[Amps] complicated Bias circuit

[Steve Gorecki VE3RX]

Paul, it looks like +ve bias for the cathode of the tube, meaning it is
likely a grounded grid design. R20 is for RX as well, I suspect, providing
further bias to cut off the tube idle current (reduce idle current to a
smaller value). Shorting R20 makes the two zener bias diodes (one selected
of course) set the proper bias for the mode in question.
Steve VE3RX