Charging voltage seems too high, is voltage regulator adjustable?

[car10107]

I've had my 65 C10 283 for a couple of years now. It has an aftermarket ammeter that always reads close to the center so I have never worried about the charging system. I recently started using one of those digital volt meter/USB ports plugged into my cigarette lighter socket to charge my phone. This particular tech will cause the voltage reading to flash if it goes below 11 or above 15. I notice that when I am driving my voltage runs between 15.3-15.7 and this seems a little high? Key on engine off voltage is like 12.8 and engine idling is around 13.8 but when the RPM's come up and the alternator is spinning faster, the volts goes above 15. I remember some of the older voltage regulators were adjustable but wanted to ask before buying a new one. I want to stay with the OE external regulator setup for now. Thanks