I have a 63 Fender Concert Amp. I'm not sure when this problem started, but the vibrato channel is now producing a lot less volume than it used to. I have to put it at maybe 7 to equal what the normal channel is doing at 2.5. It is also overdriving in an nasty way when pushed with too much input volume. There is a shrill, alsomost digital sounding edge. Any idea what the issue could be? Sound like a bad tube or is this a caps sort of issue?