Quote:
Originally Posted by regenfreak
I didn't know that 6CS6 heptode ended up in FM demodulation application. In fact, I built my first MW superhet using a 6CS6 as a frequency convertor.
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There’s an interesting story there. Zenith and GE developed the 6BN6 for the FM job. Then Zenith found that it made an excellent noise-gated sync separator for TV receivers, except that it was rather costly. The existing 6BE6 radio frequency changer valve was found to do a reasonable job, so was used instead. But the market was big enough to justify the development of sharp cut off heptodes for the job, and that resulted in the 6BY6 and 6CS6. These were also intended for use as colour subcarrier synchronous demodulators in colour TV receivers, but that was a much smaller market at the time. In the usual manner of things, both of these valves found application in HF receivers as mixers, I think (but I am not sure), the 6BY6 by Drake and the 6CS6 by Heathkit. In Europe, Philips used first the ECH81, then the ECH83 for the noise-gated sync separator job, eventually developing a dedicated valve, the ECH84. But when it wanted an FM quadrature demodulator, for some reason it chose the 6CS6, renamed as the EH90. Zenith and GE went on to develop the 6BU8 combined noise-gated sync separator and gated AGC generator for TV applications. Mullard used the 6BU8 in at least one FM stereo decoder circuit, and the ECH84, alternatively the EH90 in others. All of that is evidence in support of the “circular flow of valves” theory.
By the way, notwithstanding that Zenith and GE had worked well together on several prior projects, the Zenith-GE FM stereo broadcasting system was not the result of a co-operative development project. Rather, it was the FCC who merged what were two similar proposals into one. I think that there was a little cross-sniping between the two afterwards.
Re the FMT1, Radford gave the RD bandwidth as 1.5 MHz.
And here is the quieting curve:
Cheers,