[Amps] HB amps

[Peter Chadwick]

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 that 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