ChknHwk, I don't know about that. After I harvest this one in a week or two, I'll try it. I do know that HPS will burn MH bulbs with no apparent probs. I tried one of those conversion bulbs. I ordered either a Phillips (I forget what they call thiers), Or a GE Chroma-fit. Those are not supposed to have light loss. What I ended up with was an Ushio version. It worked OK, but had about 15,000 fewer lumens than a straight up HPS or MH. Thats alot of light to sacrifice for a little convenience.
I have some MH fixtures, but only 2 HPS bulbs. I aint gonna run right out and get another bulb to test it in a MH fixture, but I am gearing up for another grow. I'll be getting new lamps/bulbs for this grow, and I'll try a used one in a MH fixture.
I just wonder when, why and how this came about that it couldn't be done? I have called both GE and Hubbell lighting technicians about it and get the same pat answer. No they are not interchangable.

__________________
You can lead a Horticulture....But you can't make her think!!!!