A 12 volt source at one amp will easily drive this nixie tube supply. There is sufficient power produced by this switch-mode supply to drive at least eight of the nixie tube driver modules
(I've had 12 of the nixie tube driver modules
running off of one of these boards, that's 24 IN-12A nixie tubes!).
A typical nixie tube power supply offers 170 to 250 VDC at 10 to 50 mA. A switch-mode power supply is desirable because it's small and very efficient. You can fit it inside your clock and it won't heat up. The schematic for the project is taken directly from the MAX1771 datasheet, however, because of the large voltage jump from input to output, board layout and low ESR type components are critical.