A dielectric header sub-assembly includes a header body with opposed first and second surfaces and a side wall. The first and second surfaces define a header axis extending therebetween. The side wall extends from the first surface to the second surface. The second surface includes a tapered portion. A dielectric header sub-assembly includes a bore. The bore extends from the first surface to the second surface. A first bore opening of the bore proximate to the first surface is greater in area than a second bore opening of the bore proximate the second surface. A method of assembling a header sub-assembly includes inserting an electrical connector into a bore of a header body, applying an active braze filler material into the bore and applying heat to braze the active braze filler material to the header body and the electrical connector.