I know they are both rectifiers and i see the 5Y3WGTA is classed as military grade.
I have 2 6V6 amps, one is scrap only good for its tubes etc, my question is one amp
had the 5U4GB as the rectifier and the other 5Y3WGTA as its rectifier are these interchangeable, are the pinouts and voltages the same?
Thanks for any help.

Oh The 5U4GB is GE canada made and the 5Y3WGTA is Sylvania if that makes any difference.

5U4 can handle more current but has a 3 amp filament while a 5Y3 has 2 amp filament.

A 5U4 will replace 5Y3 if the power transformer can supply the extra current without over heating. Also 5U4 is larger.

Yea, beware. There are other tubes that can drop-in replace a 5Y3 but the extra filiment draw of some like the 5U4 have been reputed to fry transformers, eventually. Scarce old irreplaceable transformers. Look in the tube specs of a tube manual for the other 5 Volt rectifiers. I have seemed to have seen some other 2 Amp filiment types that could work, but they have different characteristics, like maybe less voltage drop, different peak Voltage and output current ratings, etc. Maybe the metal 5T4, and the like? High voltages might vary some with different tubes.

Best to go with whatever the schematic/manual specified for that particular model.

A 5Y3 is typically more than enough to handle a pair of 6V6 tubes. With four or more, then the current draw would call for a rectifier like the 5U4.

Larry