8 pin vacuum tubes have what is called a guide pin located in the center and it is a little longer than the metal pins so that it will hold the metal pins out of the socket until the tube has been properly rotated to allow for that guide pin to drop into the socket.

As you can see, the center hole in the socket that receives the large plastic guide pin has a keyway.



Over the years I have found many of these plastic guide pins rolling around inside the amplifier chassis because it broke off during installation or removal of the tube.

The real problem comes in when one of these has broken off and it goes un-noticed. Now the tube can be installed in any rotation so of the eight possible  ways, seven are wrong. In three of those wrong rotations, nothing will happen. The other positions will cause the tube to be miss-connected and or connected backwards ensuring certain failure.



Also, this is a good time to mention that rectifier tubes only have four active metal pins. That means some rectifiers will be made with only 4 pins, the rest will just be missing.  Some will use more, like the picture above, and some will use all eight.  In the case where a rectifier has all eight pins, despite four of them being unconnected internally, it will help the tube set firmer in the socket.

Steve

Thanks for the info. I recently suffered a broken guide pin on a cheap Chinese 807 to 6L6 converter. I looked at the other one and saw that the guide nub fell between pin 1 and pin 8. So I just pushed what was left of the broken guide pin into the chassis and inserted the converter in the proper position.