An ultrasonic welding apparatus for electrical conductors comprising strands of wire comprising a sonotrode for generating ultrasonic vibrations and a cooperating anvil having movable side walls to compress the wire strands against the head of the sonotrode. The anvil has two moving parts, one moving back and forth parallel to the fixed sonotrode face, and the other moving up and down normal to the fixed face. The moving parts carry the side walls of the anvil and with the fixed face provide an adjustable space for wires of various sizes, the space having a rectangular cross section. The gap between the moving anvil parts and the adjacent fixed face of the sonotrode are accurately controlled.