Electron tube 6Z31 Tesla is a duodiode with indirectly heated oxide cathode, originally designed for rectifiers in the receivers. It is direct replacement for the 6X4 tube and after mechanical adjustments it may be used instead of the 6C4P (6Ц4П) and EZ80 as well. New old stock tubes. Year of production: 1962 (BF code). Tested on BM215A.