[jerryhawthorne]

Mike is correct on size and possible grid cap wire routing if the tube has one. There is another concern and that is pin 1 on the metal tubes is used as a ground pin for the tube's metal jacket to provide shielding. It is not used on glass tubes as if they need to be shielded there will be an external shield that connects to the chassis. In this configuration pin 1 on the socket is not needed. The potential problem is that in some cases manufactures use pin 1 on the socket (if they use glass tubes) as a tie point for other wiring. OK, they are cheap but it was free and available on the tube socket. In order to have interchangeability between the two types of tubes, pin one on the socket needs to go to chassis ground as otherwise the tube shield is not grounded. If not used, you need to add a ground wire. If used for something else for a tie point, well I would go with "G" tubes. I like them as they add to the glow of a radio working. Plugging a metal tube in there when it is used as a tie point on socket pin 1 will allow no shielding on the tube and put what ever voltage was applied to pin one as a connection point on the metal surrounding the tube itself. If it was used as a tie point for something with high voltage that tube could give you a jolt if you try removing it or touching it with power on.
Hope this makes sense.
Best, Jerry