Need info on the headlight electrics on a 1981 Honda XL500s road trial

Over some 10 years I have been rebuilding a 1981 Honda XL500s motorcycle and most of the bike is finished and ready for roadworthy certificate.

My trouble is the headlight bulb is being blown by the AC system that powers the headlight, the bike has 3 electrical systems on board, the charging system that goes through the regulator/rectifier and converts from AC to DC to charge the battery and run the indicators, tail/stop/park lamp and instrument lights, and the dedicated headlight circuit that runs the high/low beam and the high beam instrument light, and the pulser circuit to the CDI unit for ignition, the system is rated at 6 volts.

I have tested the AC voltage output for the headlight and this reads approximately 48 volts no load at around 4000 RPM and about 10 volts at idle connected to a 6 volt 25/25w bulb, increase the engine revs and the bulb is then blown.

I have removed the seat and fuel tank and rechecked the wiring connections and all appear to be correct, I cannot fathom why the headlight system is producing such large voltages !!, and the bike being 32 years old, one would think the electrics would be starting to fade a little.

Thank you for any help