[AMPS] Grid Swamping resistor for 4CX250B

Ian White, G3SEK G3SEK@ifwtech.demon.co.uk
Tue, 30 Sep 1997 19:26:03 +0100


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

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