I feel that we should remember that the 'grandfather' of this series was
the 4X150, which was a 1945/6 design, intended for Class C service, and the
4X250 followed a few years later. (My 1946 ARRL handbook lists the 4X150 in
the Eimac advert, and the 250 is advertised in the '52 book - I'll have to
look and see what's advertised in the 1949 one). This series wasn't really
designed for SSB use - unlike the 300 (or was it 350 series?). It was in
the late 1950's/ early 60's that Marconi started using them in large
quantities in distributed amplifiers, and even then, they tended to be
selected ones. I certainly knew a guy who had been at the M-O Valve company
when they could not produce a 4CX250B that worked in the Marconi amps, but
was more than good enough for all the other applications. A story I've
heard, but don't know if there's any truth in at all, was that 4CX250Bs
were in use in other equipment that the Royal Navy were using, so they
wanted commonality, and as they paid for the development (NT203/4
transmitters) and the H1100 1kW wideband amp, that was why 4CX250B s were
used - 16 of them per amplifier, for 1 kW PEP up to 27.5MHz. The UK
Ministry of Defence was a very good customer of Eimac's! Even a small ship
could be using 64 of them for the HF transmitters alone.
The H1100 distributed amplifier had its funnies. At 28.5MHz, the delay
lines were known to catch fire...........and it needed somewhere around
7.5kW out of the 3 phase mains for 1kW PEP. It had a big blower, as you
might guess.
But they are relatively cheap and available, and limiting them to around
200 to 250 watts output, will be reasonably linear.
I suppose I ought to get a new ARRL handbook.........
73
Peter G3RZP
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