I don't know what conversions you've seen, but I've converted three SB-220s
to six meters over the years, and each the same way. In all cases, I got
rid of the bandswitch entirely and used that hole to mount a 2-30pF vacuum
variable capacitor; got rid of the LOAD cap entirely and used the original
TUNE cap as the new 6m LOAD capacitor; re-wound the plate RFC with many
fewer turns (only need about 1/4 the original winding length for 6m);
eliminated the original bandswitched input networks and replaced them with a
single network for 6m only; added more front panel grounding (aluminum
bracket) for the new plate tune & load caps (the inductance via the front
panel is too high); replaced the original parasitic suppressors with new
ones having the carbon resistor soldered across 1/2-turn of copper 1/4" wide
strap; stuff like that. The conversion takes about one full day (8 hours)
and works well.
With good tubes, I get about 900W output on six. The original T-R relay
isn't the greatest on 50 MHz, and I often go to a conventional Dow-Key
dual-pole coaxial relay like the DK-260 to pick up about a dB or so in
performance (even on receive). Of course, I add the "standby switch" mod
and also the "low voltage keying mod" to any SB-220 before I even get
started!
Are you familiar with this stuff?
73 de Steve WB2WIK/6