seperate ballast??

so a quick question i have a 400w hps lamp with a ballast and was thinking of getting a mh bulb for vegging but would i need a seperate ballast for the mh or could i just use the same ballast i use for my hps?

answers on a postcard please

conversion bulb ,

sorry here's the rest http://www.4hydroponics.com/lighting/metalace400.asp