Process for preparing a vacuum switch tube

A pre-assembled vacuum switch tube having a clearance between a stationary electrode rod and end plate is firstly prepared by forming an enlarged diameter part in the stationary electrode rod at a position where the rod extends from the end plate to the outside of the vacuum envelope and by arranging at least one piece of plate-like brazing material having a flat surface. The pre-assembled vacuum switch tube is put into a vacuum furnace; the interior of the vacuum switch tube is evacuated through the clearance; temperature in the furnace is elevated to melt the brazing material thereby causing the descending of the enlarged diameter part due to the dead weight of the stationary electrode rod and the vacuum switch tube is sealed by bonding of the end plate and the rod. Alternatively, the clearance is formed in such a manner that a supporting member having a through hole into which the enlarged diameter part of the stationary electrode rod is inserted is placed on the end plate and at least one piece of plate-like brazing material is arranged between the supporting member and the enlarged diameter part of the stationary electrode rod.