When to replace preamp tubes?

Yes, elevick and gs5556, that’s why I posed the question. They rarely blow and if I’m hearing noise that I can isolate to an individual tube, I’ll replace it. But tubes do degrade. Over time the degradation isn’t obvious. I wanted to hear what other people were doing. The prices of NOS tubes are very strong. So I don’t want to replace them if it’s not necessary but I don’t want to sacrifice the sound quality to save a few bucks either.

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.

^ Spot on. Tubes just lose their mojo very slowly. In my preamp new Golden Dragons last for about ten thousand in great working order but after that they just start to fade away. They simply start lose their dynamics and detail so the sound becomes weak and eventually just dull. Once I used for twelve thousand hours and went surprised how anemic the sound actually had been when comparing to a new set of tubes. I have never experienced microphonic nor blows in my preamp.

Tubes do not age like light bulbs at all. Light bulbs are usually on, until they fail. Tubes age more like tires.