| 摘要 |
PROBLEM TO BE SOLVED: To surely prevent a discharge generated between a junction face of a cylinder-like metal vessel of a stem insulator and a junction face of a cathode lead. SOLUTION: An annular groove 11 is formed in a junction face 10 of a cylinder-like metal vessel 16 of a stem insulator 6, and in a junction face 9 of cathode leads 2a, 2b. A metallized layer 8 formed on the above junction faces 9, 10, is formed separately from the circumferences of the edge parts 12, 13 of the annular groove 11.
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