Does tube testers tell how much life is left for tubes?


Let's say that you have new tubes, very old tubes that are almost to die, and something in the middle.
What kinds of reading do you expect from these three kinds?
Can you usually tell the life of tubes from tube testers?
How about the color? Do those three kinds of tubes have different colors when they were turned on?
I would like to know when to change tubes before it gets too late.
Some says if it sounds good, don't bother to change. 
Some brands of power/pre amps consume more on tubes than other brands and their life seems varies brand by brand. 
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Showing 2 responses by mijostyn

RIght, the testers can tell you whether or not the tube is OK. They can not tell you how much time is left on the tube until the tube starts to measure poorly which means the tube is at deaths door. I use tubes until they fail and in my experience they sound just fine until they do fail. 
I always keep a full set in reserve. If one tube fails I replace the whole lot unless that particular tube was obviously defective. I keep my phono stage on in standby. The tubes, 6922's last an average of 6 years.