Be that as it may in overall terms, the arrival of FM was used as an excuse for some of the equipment makers, as it were, to “demote” AM (of the MF and LF variety, that is) and “cut corners” on the AM sections of combined FM-AM receivers. Delayed AGC was one of the early casualties, which one may attribute to the widespread adoption of triple diode-triode valves to do both the FM and AM demodulation jobs – these did not have the extra diode required for either a separate AGC rectifier with included delay or a separate AGC delay diode. And in the UK case, one may make a rather extreme comparison between say Murphy’s late AM-only receivers, the A186 and A188C, which had bandpass input tuning, variable selectivity and 9 kHz adjacent channel notch filters, with the less-well-specified AM sections of its subsequent FM-AM receivers. There is an irony there. With the early FM-AM receivers, the setmakers probably paid more attention – within cost constraints – to the distortion and bandwidth performance of the amplifier sections, and also to the speaker and cabinet interaction and performance, as compared with preceding “run-of-the-mill” AM-only units, in order to get the best out of FM. AM RF side performance was going backwards whilst the AF side was improving.
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Another oddity was that in the USA, in the late 1950s, in the run-up to stereo broadcasting, the NSRC (National Stereo Radio Committee) considered stereo systems for both FM and AM, with quite a bit of work done on the latter by RCA amongst others. (In fact, TV sound stereo systems were also within its ambit.) But the corresponding FCC docket related only to FM stereo. This looked somewhat like an institutional bias in favour of FM.
In respect of the UK VHF-AM proposal, for the comparative AM vs FM tests, Ambassador Radio designed and built a number of FM-AM “Comparator” receivers to the BBC’s specification. This receiver did not look to be particularly constrained on valve count, and was required to have a low-distortion AM section. It is easy to see it as an ideal case for the use of an infinite-impedance demodulator. This could be tapped off the IF strip ahead of the FM limiter whilst imposing negligible load, and AGC could be taken from the FM 1st limiter grid, as was done for some FM-only receivers. But Ambassador chose not to do it that way. Rather it used the diode formed by the grid and cathode of the 1st limiter as an AM demodulator, claiming that the clamping of the limiter valve anode and screen potentials (by voltage dividers) improved diode linearity. The post-demodulator circuitry was arranged to provide an AC-to-DC load ratio of 0.998. Delayed AGC was provided by a separate diode fed from the anode of the 3rd IF stage. Here is the AM demodulator circuit:
It is almost as if the infinite impedance demodulator was generally avoided if the makers could find other ways of getting the desired results with diodes of some kind.
Some of the early UK FM tuners, from c.1953, are known to have also had provision for VHF-AM reception, including those from Chapman, Lowther and Sound Sales. The prototype Quad FM of 1952 might have had this facility, but that is unconfirmed. What form of AM demodulators they had is unknown. Later, in 1958 Jason used the limiter grid-cathode diode as an AM demodulator in its JTV FM and (AM) TV sound tuner.
Staying with AM demodulators, this Murphy item from 1949 discusses (AM) demodulator distortion and address the AC/DC load ratio issue, but all of this is in terms of the diode type, there being no mention of other types such as infinite impedance.
Cheers,