Output Tube Bias Adjustment As shipped from the factory, the output ‘bias’ adjustments are set for a nominal 65mA per KT120 tube. Under these idle conditions the tubes are each dissipating approximately 27 watts of their 60 watt rating. This point of operation provides ‘enriched ’Class AB1, and will satisfy the most critical listener.For best results, operate and adjust the Reference75at 117.5 VAC(230 VAC). Adjustment must be made under zero-signal conditions after at least 15-20 minutes of uninterrupted stabilization time.No doubt ARC would like the home owner’s main voltage to be 117.5 volts. Arc needs to retool for the 21 first century. I would bet most homes today have a mains voltage in the 120Vac to 122Vac range.
You want to set the bias at 65mA per KT120 tube per ARC specs using the AC mains voltage at your house.