The purpose of the DC Balance adjustment is to minimize the 120 Hz power
supply ripple (noise) at the final output of the amplifier. The channel's
phase-splitter tube is always removed during this test. This eliminates the negative
feedback loop, permitting a more precise DC Balance adjustment. If you reach the
end of your DC Balance adjustment before nulling out the 120 Hz hum, your output
tubes are more than likely mis-matched.
For many years the H.H. Scott "factory-recommended" DC Balance
procedure required using an oscilloscope
connected across a load resistor on the speaker terminals. Others have reported
good results using accurate AC multimeters.
Also included is the later factory procedure from a 222D Service
Bulletin which recommends using your ears and