[Heathkit] SB-220

[Jim Shorney]

Near the bottom of this page under the heading  "A Common "Bad Grid Idea" Super
Cathode Drive" you will find a good explanation of this issue. The bottom line
for me is that with the cap/choke arrangement you will have a circuit that is
parallel resonant at some frequency, which will place your grid above ground
for RF at that frequency. That can't be good. 

http://www.w8ji.com/vhf_stability.htm

Some people advocate grounding the grid through a small value (33 Ohms or
thereabouts) resistor to act as a grid "fuse" in parallel wiih a bypass cap.
I'm not completely convinced about that idea either. It seems like a bad idea
to float the grid - ever.

Eimac designed the tubes with direct grounding in mind. I like to think that
they knew what they were doing. I grounded the grids in my L-7 with strips cut
from a copper hobby sheet from Ace Hardware and it works very well.

73

-Jim
NU0C


On Fri, 31 Jan 2014 21:49:13 -0600, KA9EGW wrote:

>So here's this SB-221 [which probably should be called an "SB-211" in it's
>post-CBer butchered form.] that I am turning into a proper SB220 via input-
>and output-network implants and unbutchering of the other stuff..  Something
>I don't get, is what's the point of having 3x220pf and 1mH at each grid to
>ground?  If they should be grounded for both RF through the 660pF and for DC
>through the 1mH, why not just ground them directly with some strapping or
>braid?  One of the 1mH's grenaded at some point in this amp's life.
>
>73, Brian KA9EGW
>
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--
Ham Radio NU0C
Lincoln, Nebraska, U.S.S.A.
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