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[Amps] RE: HF and 4cx250b's

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Subject: [Amps] RE: HF and 4cx250b's
From: Will Matney <>
Date: Mon, 19 Jul 2004 00:46:21 -0400
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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?



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