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Unless you have a dual ballast, ballasts are designed to only fire either HPS or MH bulbs. You also should remember that a 400w ballast can only fire a 400w bulb, don't try to swap out different wattages. There are also "conversion" MH bulbs that can run on HPS ballasts. What kind of ballast do you have? How many watts is it designed to fire? Either way I would probably use the right bulb for your ballast.
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