[AMPS] Grid Swamping resistor for 4CX250B

[km1h @ juno.com]

On Tue, 30 Sep 1997 19:26:03 +0100 "Ian White, G3SEK"
<G3SEK@ifwtech.demon.co.uk> writes:
>Walter Dail wrote:
>>Hello All,
>>  I have a question regarding using a grid swamping resistor
>>on a tetrode amp. I have been building a twin cavity amp for 
>>432 Mhz using a 4CX250B tube. I understand that instead of 
>>neutralizing a grid driven amp a grid swamping resistor may be used. 
>>These are found in all the handbooks, etc. showing 2m amp
>>schematics. However, in the 1985 ARRL Handbook, page 32-8,
>>"A Tetrode UHF Amplifier" it does not show such a resistor
>>in the input compartment. They are using 10W drive also.
>>But I noticed in the grid PS they have a 100 ohm 2W resistor in the 
>>line that feeds the grid bias. Is this what they are using? What 
>gives?
>>Is the Handbook wrong and a grid swamping resistor should be added?
>
>Grid swamping is usually an HF or VHF technique, but it should not be
>needed with 4CX250Bs at 432MHz. For example, the K2RIW stripline PA 
>uses
>no grid swamping and is quite stable with the correct sockets (SK620A)
>and in the absence of any stray feedback paths. It isn't necessarily
>perfectly neutralized, but it is stable.
>
>In class-AB1 operation of a K2RIW, the input match is excellent with 
>no
>grid swamping at all. Something like 7-8W of grid drive is typically
>absorbed by a combination of circuit losses and power feedthrough.
>
>It is possible to screen-grid neutralize the tubes by inserting PTFE
>sheet between the socket contacts and the screen ring on the tube, so
>that fewer contacts are operative. This increases the inductance from
>screen to ground and series-resonates the RF path. K1FO describes this
>technique for control-grid neutralization of GG 4CX800 PAs for 432 in
>the ARRL UHF/Microwave Projects Manual (the grey book, not the black
>Experimenter's Manual).
>
>
>73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
>                          'In Practice' columnist for RadCom (RSGB)
>                           http://www.ifwtech.demon.co.uk/g3sek


The 4CX-250 family rarely requires neutralization above 2M in a good
layout design.
The 4CX1000A should always be neutralized at any rated  frequencies as an
added safety precaution to the 0 Watt  Ig rating. 

Adding a swamping resistor is likely to cause as many problems as it
cures at 70cm. There is no such thing as a non-inductive carbon comp
resistor at those frequencies and any L must be dealt with. 

The 100 Ohm resistor in the bias lead takes the place of a RFC and can
enhance stability over an improperly selected RFC. It does require very
careful consideration of the bypass cap. Some use a series combination of
the resistor/bypass  at the socket and then the RFC at the grid
compartment feedthru. 

GL   Carl  KM1H

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