Does it hurt a tube amp, to leave it on standby mode for a couple hours?

My understanding is that standby vs. fully on is just the standby cuts off a high voltage circuit in the amp. The low voltage circuit that powers the tube heating elements stays on though. The intention is to not turn that high voltage on until the tubes are warmed. So the car analogy is like a block heater prior to starting up. Putting it in standby afterwards blocks the high voltage again, which cuts out the amplified current, therefore no sound. But the tubes are still staying heated.

I will admit I don't have personal experience. Just an engineer with some (very little) electrical background knowledge that nerds out and watches a lot of youtube videos . I was curious when my tube amp didn't have standby and saw that some people added it afterwards. Wanted to know why.

Standby is just the tube heaters, as Jonathan said. Basically, you’re just warming them up for the show. After you remove standby out of the circuit, you are idling. With the volume down, you would set the bias at this point. When you plug in and play, that is full throttle on the tubes.

You could leave the amp on indefinitely without running a signal through it, and you’re still good. In standby, it’s just keeping the tubes warmed up.

The reason for all of this is to keep from stripping electrons from the cathode/s prematurely which degrades the life of the tubes.