[Amps] tubes

R at contesting.com R at contesting.com
Tue Apr 4 13:04:04 EDT 2006


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



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