If you are going to make a cooltube out of a pyrex bread baking tube, be advised. --- The bread tubes are only 3.5 inside dia. ,that means you need a t15 size bulb. I think that for hps bulbs that is not much of a deal, but for my 250 m/h , Im having a hard time finding a t15 bulb with 5k color. They exist but cost about 120 bucks. For that price I can get a real 6" cooltube and then use any bulb I want. Just thought Id tell you about my experience.