Finally, my 6m-converted SB-220 (which is now beginning to resemble a homebrew
amplifier) is back in service -- after repairing the burned-out plate RF choke;
replacing the welded-closed bias relay (along with its associated dropping and
shunt resistors) with a faster and quieter reed relay; replacing the smoked
bias diode string with bigger diodes; replacing the fractured rotary meter
switch with a cool 3-position toggle switch; adding a B+ glitch resistor;
and... guess what? The 3-500ZGs apparently emerged undamaged from this episode,
and are both producing full output, very stable. In fact the amp feels more
stable than before it blew up last month. Last thing I'm still planning to do
is put a very fast 1A fuse (with 50K ohm shunt resistor) in the bias supply
line, which will hopefully protect the diodes and relay contacts from future
glitches.
Thanks to everybody whose suggestions and tutorials contributed to my learning
and success with this project. It was fun.
Bill W5WVO
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