Galaxian (like Pac-Man) has AC voltages on the harness. So the adaptor has to have a AC->DC power supply built into it (you'll find old ones like this called the "Li'l Green Giant"), or it has to have inputs for hooking up a switching power supply instead. You'll probably find plenty of the latter type by looking for Pac-to-JAMMA. You could buy one of the Pac kind and modify it for Galaxian (you need to fix the speaker connections).