Errr, You CAN interchange between MH & HPS bulbs providing you ENSURE the Power (watts) rating of the bulb is identical to the Power distribution of the ballast. I'm sure that was your initial question!! You do not need to by different ballasts for MH & HPS as far as I am aware

Or did u make an error
SpiritLevel Reviewed by SpiritLevel on . Ballast question Hi all I got a 400W MH lamp with a ballast. Also have a 600W HPS for flowering, also with a ballast. Problem is, I'm vegging with the 400W but the weather is just too hot. I can manage temps with difficulty. If the weather doesnt change soon, I will have major problems with flowering once I switch to 600HPS. What I want to do is to get a 400W HPS bulb for flowering, I'll use the 600W exclusively in winter. Question is, can a 400W HPS bulb be safely connected to the ballast of a 400W Rating: 5