old 400w bulbs

[Bryan]

Yes, but some 10K bulbs will start and run better under specific ballasts they were designed for. Problem is trying to find the correct ballast for the bulbs as many are renamed and repackaged so we have no idea who actually made them. But to reiterate, the 400 MH you have should have no problems with any of the 10k bulbs except a double ended bulb which is a completely different animal.