The only time to use an MH lamp is during the vegetative stage. An HPS is better suited for the flowering stage because it contains the right kelvin temperature for when a cannabis plant is flowering, making the all important THC. If you were to to buy an MH/HPS conversion lamp, then you could use both a regular MH or HPS bulb. But if not, putting a regular MH into an HPS lamp wouldn't work, unless it was an MH-conversion bulb made to work in an HPS lamp.