Mid set my tone goes all soft and muffled, so i peek around the back of the amp and one power tube is not lit up at all. Amp was still making noise so i finished the set. today I bought a pair of 6l6 groove tubes and im gonna have to replace them as soon as we get to the next show. Is it as easy as removing the dead tube, and popping in a new one? Do I need to replace at least two? The amp has a fixed bias so is it that simple?