Complex sentence: Presently, there have been no successful fourth generation tubes as the tube life (Mean Time To Failure) of prototypes did not meet military requirements; however, several notable manufacturers such as ITT have produced "Gen 4" tubes.
Split into simpler sentences: Presently, there have been no successful fourth generation tubes as the tube life (Mean Time To Failure) of prototypes did not meet military requirements. However several notable manufacturers such as ITT have produced "Gen 4" tubes.