Sat, 5 May 2001 15:20:37

Hi Dave,

Your web page was pointed out to me after I recently posted a question in the usenet newsgroup asking if anyone knew what a GL-2H21 was.  As a result of my experience with General Electric FM transmitters in the early 1960's I already knew what they were, and was just curious what people would come up with as answers.  I guess one thing I didn't know after all these years was the correct spelling of Phasitron, otherwise I would have found your web site sooner.

You mentioned the frequency control systems used in direct FM transmitters were a "kludge", and mentioned the RCA motor driven capacitor, which by the way was also used in RCA's early Television transmitters.  An interesting point is that the Western Electric FM transmitters used a very similar motorized capacitor design for frequency control, so it wasn't an RCA exclusive.  The RCA frequency control system actually used a number of dividers, so it was not from "the days before dividers and phase locked loops" as stated on your web page.  The Western Electric design may not have used any dividers, and may have been similar to a preliminary RCA design, I have an article on it buried somewhere.  The original pre war RCA FM exciter used an all electronic discriminator type AFC, as did RCA's last vacuum tube exciter introduced in the mid 1960's.  RCA's motor driven capacitor was replaced in the late 1950's by an all electronic phase locked loop system.  I think those old motor driven frequency control systems used by RCA and Western Electric are even cooler than the phasitron tube.

Your date of "sometime around 1960" for the serrasoid modulator is way late.  I believe the serrasoid modulator was introduced in the late 1940's, shortly after the phasitron came out.  It wouldn't surprise me to learn that they both were outgrowths of the war effort.  J.R. Day was the developer of the serrasoid modulator, and I have a paper from the late 1940's, or 1950, describing its operation, buried somewhere.  By the time I was involved in FM broadcasting in the early 1960's, it seemed every FM transmitter that I ran into, with the exception of the RCA's, and old Western Electric's, had been converted to use serrasoid modulators, mostly built by REL and Gates.  As you mentioned, virtually every maker of FM transmitters, except RCA, used serrasoid modulators in the early 1960's.  It was my impression that the serrasoid modulators were a "kludge" to convert to stereo, while the RCA system lent itself to a less complex stereo implementation.  I think as the 1960's progressed, and I lost track of the broadcast biz, most transmitters went to direct FM systems similar in concept to what RCA had used all along, but you would surely know more about that later period.

Another e-mail from John Byrns:

Sun, 29 Jul 2001 18:31:34

With respect to the date of introduction of the Serrasoid modulator, I finally located the photocopy of the J.R. Day article on the development of the Serrasoid F-M Modulator.  It is in the October, 1948 Issue of Electronics magazine on pages 72-76.  James R. Day worked for REL (Radio Engineering Labs) at the time the article was published.  The article includes a sidebar by E.H. Armstrong giving a glowing endorsement to the Serrasoid Modulator, and saying "I have always felt that the phase-shift method would be the surviving method".  Armstrong clearly felt the development of the Serrasoid Modulator had settled the direct FM vs. indirect FM issue for once and for all.  The station group I worked for had a number of these REL Serrasoid Modulators, which replaced the Phasitron Modulators in their GE transmitters, the Armstrong Dual Channel Modulators in their REL transmitters, and the "resistance" tube modulators in their Westinghouse transmitters.  While it was in the early 60's that I worked there, I would guess that the REL Serrasoid Modulators had been installed in the late 1940's time frame, around the time that the Electronics magazine article was published.  The date for the introduction of the Serrasoid Modulator appears to be the late 1940's, rather than "sometime around 1960" as it says on your web page.  I am expecting the arrival of some old 1940's issues of Electronics magazine any day now, and I hope this issue is included, along with the one on the GE transmitters referenced by Adler in his IRE paper.

With respect to RCA and their motorized capacitor AFC system, RCA didn't use that system until after WW2.  Western Electric first used the motorized capacitor AFC in their pre war FM transmitters, and also used it in their post war FM transmitters.  I have never actually seen one of the RCA FM transmitters with the motorized AFC, but have seen several of the Western Electric 10 kW transmitters with this type of AFC.  The RCA pre war FM transmitters used balanced reactance tube modulators, with a discriminator feeding an AFC correction Voltage back to the reactance tubes.  RCA used the motorized AFC in their postwar FM transmitters, which retained the balanced reactance tube modulators, but used the motorized capacitor to control the carrier frequency, in place of the earlier discriminator/reactance tube AFC.  In the later part of the 1950's RCA went back to an all electronic AFC system in the new BTE-10B exciter, using a chain of frequency dividers, similar to that in the motorized system, to drive a phase detector which provided the correction signal to one of the reactance tubes.  When FM stereo came out, RCA provided a modification kit for the BTF-10B that replaced the input transformer that drove the balanced reactance tube modulator, with a vacuum tube phase splitter that extended the response down to something like 3 Hz, and up high enough to accommodate a composite FM stereo signal, the wide bandwidth being necessary to keep the phase shifts within the spec. for the composite signal.  The instruction manual for this modification kit did warn about not testing the exciter with high amplitude low frequency signals, as the AFC could be driven out of lock with the extended response.  RCA claimed that their earlier post war exciter, with the motorized AFC, could also be modified for stereo in the same manner, I would really have liked to see that, if anyone actually did it.  In the mid 1960's RCA replaced the BTE-10B with the BTE-10C which was a vacuum tube exciter with varactor diode modulators, and a pulse count discriminator for AFC.  I think that the BTE-10C was RCA's last vacuum tube exciter.  I hope to get motivated some day to create a web page with photos and descriptions of these RCA and Western Electric direct FM exciters.


John Byrns

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