B&W 850A taps are 0.8, 1.0, 1.75, 6.5, and 13.6 uh per catalog values. I can
not seem to locate my actual mesurements. Maybbe John can find his. Catalog
also states VDC 2000-4000, RL 2500-5000 ohms, output impedance 50-75 ohms,
capacity to resonate each band: 50, 55, 70, 80, 150.
As John says the coil is not tapped correctly. Tru for use at a single V/I
ratio if one looks at the RL charts (ARRL handbook has Pi charts). At
RL=5000 will need high Ep and low I.
My 4-1000A built years ago was a single band (had plug in coils, untuned
cathode), had 6KV noload and about 5300 load (.95A). Plate would get orange,
used LOT of air to keep cool. That amp is now pieces I still have here.
I have seen the Calif KWs where a 4-1000A was used at 9KV at just under
2A. A supply of tubes was very plentiful and cheap then. In 1968 at CA DX
convention a 2x4-1000A amp was shown in slide presentation (though it was
from Oregon, so CA does not all lthe credit).
If the B&W 850A is to be used with 2x3-500Z then it most certainly must be
retapped. Lets say 3500VDC at .8A (thats 2800 Win). At 60% efficiency thats
1680 Wout. RL (assuming k=1.8) would be (3500V)/(1.8)(.8A)=2430ohms. ARRL
handbook shows coil taps reqd as 0.98, 1.77, 2.75, 5.5, 10.4 (Q is high on
10M but about 12 on other bands).
This is just published info to give an idea of stated values. I did not go
through teh calculations and not anywhere as technically knowledged as a
great many on here. But these values are a starting point. Use the B&W 850A
but retap it for your needs, it has nice switch.
73 de Arne N7KA
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