kt88 tube placement

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.

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?

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).

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.