Tubes sound so much better after 30 minutes of warmup. The difference is so startling I take notice and think OH WOW!

I know this question has been asked since the begining. Why do tubes sound remarkably better after 30 minutes?

they must have warmed up

In simple terms, the cathode emits electrons and the anode receives those electrons and the purpose of the grid is to control the current flow from cathode to anode. So, the cathode cannot emit electrons until the heater has warmed and that's the reason why valves need a warm-up period to get started.

I've noticed the 30 minute thing with both my tube amps. After an hour, things are really cooking and they sound even better. I've been playing my 500-C all morning. It's so good, it grabs my attention from what I'm doing.

That's the valves' magic.