00A - 99Z

The earliest of the tubes had just a two digit designation. Things can get confusing with these tubes though as some use a three digit designation with the first digit just used for brand identification. As a general tube, if your tube is the earliest (globe/balloon) shape and it has a three digit number, drop the first digit. So a UX-301A is really just an 01A. Or a 450 is really a type 50.