On the side of the light it says Hydrofarm, 400watt 120v 4.0a 60z and says use only 400watt Metal Halide bulbs M59 type lamp. When I also got it from the friend he still had a old burned out 400watt metal halide bulb in there, i bought the same bulb that was in it. What is the cap and how can I replace it? Unsure of that one.