FWIW:
Early this Spring, I started to convert my SB220 to 6 meters. It became more
complicated than I wanted (i wanted an easy reversal back to the original) so I
abandoned my efforts and turned to my SB200 for a 6 meter conversion. It was
easy and has worked well.
The conversion of the SB200 to 6 meters took less than 4 hours. I pulled the
plate tuning cap and replaced it with an old Bud 55 pf unit I had in my junk
box. I also removed the 10-20 meter coil (left the 80/40 coil in place) and
wound a new 2 1/2 turn coil from a copper thermocouple. To reduce stress on the
bandswitch, I jumped the switch contacts with a piece of copper ribbon. The
plate coupling cap is mounted directly on the plate variable.
One important change is to disconnect one section of the loading capacitor. It
makes it much easier to tune and is easily reversible.
I removed one turn from each parasitic suppressor per W7MY's web page. For the
input, I tack soldered a 50 pf mica cap on the cathode side of the ten meter
input coil and a 68 pf capacitor on the input side. The SWR came right on down
to almost 1 to 1. The stock antenna relay does not seem to be a problem. There
is a slight bit of instability without an input coax connected but once the amp
was connected to my Icom 706, it was unconditionallly stable.
I get 725 watts out with about 80 watts of drive from the IC706. Signal reports
have been excellent with no distortion reported. I key the amp with a
footswitch to avoid any issues with the relay system in the 706.
I highly recommend the SB200 for conversion to 6 meters as long as it has the
Cetron tubes. I have no experience with the Chinese 572B's. My grid and plate
current meters read in the normal ranges.
I have photos of the plate compartment and cathode coil if anyone is interested.
73, Dennis W0JX/8
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