What Viridian and some others said. Tubes usually just fade away. You may notice a loss of gain, first of all, with tubes in a phono stage. Sometimes tubes also become noisy, but if no catastrophic event occurs, they just lose their mojo very slowly and gradually. I think listening test is better than testing in a tube tester for determining when to change tubes. Trust your ears first of all. Some tubes can look marginal in a tester and actually sound fine. This is because 99% of tube testers are incapable of testing tubes at the actual voltages and currents to which they are subjected in the context of a working circuit.
When to replace preamp tubes?
When do you replace preamp tubes like 12AU7sand 12AX7? Do you wait till they blow or start making noise? It seems I’ve had them in for a long time and are in use everyday. I have replaced them in the past when they become microphonic but should I wait that long, should I just replace them after a certain amount of hours?