some use an old 9v battery that has drained down to 5 or six volts . I have an old 120 to 12v radio shack transformer that is center tapped ...6v per side that is in a project box with a fuse on the mains and a switch that goes to a light socket. Any 120 to 6Vac step-down transformer will do. Most of the bulbs ...at least on Gottlieb machines are 6.3Vac and so are your 555 bulbs. The 555 bulbs have a strip of metal that heats up and cools down to provide the flash, Kinda like the old turn signals in cars of the 70's era.....they have to be hooked up to 6.3Vac to work
this setup will work with led playfield lights also. Led's typically come on at around 3 V dc and all are made with a resistor in the case .they work on AC input the led itself rectifies the AC to dc and it lights therefore this set up works on all

The 555 s have to heat up to work above quote is also true but a 6 volt ac supply will work on either polarity doesn't matter because the 60 hz sine wave flips from negative to positive

sorry but I had 455 bulbs mixed up with 555 .....455s are flasher bulbs 555's are wedge type my bust