When should you change vacuum tubes?

Vacuum tubes sound different over time and need to be exchanged for new ones. Is there an easy way to tell if they need to be replaced? Have a question you want to ask Paul? http://www.psaudio.com/ask-paul/ I am getting close to publishing my memoir! It's called 99% True and it is chock full of adventures, debauchery, struggles, heartwarming stories, triumphs and failures, great belly laughs, and a peek inside the high-end audio industry you've never known before. I plan a few surprises for early adopters, so go to https://www.paulmcgowan.com and add your name to the list of interested readers. There's an entire gallery of never before seen photos too.