A thermostat is preferable. Try this one: http://www.amazon.com/Hydrofarm-MTPR...0858892&sr=8-1
You want the surface temp where the UTH is (right on the glass under the substrate) to be between 90-95*F. You must measure this spot with a digital probed thermometer or a temp gun. If you aren't using a digital thermometer with a probe then you aren't getting an accurate reading.
If your ambient temperatures are above 75*F then you really don't need a heat lamp. If you want to raise it a bit, use a low wattage red bulb. Any heat lamp will lower the humidity so avoid it if you can.