It does sound like a tube on its last legs. If you're new to tubes, I do recommend getting help from Upscale Audio. Bill from Morningstar Audio is another helpful resource (they're a dealer for Eastern Electric components). Vintage tube services (http://vintagetubeservices.com/Default.htm) is another useful resource, and the tubes are rated for use as Rodman suggests.
To completely retube, you'll need 2 12AX7's, 1 5751, and 3 12AU7's (unless you have the PV-llL).
To completely retube, you'll need 2 12AX7's, 1 5751, and 3 12AU7's (unless you have the PV-llL).