I would suggest that you plug the guitar into the effects return socket on the back, ideally with some boost or overdrive pedal to increase the level of the guitar a bit but it works ok without. If you get a good clean sound (it will be a bit quieter than normal if you dont have a booost) then the power amp and power tubes are all ok - then you know that the problem is in the preamp (or those effect loop jacks). If the sound is distorted/weak this way then you know that the problem is in the power amp or power supply area.
In an amp that has been left standing for a while corrosion of contacts is a quite likely cause. You can do a basic job of cleaning the loop jacks by spraying some contact cleaner onto a jack and plugging it in and out of both send and retn sockets a few times. You could also try reseating alll the tubes by rocking them a little in the sockets in case there is a bad connection there.
