i use my reostat i build and they work great
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cheep easy to build, i cant rememeber who it was on this site that first gave me the link but cheer to him/her if it was you reading this
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but its not a constant temp like all reostats are, if its hot outside it still puts that much volt you have the dial set to output so you temps go up, thermostats are the way to go when you get the extra cash to buy one