I don't think I'd go for just 2 - 3 or 4 is likely a better bet. A pair of them
are rated for about 2400watts PEP output in plate and screen modulated AM, so
provided they still have emission, running them at around 3000 or a bit over on
the plate should give some output. Cooling is simple, and I don't believe that
the IMD figures published in various places suggest that they're that bad. Run
them in passive grid AB2 with plenty of negative feedback, well regulated
(shunt regulator) bias and a regulated screen supply should give pretty good
results. Down side is that they are getting past it by 25MHz, getting cheap
ones with plenty of emission might not be so easy, and if they are going to get
hit hard, the you definitely want the carbon anode ones. And a lot of tubes
built in WW2 may well be more than a bit soft by this time - I'm damn sure most
of the 829s and 832s you see for sale will be. Many of the tubes made in WW2
were meant for a relatively hard but short life, so th
at they could be produced in quantity to a reasonable reliability spec without
going overboard on costs. For real fun though, run the 813s in Class AB/C in a
polar loop amplifier, and IMD of better than -50dB should be achievable.....
However, you could probably get some 4-400 or 4-250 pulls for similar prices,
and they are much more efficient at 25 or 30MHz.
73
Peter G3RZP
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