If you have a lot of drive you can connect the control grid to the cathode,
ground the screen grid and run grounded grid. The linearity will put high
mu triodes to shame since there will be very little gird current. The power
gain will only be about 5. For 500 watts output you will need to run two
tubes in parallel with about 100 watts drive. For more gain in cathode
driven mode you can apply the same grid biases as you would in grid driven
operation and RF ground the grids. This will produce less IMD than a pair
of grid driven tubes due to the inherent negative feedback in the cathode
driven amplifier.
73
Bill wa4lav
At 02:13 PM 4/3/2006 -0700, Colin Lamb wrote:
>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@tampabay.rr.com>
> >Sent: Apr 3, 2006 1:20 PM
> >To: Amps@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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