[Gunny]

with the amp's power supply caps discharged, you can pull the tubes and re-tension the socket pins. A small pointed awl or equivalent tool is used to reduce the diameter, hence improving the gripping surface, of the pins.
I've known people to also take a round wound guitar string (not a new one) and run it through the pin to polish oxidation off the metal surface. If you try that, a shot of contact cleaner wouldn't hurt while doing it.