kt88 tube placement

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.

Is this the only reason, I thought is has to do with the getters, because when you mount the tube horizontal and pin 4 and 8 also horizontal then you have one getter on top and one getter facing down. So the getter on top will be hotter then the other getter facing down.

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.

If you draw a line between pin 4 and pin 8, the line should be horizontal to the ground (left to right) and oppose to vertical line (up and down).

Thanks, but I don't understand the meaning of a vertical plane. English is not my first language. Does it mean in a vertical line, so pin 4 is pointing down and pin 8 up in a vertical line?

I would assume that they want you to make sure that the tube sockets in your amp are oriented in such a way that the pin guide will ensure that pins 4 and 8 are on a horizontal plane and not vertically. If not, you might have to change the orientation of the amps pin sockets.