[AMPS] SB 220 vice SB221

[km1h@juno.com (km1h @ juno.com)]

No need to repeat the long stuff from Steve but I feel that he made a few
points that I think are wrong.

1.  Once a sustained arc occurs in the Tune cap or across the bandswitch
it becomes a very wideband signal source and of course it will be heard
all over the spectrum. It is called a spark gap.  However that arc is not
a parasitic. It MAY have been started by the parasitic or something
entirely else. 

2. A true parasitic will take place with the amp terminated in a perfect
50 Ohm load...DC to light...since it never gets past the tank circuit in
the first place. Try it yourself; drive the pi-net from either direction
with a 120-200MHz signal and see what comes out the other end. 

3. What you see in the SB-220 and many other amps is what I call a
secondary reasonant circuit formed by the shorted turn effect of the tank
coil - bandswitch combination. Stray C and L set up this massive
secondary potential that is just looking for a path to dissapate thru. In
many cases you can de-Q this circuit by placing a low value HV snubber
cap across the bandswitch contacts. I have found 10-15pf to be sufficient
and across the 40-20M contacts right at the switch. Other amps with
shorting or non-shorting switches will require different cap placement
but I have yet to find a combination that does not work. 
The snubber will affect tuning a bit on the highest bands so it does
require a Tune cap with sufficient minimum C to compensate on 10M....the
SB-220 cap is OK.   

I may not have all the terminology correct since "I aint no durn
injuneer" but I do feel that the basic idea is correct.
I have run SB-220's with the wide spaced tune cap and an experimental
plate xfmr at  power levels of 1500-1800W with nary a trace of arcs once
the snubber is installed. This also assumes that the parasitic
suppressors are replaced with improved versions since those original 2W
carbons just cant take the 10-15M RF at those power levels. 

73   Carl   KM1H

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