[Amps] SB-220 6 meter conversion parasites

[Steve London]

First, I want to thank everyone for their comments and suggestions, 
which were mostly sent privately. They were very helpful.

The parasites/self-oscillation is fixed. That simply involved changing 
the R in the parasitic suppressor to about 65 ohms. Still using copper 
strap for the L. Nichrome ribbon not required in my conversion.

Getting the amp to produce significant gain was an interesting exercise. 
Surplus Sales was sold out of the smallest value of capacitor to use for 
the tune control. I bought the next size up, knowing I would have to 
remove some plates. I ended up removing half of the stator plates. Not 
too difficult, but don't lose the ball bearings ! That still didn't do 
the job, and had to remove some L from the pi-network. Basically, it 
ended up looking like the April 2008 QST conversion with about 2 3/4 
turns of 1/4" copper tubing.

The input network was also from the QST article. Along with the original 
T/R relay, it still needs some tweaking, but the input SWR is acceptable.

The only other electrical change I made was the bias zener diode. With 
the original 5.1 volt zener, the resting plate current was 170 mA in the 
SSB position, which seemed high. I changed to a 5.6 volt zener, and the 
resting plate current is now 90 mA. Surprised it went down that much. 
Might be pushing it a little too much towards Class B or C now ?

The converted amp works great now - 70 watts input produces 1150 watts 
output in the SSB position.

If I was doing it over, one change I might make is to swap the tune and 
load capacitors, like the W1QJ King Conversions. That would reduce the 
lead length from the RF choke to the blocking capacitor.

Thanks again for the suggestions. I appreciate there are many different 
perspectives on this, and probably no one perfect answer, but many that 
work well enough.

73,
Steve, N2IC