Let me take a stab at providing some useful info. As a start, GZ34 and 5Ar4 are different designations for the same tube. It is a rectifier tube. The most popular old tube from the glory days were Millard GZ34 tube made at the Blackburn plant in England. Good ones are hard to find and expensive. A very good new production tube is the Gold Lion 5AR4. They go for about $40.
Is this the kind of info you want.
Is this the kind of info you want.