From MetroAmp Wiki
Metroamp 100 watt power transformers are wound with an extra accommodation in the high voltage secondary which allows switching the B+ voltage in the amp. Specifically, the HT secondary has 80% taps added.
To properly switch between these voltages and adjust bias voltage requires a triple pole, double throw, three position switch.
The theory here is to direct either 100% or 80% of the secondary voltage to the diode bridge. Which is accomplished using two of the three available poles on the switch. The third pole is used to add series resistance to the raw bias voltage supply when the switch is in the 80% position.
Since the switch also has a middle position, this is used as standby mode with no voltage directed to the diodes.
Wiring for our 1203-80-ML or 1203-80-MS is as shown in both schematic and layout form:
The value of the resistor on the switch will typically be in the 15-33k ohm range. Alternatively, a small Bias Pot can be mounted on the switch and adjusted to exact preference. The pot is shown in the schematic view above and is labeled "VR2 25K Low Voltage Bias Adjust".