I agree with Eldartford, most often tube performance deteriorates slowly. Some can last a year while others may survive 20 years. Too many variables about circuit application, user care, etc. so time alone cannot be the measure.
I believe anyone with more than say 2-3 tubes in their system should try to find a tube tester and learn to use it. I'm not knowledgeable about electronics but I found a B&K tester at a swap meet and bought the tube charts and calibration instructions from them. That was a few years ago but the $ and effort paid for itself many times over.
I believe anyone with more than say 2-3 tubes in their system should try to find a tube tester and learn to use it. I'm not knowledgeable about electronics but I found a B&K tester at a swap meet and bought the tube charts and calibration instructions from them. That was a few years ago but the $ and effort paid for itself many times over.