplifier circuit in KROF's Western Electric transmitter. The more common plate (anode) modulated AM transmitters operated with only moderate overall efficiency (RF power output divided by AC line power consumed), but their tuning was not critical and unlikely to produce adjacent channel interference unless over modulated. By contrast, the high efficiency Doherty amplifier tuning was critical and required careful measurement techniques with an oscilloscope to produce a proper output without spurious, off-frequency interference.

In another letter (dated circa early 1950s) from a Consulting Radio Engineering firm (probably duTreil & Associates), the writer stated that Western Electric transmitters, once they had a few years of age, "gave lots of trouble."

The 1936 Doherty patent expired in 1953, the same year Western Electric closed down its broadcast division. In 1958, James Weldon modified the Doherty amplifier circuit for improved stability by operating the the carrier tube as  a grounded grid (cathode driven) triode amplifier, and the peak tube as as a grounded cathode (grid driven) triode amplifier. Unlike the earlier Western Electric designs, the Weldon design in Continental Electronics 50 KW AM transmitters seldom required adjustments and was extremely stable and reliable. The Continental 317B installed at KEEL Radio, Shreveport LA (50KW day, 5KW night) in 1962 was typical of Continental Electronics products of that era.

Edited by k5jri radio
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Addendum concerning 317B: Aside from its long term stability and reliability, the Continental 317B transmitter was very compact (by 1962 standards) for its 50 KW power output. Everything but the final amplifier blower was self contained inside a set of cabinets only 12 ft. wide x 6 ft. deep x 6.5 ft. high. Its two 6673 final amplifier tubes weighed 29 lbs, each. By comparison, the RCA 50KW transmitter (installed 1949) at nearby KWKH had a set of cabinets 36 ft. wide with an external concrete vault (for fire safety) containing the 3 plate supply transformers, plate supply filter choke, modulation transformer, modulation reactor choke, and probably several other large items outside the cabinets but not in the vault including blowers, circuit breakers, etc. Its four 5671 tubes (two final amplifier tubes plus two modulator tubes) weighed 228 lbs. each.

The next Continental iteration of the the 317 series, the 317C had two 4CX-35000C tetrode tubes serving as screen modulated peak and carrier final amplifiers. The 317C was the largest selling and arguably the ultimate 50 KW tube transmitter.

The next RCA iteration of their 50 KW transmitter product was based on the 1936 "outphasing" modulation system invented by Chirex and marketed by RCA as "Ampliphase." This designed eliminated the modulation transformer and modulation reactor choke, and required only two 5671 tubes. Later 50 KW "Ampliphase" models had two 6673 tubes.

Beginning in the 1930s, Mexican licensed "border blaster" AM radio stations operated from just across the border separating Mexico from the United States. One of these stations, XERF, Ciudad Acuna, Coahuila, Mexico (250 KW on 1570 KHz) installed a 250 KW "Ampliphase" transmitter circa early 1950s at its transmitting site across the Rio Grande from Del Rio, TX. The former XERF, now owned by the Mexican government, currently operates as the Spanish language station "La Poderosa" (100 KW on 1570 KHz).

RCA also manufactured 5 KW and 10 KW "Ampliphase" transmitters. KNOE Radio, Monroe, LA  (5 KW on 540 KHz) installed a 5 KW "Ampliphase" transmitter circa 1971 and promoted themselves on the air as "the sound quality leader of northeast Louisiana." 

Edited by k5jri radio
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