I have a 1965 Twin Reverb. I did take the amp to an amp tech and he re capped what needed to be redone and changed the power tubes. The reverb was making noise when the amp was on and sitting idle. It was kind of a popping noise. I just purchased the amp and it still had the original tubes so I changed the tubes and the popping noise is gone but now it squeals or howls. The howl goes away when I turn the foot switch off. Any Ideas???