That sounds like a bad tube. You can try cleaning the sockets and pins first tho.
If you plan to keep your AI in stock form, I suggest only using Russian tubes, Sovtek or the 1975 Voskhod 6N23P.
Do NOT try any others especially not an NOS type tube.
I've been repairing and referbishing AIs for around 15yrs.
Feel free to email; hope I've helped.