[captainclock1988]

Except for looks they are the same , BUT, if there is NO SUFFIX, and the tube has a metal outside, it can be a problem in Philco radios. These metal tubes use the case as an RF shield, and it connects to a pin that is connected to the ground plane of the radio. Philco radios use the G or GT types which do not use this pin, and so use the socket solder lug as a tie point, which can be live, so you can't use a metal tube in Philcos safely, metal does not substitute for a G or GT type. This is why many RCA radios don't use tube shields for their RF, oscillator, and IF tubes, the metal tube IS the shield, but Philcos and other radios use those shields and G or GT tubes.

Hi Mike, does this also apply to Zenith radios as well? 
I'm wondering because I have a 1942 Zenith 10S690 Radio that all of the tubes in the radio are of the G type variety of tubes and all of the critical tubes in the set had shields around them, there was one tube in my Zenith that was replaced at one point in time that was replaced with a metal cased tube and the original tube shield was tossed when they installed that metal cased tube into the radio (the radio other than the 6A8 tube and the 5Y4G tube has all of its original Zenith tubes in it yet).