It's an oldie but a goodie, and I love this amp. Lately, it's developed a crackle when I play the low E string. It's not the reverb, it's not the speaker - I've swapped parts and tested for that. The crackle stays with the amp. Everything else seems OK. I'm no kind of a techie as far as amps go, but I did replace the (dead) input jack in this one, years ago. I'm a-wishing and hoping this is a common issue with a simple fix... Any ideas?