PURPOSE: To check the variations of cathode cut-off voltage in a highly accurate manner as well as to aim at improvement in the voltage characteristic, by inserting a contact interposing member into a space between an upper plane surface and a cathode electron radiating surface outside the electron structure after setting the distance between them, while shifting an internal cylindrical supporter body from the lower part of an external cylindrical cathode supporter body after locking both supporter bodies together by welding.

CONSTITUTION: A distance l between the upper surface of an internal cylindrical cathode supporter body 14 and a cathode 1 can be found by using the thickness t1 of a reference plate 13 and a specified value A, then a contact interposing member 16 is contacted with the surface of an opening part at the cathode side of a G1 electrode 3, and the internal cylindrical supporter body 14 where the cathode is secured tight is inserted into an external cylindrical supporter body 15 from the lower end while the upper plane surface of the internal cylindrical supporter body 14 is shifted to the extent of being forcibly pressed to the under surface of the contact interposing member 16. Afterward, both supporter bodies 14 and 15 are locked together by welding and then the contact interposing member 16 is so designed as to be removed. A distance d between the cathode and the G1 electrode's opening part surface can be found by the distance l between the thickness 12 of the contact interposing member 16 and the upper plane surface of the internal cylindrical supporter body and the cathode.