Abstract: An evacuated envelope having substantially flat, spaced apart front and back walls and support walls extending between the front and back walls forming a plurality of parallel channels extending along the front and back walls. In each of the channels is an electron beam guide assembly which includes a pair of spaced, parallel plates fixedly secured together along parallel elongated edges by spacer members which are between the plates and to which the plates are mechanically secured. The plates have a plurality of openings therethrough arranged in rows longitudinally along and transversely across the plates with the openings in one plate being aligned with the openings in the other plate. The beam guide assembly fits in tracks formed along the support wall adjacent the back wall so as to support the beam guide assembly in fixed, spaced relation to the back wall.