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).