A lamp assembly having a lamp base and an incandescent or electrodeless gas discharge light source mounted on the lamp base. The light source includes a light-transmissive envelope having a groove extending around the envelope at one end, and this groove is used to retain the envelope in place against a mounting surface of the lamp base. The mounting surface has a plurality of fingers circumferentially spaced about a central insertion axis of the light-transmissive envelope. When the envelope is inserted into the lamp base, the fingers deflect outwardly away from the insertion axis and then return at least partially to be received in the groove of the lamp envelope. The fingers provide a radially, inwardly directed force on the lamp envelope to inhibit relative movement between the envelope and the lamp base.