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