[AMPS] Grid Swamping resistor for 4CX250B

[Rich Measures]

.....
>  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. 

-  A grid termination/swamping R [GTR] always improves stability.  An 
open grid is an invitation to oscillation.  

>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. 

If the cold end of the 100 ohm grid resistor is bypassed for 432MHz, and 
there is virtually no L between the resistor, the grid, and chassis 
ground, then the grid would be terminated by the resistor.  (note: a 
decent 432MHz bypass is a 25 - 50pF doorknob capacitor. [sic]   More C is 
not better.) 

>Is this what they are using? What gives?
>Is the Handbook wrong and a grid swamping resistor should be added?

-  IMO, part of the problem is Newington's drafting department.  When I 
was writing 
*QST* articles, I would send in schematic diagrams that I checked for 
errata.  However, when the drafting department redrew a diagram they 
occasionally Added errata.  It was frustrating, Walter.  My one-time only 
(thank you) experience of writing for *QEX* was even more frustrating.  
No less than 6 errors were introduced into the published version of the 
article.  When I informed Newington that there were 6 errors in the 
published version, they said they were sorry and that they would correct 
them in a future "Strays".    They did not. 

Rich---

R. L. Measures, 805-386-3734, AG6K   


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