Tubes use a lot more energy than transistors and consequently produce quite a bit of heat. I would turn the amp off if I was not going to use it for a while. I don't know if the additional heating/cooling cycles would shorten the life of the tube or not. With most tubes I figure 30 hours of running to get them pretty close to their potential, however, I don't have a lot of experience burning in tubes beyond that point.
3/9/13 at 6:27pm