Just acquired #25 of my cobrahead collection from a guy that owns a small hardware store with two of these mounted. This was a typical clean, repaint, and reballasting job as this luminaire had a 240 volt ballast and I needed a 120. So off to Shanor Light and Supply I went and bought a new ballast that burns both MH and MV.
NICE! My M-250A from October 1967 had a 240V 100W MV ballast (with PC socket wired for 120V) but fortunately Joe Maurath Jr. had an extra 100W 120V powr/module so I have the original 240V door and the 120V door which I use to light it up. The 240V is just a two wire choke so I'm sure it works.
For my silver M-250R, it was also 100W MV with a 240V choke and 120V PC but I had an OEM 175W 120/240V GE ballast from a 1966 M-250R that I stuck in so it's an OEM ballast replacement. I kept the 240V 100W choke to either put back in the fixture or stick in another fixture when I get my own place and run a dedicated 240V circuit.
All of my ballasts are set up for 120V. If I were to set up a 240, I'd have to run an additional two lines from my subpanel and install a two pole breaker direct to the luminaire arm. Too much hassle for me. BTW, HID ballasts with multitap wiring are fairly reasonably priced at any supply house.
Yeah I've seen inexpensive multitap ballasts online but I prefer to have OEM ballasts in my lights, partly for authenticity and partly because OEM ballasts tend t fit better than replacement ballasts. GE used to sell OEM ballast kits but now I think they just sell rebanded Universal HID ballasts with labels similar to their fluorescent ballasts.
Ah yeah I wouldn't want to make an M-400 or OV-25 175W or something. It messes up the distribution and a lot of the light gets "lost" in the larger optical. My OV-25 remote ballast was wired to a 175W MH/MV ballast box for awhile though so I can't talk lol.
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For my silver M-250R, it was also 100W MV with a 240V choke and 120V PC but I had an OEM 175W 120/240V GE ballast from a 1966 M-250R that I stuck in so it's an OEM ballast replacement. I kept the 240V 100W choke to either put back in the fixture or stick in another fixture when I get my own place and run a dedicated 240V circuit.