age does seem to dim down old eye tubes, but I just thought I’d mention in case you didn’t know, that a lot of times there is a resistor inside the eye tube socket. Someone else may be able to enlighten you on what that resistor does exactly, but I think it is normally a high resistance , like 10 or 20 K or something. i don’t know if changing it will help or not but in case you find yourself wondering where it is , look in the socket. You can probably measure it without even opening up the socket if you cn figure out which pins it connects between.
Phil