I have a fender frontman 25r amp that is squealing like a stuck pig!! When it's on the dirty channel it starts squealing as soon as you turn the volume past 2 or 3 and the squeal changes pitch as you turn the knob further. Even without a guitar plugged in!

Also, when on the clean channel the volume is really low and when you turn it up it barely changes.

I've had this amp for years and it's been a great practice amp. I really want to fix it. Anyone know what might be causing this?