[Amps] RE: HF and 4cx250b's

[Will Matney]

Jerry,
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.

Will Matney

Hi all,

>
>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?
>
>
>Cheers
>
>Jerry