[AMPS] Grid Swamping resistor for 4CX250B

[Shawn Tayler]

AT this frequency you might be better off trying to use the screen to give you the isolation from plate to grid.  
Several broadcasters do this with big tetrodes on the FM band.  There is a design using a 4CX1000 on 2M in 
one of the handbooks that uses this approach.  It complicates the metering and power supply setup but it can 
work.  

The swamping resistor doesn't always work.  We had a devil of a time getting a 4-1000 to settle down at 6M.  
Turns out the problem was a resonance from the plat tune cap thru the plate to the grid.  The resistor actually 
helped it, at 110Mhz.  We fixed this one by using a stainless steel strp from a hose clamp, the slots added a 
small amount of inductance that swamped the 110Mhz energy real nice.  Grid resistors are more for matching 
than helping stability, at least in my experience.  They do help but they are not the end all fix.



On Tue, 30 Sep 1997 09:36:55 -0400, 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?
>
>Comments please,
>Walter Dail -KT4JA
>
>
>
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de Shawn
 N7LQ (formerly WB7TDI)

DM09dn & DN01fa



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