kt88 tube placement

Lemme 'splain Lucy! If you take the tube in your hand and hold it horizontally, with pin 4 at the top and pin 8 at the bottom, (or vice versa!) you will notice that the plate inside the bottle has its narrow face(s) on the 'top' and 'bottom', while the wide(er) faces of the plate are facing 'front' and 'back'.

This is the strongest orientation (relative to gravity) for the internal structure, especially the filament/cathode which is inside and which you can't see.

Rojo, There is no convection inside a vacuum tube because there is no air or other gas to convect, so all internal elements (except the filament/cathode) are at the same temp, assuming all the operating voltages are within spec.

Besides, not all KT88's even have side getters.