RE: [BULK] - [Amps] Convert Heathkit SB-220 to 50Mhz

["David A. Pruett" <k8cc@comcast.net>]

Back in 1992 I converted an SB-220 to 50 MHz following much the same path 
as WB2WIK.  However, I was able to achieve much the same result with 
less-intrusive mods.  100W of drive from an IC-756 yielded 1000W out on a 
Bird 43, the efficiency seemed reasonable and the tubes ran pretty cool.

Here's what I did:

- Replaced the original Johnson tuning cap with a 55 pF version from the 
same family.  The cap mounted to the original front mounting hole in the 
chassis, and I drilled another 5/32" hole for the rear mount.

- The original load capacitor was retained.

- Removed the 10/15 tank coil and replaced it with 1-3/4 turns of 3/16" 
tubing, 1-1/2" inside diameter.

- Set the bandswitch to "10M", which bypasses the 80-20 tank coil.

- Replaced the parasitic suppressors with similar ones but with only 1-1/2 
turns.

- Plate choke was left stock.

- Rather than ripping out the input network, I simply disconnected both 
coax coming from the input (from the T/R relay) and from the output (tubes) 
and tie-wrapped them out of the way.  I then built a pi-network out of the 
QST article using a slug tuned coil and a couple of mica capacitors.  Got 
lucky - 1:1 input SWR first try.

In 1997, I needed an amp for an HF contesting trip to WP3, so since I had 
no 6M antennas in the air at the time decided to convert the amp back to 
HF.  This took all of about an hour, consisting of re-installing the 
original plate tune cap, 10/15 tank coil and parasitic suppressors, then 
removing the 50 MHz input network and re-connecting the original 
bandswitched input network.

A year or so later, KE9I was looking for a SB-220 to convert to 6M so I 
re-did the conversion and sold the amp to him, so the design should be 
reproducable.

Back when I had a personal web page, I had this all documented on-line.  I 
do have a .PDF file detailing the whole thing with pictures.  It's about a 
600K file, and I'd be glad to share it with anyone who can handle such a file.

Dave/K8CC


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