Hello to the group--
I've also modified a SB-220 for full WARC coverage and 160 meters,
to the extent of building up a new bandswitch assembly and
using a TU-6 board for the cathode circuit. An Ameritron
plate choke was also incorporated. I did not follow Rick Meausure's
design...
Two concerns from what I am reading: Did you check the
balancing resistors on the SB-220 capacitor bank, and how
old are the caps? Hum indicates a rather serious problem,
or it could simply be RF pickup back into the mike cable to the
HF rig when QRO on 160.
Secondly, your power output is rather low, so I am wondering
if your loading capacitor tuning range is adequate. What is
your grid current with the amp fully loaded? Grid current
tells all in the SB-220. My SB-220 will do about 1kW output
with around 80 watts drive. But the loading control really
needs more authority for 160 meters. Generally the
tank is designed to match as best possible for a 50
ohm system, there is not a lot of margin for a wide
impedance matching range on that band.
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