Abstract: A body of electrically insulating material is provided with a plurality of apertures extending between two surfaces thereof to accommodate an equal number of electrically conductive pins extending from the tube. One of the surfaces of the body of the base is recessed at the location of one of the apertures. The recess communicates with at least one elongated cavity adjacent the associated aperture. The cavity extends at least partially between the two surfaces of the body. The assembly includes an electrically insulative elastic gasket disposed within the recess. A first portion of the gasket is preferably of tubular shape and has greater length than the depth of the recess. The longitudinal axis of the first portion is aligned with the longitudinal axis of the associated aperture. A second portion of the gasket generally matches the shape of the recess in the base body. A third portion of the gasket is seated in the body cavity for maintaining the position of the gasket relative to the body.