[Amps] tubes

[Colin Lamb]

The 4CX250B is a 250 watt ceramic tube based upon the older 4X150 design.  It was not designed for grounded grid service.  Like the 4CX1000, the grid is fragile. 

In the 4-1000, the grid is so stout that you can simply connect the grids together and ground them.  If you do that you will destroy the tube on the first transmission.  You can use cathode drive, but the grid and screen must be at their normal value.

You can hang a low value resistor on the 4CX250B grid and drive the grid, but the tube is not great on distortion so it should only be done if you have some sort of feedback.  Cathode drive has an advantage in that there is some inherent feedback.  

With the supply of Russian tubes available for grounded grid, there is not a good reason to use the 4CX250B tube in a high power low band amplifier - except for the ham that has either no funds or no source of other parts.  They are still useful, however, as an amp for a low power transmitter (like 5 watts).

They used to be useful on vhf, because they work fairly well at vhf and low uhf - but the newer Russian tubes have much more dissipation that it hardly makes sense to make a high power amp using these old relics.

Colin  K7FM

-----Original Message-----
>From: "carl s." <carlseye at tampabay.rr.com>
>Sent: Apr 3, 2006 1:20 PM
>To: Amps at contesting.com
>Subject: [Amps] tubes
>
>      Who can tell me about the 4CX250B tube.???
>    power ? GG. capabilities, Etc.:: (hf only )
>      just want to know !!!!!
> carl / kz5ca
>
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