If the radio is similar to a Military Radio or most other ordinary tube
type radios,you don't need to wait untill it warms up to use it.If you
are going to stack tube type radios on top of tube type radios,I think
it is a good idea to use some one inch by two inch strips of wood
between the radios for air ventilation or you could buy or build some
sort of "book cases" for the radios.If you are worried about too much
heat,set an electric fan to blow on them.Tube type radios are not
similar to computers,computers can't handle as much heat as tube type
radios can.
cuhulin