My Fender Blues Deville 212 has a very "ratty" tone on clean...

I am guessing on what the problem is , the post is very vague. i am guessing a channel switching issue. you need to check the "sugar cube" looking power resistors solder welds. they are probably cracked and arent making good contact. if you tap on the amp it will cut in and out, its probably these two resistors. they are usually 470ohm 5 watt cement resistors. what happens is they get real hot, and as they cool down they crack the little solder welds. i usually replace them with new and keep them floating up over the pcb to help them stay cool and off of the board to prevent further solder weld cracking, stock they are real close to the board. then put a little silicon under the resistor to hold it in place and to keep it still on the board. i just fixed a blues deluxe with this issue. while you are in there, cut out the resistor sitting right behind the reverb pot, this will make the reverb awesome. also solder the mid pot center leg to the mid pot leg that comes off of the bass pot leg to get better midrange control. very easy mods. check the master volume pot, is it a B100K? pull it and put an A250k in its place for better volume control. bias pots , etc. check my website yostamplifier.com.