An LED light source is disclosed that is configured to be retrofit to an existing lamp fixture. The existing lamp fixture is of the type including a ballast housing for providing electrical power. The retrofit assembly includes a central support connectable to the ballast housing. A circuit board is mounted to the support. The circuit board includes one or more LEDs. A dome is mounted to the support and covers the LEDs. A circumferential heat sink is mounted around the support and extends radially beyond the ballast housing to carry heat directly away from the circuit board and support.