Len W- not to be contrary, but what you are hearing is microphonics. Some tubes are microphonic when new, some are not. I don't know if this can also occur due to aging. It may be sign that the tube needs to be replaced, but it is NOT a reliable indication of aging. BTW, some microphonic tubes can benefit greatly from a tube damper.
Listening for tube degradation
Hey, just wondering what specifically there is to listen for in regards to tube degradation. I don't have access to test equipment so I have to rely on my ear to determine if certain tubes are on their last legs. I've been able to notice a definite "scratching" or muddied sound whenever one or more of the tubes are in need of replacement. However, I find myself replacing ALL of the tubes since I can never pinpoint which exact tube or tubes needs replacing. Also, are there any visual cues to look for (ie.output tubes glowing brighter than others)? I have a 15W integrated tube amp that you can look at on my system page.