300Bs - does leaving them on shorten their life?

My 300B SET amp has a standby function that allows me to warm the tubes up gradually. I think when they are in standby, at what is essentially a very low idle, it's not using up any or very much life of the tube. This is why, if I have another custom power amp made in the future I plan to request a standby switch.

After you break them in, say 60 hours (no content needed for break-in, just leave them on), IMO it is STUPIDO to leave them on. Other than that I have no opinion on the matter.

Turn em on, go do something else for a few minutes, play, OFF.

STUPIDO - Noted!!! Thanks 

I’ll be turning them off.  heard one post about the trauma of powering up, but may live with that risk. It’s and expensive fix if I hasten their demise.

If you have some uber rare and expensive 300Bs, you could use a device called a "Variac". This device limits voltage to reduce start-up stress on your tubes.

This is a frequent debate that is never settled.

One side believes leaving tubes on is a fire hazard and uses up the tube faster and the heat also wears out the rest of the amp sooner.

The other side believes that turning your tube amp on and off is the most destructive thing you can do to it. Of course there are others with opinions all over the spectrum between these opposing poles.

My advice is to call up the manufacturer of your amp (even if you didn’t buy it new) and do whatever he advises.