[Amps] tubes

[Bill Fuqua]

At 10:04 AM 4/4/2006 -0700, R; Measures wrote:

>On Apr 4, 2006, at 6:56 AM, Bill Fuqua wrote:
>
>>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.
>
>With the grid wired to the cathode would there not be zero grid current?

There are two grids. One connected to the cathode and one to ground.
The screen grid becomes the control grid. But the geometry of the 4cx250 is 
such
that electrons have a tough time hitting the screen grid due to the careful 
alignment
of its elements to the control grid.


>>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 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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>>>
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>>
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>
>
>
>Rich Measures, 805.386.3734, AG6K, www.somis.org
>