So, I have a Hartke 210 combo bass amp. One of its features is that it has both a tube preamp and a solidstate preamp built in. However, the tube preamp does not seem to be working at all. If the solidstate preamp knob is turned all the way down, no sound will come out of the amp at all, even if the tube preamp knob is all the way up. I was given this amp used a while back, and the tube preamp function hasnt worked since ive had it. Is it possible that I need a new vacuum tube? And if so does anyone know how to go about replacing the tube?