Oldest tube in use... I guess the RCA 5693 from 1948. I use them in a Lambda 25 power supply to supply regulated 325volts to a slightly modified Croft Micro 25R preamp.
oldest tube(s) in your system
Please let us know what they are running in too. Thank you for your attention to this matter...ha!
2 x 1949 Sylvania ladder plate 6BL7GT (in monoblock amps, custom built 2 x 6BL7GT + 4 x KT77)