Quote:
Originally Posted by Boater Sam
Here is another common continental valve that doesn't grace our UK shores.
PM84 Indicator, 300mA series heater chain version of EM84.
How come the continental TVs have indicators and not ours?
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I am not sure that tuning indicators were that common on European TV receivers. But I have the impression that the European setmakers were less occupied with corner-cutting than their British counterparts, and did offer some models that had various circuit refinements.
The TV tuning indicator did involve some complexity, in that the magic eye could not be fed direct from say the vision AGC line. Typically, a sidechain vision IF stage was required, with a relatively narrow bandwidth centred on the vision IF. But the tuning of this stage was not quite straightforward, as the tuning needed to adjusted to offset the effect of the Nyquist slope so that the peak was at the vision carrier and not displaced from it, and also was reasonably symmetrical about it. Then a rectifier was required to provide a negative DC bias to the magic eye.
Quote:
Originally Posted by FERNSEH
The PM84 was used as the FM radio tuning eye in the Ultra VR17-64 TV receiver.DFWB.
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I guess a corollary question is why not also use it on TV by taking a feed from the sound IF AGC line. In turn the answer to that is probably that the sound IF channel was too broad to provide a meaningful peak indication. AM TV sound IF channels had to be quite broad in any case in order to ensure proper functioning of the noise limiters, and often they were wider than that required to allow for oscillator drift. Even with FM receivers, the IF bandwidth was really too broad to provide a useful peak reading tuning indication, and one “fix”, as recommended by Mullard, was to put a small peak in the IF curve. But this approach was also deprecated by those interested in high quality, who usually advocated the use of some form of null-reading indicator. GE developed the 6AL7GT magic eye for this purpose just after WWII, but it seemed to be rarely used in the UK. The RCA UK New Orthophonic FM tuner is the only example that comes to mind. So maybe the 6AL7GT qualifies as an odd valve in a UK context. The 6AL7GT also had some use as a TV tuning indicator in the USA, driven from the FM discriminator output of split-sound receivers.
Cheers,