How does sound change when tubes start to fail?

My experience is with 300B tubes in my VAC Renaissance 70/70. When tubes started to “give up” I have heard the following: decrease in dynamics, lowered high and low frequency output, “muddy” sound. After re-tubing I was amazed by what’s been missing.
I hope this helps…

Hi Daka. Meta's description is a good one. Another tell tale sign of a spent 7868...... look at the silver flashing[gettering] on the top of the tube. It should look as solid silver down below the shoulder of the dommed top. If you can see the getter or there is very little silver flashing left, the tube is just about spent or should I say tired. As to your techies comment: I am certain that your techie must have noticed that there was very little getter flashing left to make a statement like that. I have seen original Fisher labelled sylvania 7868's with almost no flashing left but still test quite strong. Your Fisher 400 is truely one of the great ones and with a top shelf tuner to boot. Perhaps it's time to retube it. Best of luck!

I think you have bad tubes if you even have to ask... Any signs of disinterest in the music is usually one of the signs of tubes on their last legs as they tend to sound flat. I think you can typically expect about 2 - 3 years from a set of tubes in normal listening conditions...