Ecx123 2-25-2017
The Franks use 5U4GB and Isreal says they are different than even 5U4G. I believe he said 5U4GB takes more current.
A 5U4GB can **handle** a somewhat greater output current load than a 5U4G, but their specs are otherwise pretty similar. The website description of the Franks Mk II indicates 5U4G, however. Perhaps the description isn't up to date.
A 5U4GB will just about always be a suitable substitute for a 5U4G. A 5U4G will be substitutable for a 5U4GB in some designs but not in others.
Charles1dad 2-25-2017
To the best of my knowledge the Millard GZ37 is a drop in replacement rectifier tube.
The GZ37 is equivalent to a 5AR4. It will often and perhaps usually be a suitable substitute for a 5U4G, and it will be a suitable substitute for a 5U4GB in some designs but not in others. There was some discussion of the 5AR4 near the bottom of page 9 and near the top of page 10 of this thread. (Those page numbers apply if posts are sorted most recent last).
Best regards,
-- Al