a commercial 13.8 volt supply should be fine - I was waiting for someone else to answer since I have a v4 -- but v6 should be same in this regard.
the caution is to folk who try to make a power supply with a wall-wart - and get 18 volts instead of 12 or 14 volts into their receiver (and nearly 0 volts into their transmitter if the receiver survives!).
enjoy your new rig - and I agree with your caution. some folk insist on having a fuse in the path if they connect their uBITX to a 20-40 amp power source.
aka Curtis Eugene Milton wb8yyy