The idea is to be able to compensate for low line voltage. The filament
xfmrs are fed from the 115 volt line through a rheostat to 100 volt primary.
In case of low line voltage just decrease the resistance in the rheostat
and you can bring filament voltage, which is somewhat critical with
thoriated tungsten filaments, up to the nominal value with line voltages as
low as 100 volts. If the line voltage is higher that 100v, just add
resistance in the line and bring it back down to 100v, which furnishes the
correct filament voltage to the tubes.
This is fairly common practice with many transmitters. The BC-610E has the
same arrangement.
If you can't find the correct transformer, the primary could be wired in
ahead of the rheostat directly to the a.c. mains voltage, but you will lose
the variable filament voltage control for that tube.
73,
Don k4kyv
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