You need an MVB for reptiles if you're using it for UVB output to guarantee that your tort gets UVB of the correct frequency.
I have a mercury vapor bulb over my side door and it has a light sensor so it comes on automatically at dusk and goes off in the morning. I really doubt that bulb puts out any UVB. What you want is a mercury vapor bulb that puts out ultraviolet 'b' light.
This might help explain it: