When you start hearing pops and clicks in the speakers. If you have a tube tester, that would more of a tell-all, but trust me, you'll KNOW when to change tubes when you hear those pops.
there could be some different "symptoms" when the time comes but to be more simple if you're using a regular stock tubes and listen approximately to 15...20 hours per week just replace them after one year. in my vtls that i listen to more than 15...20 hours per week i heard some clipping after 12 o'clock of preamp in the second octave of piano when i was playing Van Cliburn. fyi second octave in piano has a freequency spectrum a-bit higher than midrange since there are big, small and first octave prior to second. some other tube amp owners might say that valves tend to sound dim after their half-life. if such thing occurs with my tubes i do definitely replace them. changing tubes is an essential issue since once the tube dies while operating in the amplifier it could cause "side effects" such as blowing or damaging DC supply circuitry more or less depending on how the circuitry is designed.
You start hearing distortion at higher volume levels. and or you must turn volume control higher to get previous listening level. Sometimes the above can be reversed by re biasing the tubes .
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