[NostalgiaRadioTime]

I'd start by cleaning the tube socket. Spray some contact cleaner or DeOxit in the socket and work the tube in and out a few times. Might not be a bad idea to do this to all the tube sockets. That you can do without removing the chassis. Let the radio sit for at least 15 minutes before trying it to allow the cleaner to evaporate. If that doesn't clear up the problem, you'll have to pull the chassis and try spraying some cleaner into the volume control itself under the chassis.

These things may or may not clear up the problem but try the easy stuff first!