I just bought a Princeton '65 reissue. I tend to practice on and off during the day and will frequently take a brake to do something else and come back to it. What is your rule of thumb for leaving the amp on? I'm thinking if I know I'm going to be away more than an hour shut if off, if not leave it on, but am curious what other people do. Thanks! P.S. - I'm not worried about the cost of tubes so that's not a factor.