.....
> 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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