One quirk of the D5500 voltage regulator circuit is that when the lamp socket gets oxidized and makes poor contact, the triac goes full on. This places the full 120v line voltage on the socket. Then it arks through the oxidation and fries the bulb before the regulator circuit can cut it back to 82v. The good thing is that the lamp socket is still available and inexpensive. I don't recall but I think I got these from "Bulb Direct" or "Interlight" along with some new bulbs.