Using 4CX250B's in parallel can be done easily enough. I have seen from
1 to 4 paralleled. Run them with about +2500 Vdc plate (OCV), +300
screen, and -30 to -70 vdc control with a -90 to -100 vdc cutoff. Once
you know the currents of the plate, screen and control, it's pretty easy
to lay out. Personally if it runs over two 4CX250B tubes, I'd use a
4CX800 or 2 of them. The 4CX800 is does not cost much more than a good
quality new Eimac 4CX250B and the socket is pretty cheap also. The
4CX250B's dont need the expensive air system sockets either. Just use a
straight ceramic socket and cut the chassis out to match a chimney. You
can save a good bit of money by doing this. Use a simple regulated
screen supply (variable), and a variable bias. The bias can be ran from
a simple zener with a potentiometer in series to ground. On receive,
throw the full -90-100 Vdc on the control with a relay and on transmit,
put the zener circuit in line. It is good practice to put a parasitic
suppressor in the control grid lead also. You can match the input with a
pi network or a tuned transformer, whichever you wish. If your going to
run a neutralization circuit, use the transformer design.
Is there some reason why there seems to be very few HF amps built around
the 4cx250b's? They seem to be cheep and very plentiful.
I am thinking of building an HF amp using 3 or 4 4cx250b's. Is there
anything wrong with doing this?
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