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The ACM3 and early Band I TV Transmitters in the UK

by Ray Cooper, April, 2018.
    
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A GEC ACM3

Ray had first hand experience of the early post war transmitters and writes:

Alexandra Palace used high-level mod of course (the Crystal Palace replacement didn't) and, more to the point, so did Holme Moss. The difference between the two was that Alexandra Palace was very much a fore-runner of the Sutton Coldfield design, and used enormous water-cooled transmitting-type valves in its modulator, instead of the much smaller and more manageable ACM3s as used at Sutton (and, incidentally, also Holme Moss). The big difference between Sutton Coldfield and Holme Moss was that the Modulator at Holme Moss was arranged as a shunt-regulated cathode follower, much more efficient than the brute-force straight cathode followers used at Alexandra Palace and Sutton Coldfield (and, incidentally, a great more difficult to set up properly, too).

ACM3s were used in such small quantities - they were only used at Sutton Coldfield and Holme Moss, I believe - that they were all hand-made, a production line for such numbers being uneconomic. During the early 'seventies, I think it was, suddenly new ACM3s arriving from GEC started failing prematurely. An investigation was launched, and it was discovered that the old chap who wound the grid structures had retired, and nobody else knew how to do it properly. They had to get him out of retirement to show others how it should be done.

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