how about cleaning up the jack? As far as I know, if the center conductor and ground aren't making electrical contact it should be ok, but it couldnt hurt to start with that. You might want to check your solder joints in general too, I had a similar issue with a non working channel after changing the coupling caps to K75-10s. After a couple of rounds of troubleshooting with the bottlehead folks, it wound up being a questionable solder joint on one of the tube sockets. Do you have a solder remover?
Dave