The ornamental design for a lampbase, as shown and described.
FIGS. 1 and 2 are perspective views of a lamp base showing our new design;
FIG. 3 is a front view thereof;
FIG. 4 is a back view thereof;
FIG. 5 is a right side view thereof, the opposing left side view being a mirror image;
FIG. 6 is a bottom view thereof; and,
FIG. 7 is a top view thereof.
The broken lines shown in the drawings are included for purposes of illustrating environment or portions of the lamp base and form no part of the claimed design.