Noel,
I just sold my SB-220 converted to 50 MHz. It worked very well - 100W in
from a IC-756 resulted in 1000W out with the tubes showing no color.
Efficiency appeared to be around 50%.
In a nutshell:
1. Replace the place tuning cap. I used a Johnson 50 pF/.125 spacing unit.
Surplus Sales of Nebraska has a 9-38 pF unit that is looks identical to
the part I used.
2. New parasitic suppressors with only 1-1/2 turns.
3. New plate tank coil: 1-3/4 turns of 3/16 tubing, 1.5" ID, 1.3 inches
long.
4. New input pi-network: 2-1/2 turns on a 5/16 slug tuned coil, 39 pF input
cap, 10 pF load cap.
I do not recommend trying to convert the 10M position to 6M. Managing
stray capacitance is crucial to getting the thing to work.
The way I did it, the only permanent change was a 5/32 hole in the chassis
to hold the new plate tuning capacitor. I simply disconnected the
bandswitched input network and tied the cables out of the way. The 10/15
tank coil is removed in lieu of the 6M part, but the 80/40/20 coil is left
in place. (With the bandswitch set to 10M, the 80/40/20 coil floats and is
of no effect.) The amp can be restored to stock condition with about an
hour's worth of work.
If you need more info, let me know.
73,
Dave Pruett, K8CC
Home phone: (734) 481-0755
Home email: k8cc@ix.netcom.com
Work phone: (313) 493-2168
Work email: dap14@chrysler.com
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