Yes, one might expect all of that to have been “old hat” by the 1950s. But perhaps not. Wireless World (WW) ran a two-part article by W.T. Cocking on “Modern Detector Circuits” in the 1939 August 10 and August 17 issues. The opening summary was:
“The characteristics and circuits of the most usual forms of detector are described in this article. Commencing with the diode, the author goes on to deal with the grid, anode-bend and negative feedback detectors, and points out their advantages and their limitations. In the following article, a special method of feeding the diode will be discussed and will be shown to offer important advantages.”
This article did not include mention of a diode direct-coupled to a cathode-follower. But counter-intuitively, the special circuit turned out to be an RF cathode-follower driving a diode demodulator. The argument was that a negative feedback (infinite impedance) demodulator, with its minimal loading on the final RF stage, could be useful for a communications receiver, where high-Q, relatively narrow-band IF transformers (IFTs) were required. But to take full advantage, the final IFT could not feed the separate AGC diode directly, but needed to do so via a suitable buffer stage. If that buffer were a cathode-follower, then conditions were created for the employment of a diode demodulator whose operating environment allowed it to perform in an optimum way.
Returning to the AWV Radiotronics articles, something to bear in mind is that whilst the northern hemisphere the emphasis at the time was moving to FM, that was not the case “down under”. FM did not arrive in Australia and New Zealand until the 1980s, so getting the best out of AM remained important until then. By the 1970s, not only was high quality AM equipment from the north quite scarce, but the AM sections of hi-fi tuners and receivers had generally deteriorated, particularly those from Japan Inc. (To be fair, there were exceptions, of which Sansui was probably the most notable.) Thus, local solutions were required. By way of one example, in Australia, Allen Wright produced a couple of wideband AM tuners that were intended to match or better the performance provided by the Quad AMII and AM3 tuners. Both used precision rectifiers for AM demodulation, and the second model also had a quasi-synchronous demodulator (MC1330). Of these, the designer later said:
“Both sounded like GOOD FM, just in mono, from the superb quality non commercial government stations. Compressed commercial stations sounded just that - compressed!” (The compression problem is not new – it has just got a lot uglier since the 1970s and 1980s.)
In the USA, the interest in better quality AM reception has waxed and waned over the years, but did not disappear entirely. Certain events have resulted in peaks of activity. In the late 1950s, experimental FM-AM stereo broadcasting resulted in the production of quite a few tuners with good AM sections. Then with the end, or at least extent limitation of FM and AM simulcasting, there was another upswing in AM interest the late 1960s. In the 1980s, the arrival of AM stereo increased interest in better quality AM. In the 1990s, the introduction of the NRSC curve and the AMAX standards was another high point.
Only the first of those “peak interest” events was pertinent to the valve era. I suspect that for the most part, the makers used diode demodulators. One example was the Heathkit PT-1 FM-AM tuner. This used a push-pull crystal diode AM demodulator, which was
AC-coupled to the AM cathode follower. However, the latter was of the bootstrapped type, so would have provided negligible additional AC loading on the demodulator. The overall AM circuit was quite sophisticated considering that it was MF only, with an RF amplifier, two IF stages, with full AGC on the 1st and one third AGC on the second, and a sidechain AGC amplifier feeding a diode-strapped triode AGC rectifier. But the mixer was a self-oscillating 6BE6
The late 1960s/early 1970s solid-state models appear to have used crystal diode demodulators. In the AM stereo era, synchronous demodulators took over. The Motorola C-QUAM decoding ICs used a mix of wideband quasi-synchronous and PLL fully synchronous demodulation, which disposed of many of the problems associated with diodes and other rectifying demodulators. Nonetheless, one can find examples of synchronous AM demodulation in the valve era, more in connection with professional equipment but occasionally in consumer equipment. (Synchronous (quadrature) FM demodulation was much more common in the consumer domain, particularly for TV intercarrier sound channels.)
Outside of the hi-fi arena, some of the American domestic receivers had better-than-average AM sections, albeit not of the wideband type. Several Zenith FM-AM valved table receivers from the late 1940s onwards had MF-only AM sections with RF amplifiers and two IF amplifiers. Mostly these used one diode from a 19T8 or similar for demodulation. That one RF, two IF configuration was sustained even with Zenith’s first FM Stereo-AM table receiver, the MJ1035W, where one might have expected the emphasis on FM stereo to have justified cutting corners on the AM side. The GE “Superradio” transistor portable was said to have a good AM section.
In the UK, the two hi-fi makers who stayed longest in the wideband AM tuner business were Chapman, until c.1969, and Quad, who exited in 1973. Both used diode demodulators, with the output taken from a fraction of the load (about a quarter in the Chapman case, and around one tenth in the Quad case) in order to minimize excess AC loading. The basic Chapman design dated from 1951. The Quad AMII was new in 1960, and quite a bit different from its 1953 Quad AM predecessor, although that had also used a diode demodulator with tapped-down output. The Quad AM3 of 1969 was essentially a repackaged, self-powered AMII with some detail changes to the AF output filters.
In the Chapman design, with its sidechain AGC IF, it would have been quite easy to use an infinite impedance demodulator. And a direct-coupled cathode-follower could have been added to either, avoiding the need for tapping down on the diode load. But at the time, their AF output levels, although low by today’s standards, were fine for feeding the amplifiers of the time.
Valve count insofar as it contributed to total cost was something of an issue, and no doubt mitigated against adding say a 6C4 as an infinite impedance demodulator or as a cathode follower, particularly if it offered little material advantage in situ. Back in the 1950s and into the 1960s, tuners attracted quite high purchase tax in the UK, whereas amplifiers did not. So, there was some inclination not to include buffer stages in tuners, but to use the amplifier input stages as such. And to take HT and LT from the amplifier, rather than a built-in power supply. The Chapman tuners were originally externally powered, with a self-powered option arriving in the mid-1950s, and late production being only self-powered.
Cheers,