If you are using your guitar amplifier or bass amplifier say 4 hours a day 5 days a week = 20 hours per week. Then you should change your tubes every 6 months if you want to maintain the best sound quality. A year ( approx 1000 hours ) should be the absolute maximum on a set of tubes. Yes, they may still light up and produce sound, but it will not be a very good sound.
Randy Morgan ….. Owner & head technician at All Service Musical Electronics Repair