Large aircraft have several voltages running different systems. DC voltage is normaly 28vdc and the AC is 120 at 400hz. Many of the non primary instrument lights are 3vac. Primary lights and instuments are normaly off the battery bus, and 28vdc. Your pitot ammeter I believe is used to show the draw for the anti-ice heater built into the pitot head. This is most likely to be 120vac.