You can use a 6 volt battery charger as a source to power it up; however, you want to make sure you have a speaker hooked up prior to turning it on as some early circuits did not respond well to no load on the output side! I'd use a 8-10 ohm speaker but make sure it has a load coil (1.5 ohm, 26 mH) as there will be voltage present at the speaker terminals.

You might want to put a 10 amp fuse in live with the positive connection as well.