The plates are the black or gray (usually gray) rectangular or square objects that take up most of the space in the tube. Long plates are usually found in older tubes, early (1950s) versions of tubes like the 12AU7. They may sound different from the later short plate or box plate or square plate (three different names for the same type of plate) versions of the tube but any 12AU7 socket that can use a long plate should also be able to use a short plate.
Tell us what brand of preamp you have and we may be able to help you get the right tubes in it.
Tell us what brand of preamp you have and we may be able to help you get the right tubes in it.