Why would you want it?
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This is an old thread, and perhaps no one cares any more, but in case someone finds this in a search, I will attempt an answer...
The two small blue pots in the ARC PH7 adjust the operating current for the JFET input stage, not for any tubes. Generally you would not need to touch these controls, as the JFETs and their associated passive parts will hardly age at all. Only if you need to replace a JFET would you need to use this to adjust the drain voltages to around +12 volts. I have not seen a PH7 schematic (does anyone have one?) but the PH5 schematic is available, and the voltage gain circuits appear very similar. I measured the voltages in my PH7 and they seem to match the voltages annotated in PH5's schematic, so I feel fairly confident in the my comments above.