+++ for newbee...
I usually wait 10-15 minutes for the tubes to cool, and the power supply caps to lessen their stored voltage.
Probably longer than needed for small signal tubes (to cool), but certainly not a bad idea for power tubes (especially the 2A3/300B) I've used over the past 20 years.
I can always find something else to worry about during the long wait, so nothing is really lost (from my life).
DeKay