Went to the Fender site and did not see the schematic for the Pro Jr posted. So figure it would be similar to the Blues Jr. and by the look of it all amps have the input grounded without a cable plugged in. So I would see if you have the problem with no guitar plugged in and the volume control set at different levels. Because by your post above the noise drops with volume setting, so it sounds like you may have a bad volume pot. You could use a pot cleaner on the volume pot like this http://store.caig.com/s.nl/sc.2/category.293/.f me I use WD40 which by just saying will cause some people fits, but I find it works. Who knows what is the secret sauce in the DeoxIT stuff anyway relative to WD40. But if the problem is contact wear on the resistive element in the volume pot the final solution would be to replace the volume pot. You should look for bad wire connections from and two the volume pot along with the solder joints to make sure the problem is not there while you are checking things out. Also the solder joints on the tube sockets and components on the PCB. If you think the problem is related to a tube and you have multiple 12AX7 preamp tubes you can also swap the location of the tubes to see if this changes the problem. If the problem changes then it would look to be a tube that is bad. I did find the Pro Jr. schematic looking a little more and see that there are two 12AX7 tubes one the two preamp sections and the other the phase splitter. You can even run the amp with tube V1 removed which is the preamp section if the amp is quiet all your noise problems are in the preamp section. I think your problem will probably be the volume pot by what you have posted.