Replacing 5U4G with 5U4GB may result in higher rectified voltage, so should never be done. Replacing 5U4GB with 5U4G may result in lower rectified voltage, and may result in damage of the 5U4G rectifier.However, here are vintage datasheets for the 5U4G and the 5U4GB. Note the statement in the first paragraph of the 5U4GB datasheet that:
The 5U4-GB ... may be used as a replacement for either the 5U4-G or 5U4-GA.Also, while the detailed parameters indicated in the datasheets do suggest that substituting a 5U4GB for a 5U4G will result in higher rectified voltage, the difference does not appear likely to be major.
Finally, my older Sam’s Tube Substitution Handbook lists the 5U4GB as a direct substitute for the 5U4G (while of course not listing the reverse). I have never known its listings to be inaccurate, and I have often found them to be overly conservative (i.e., to err on the side of caution) if anything.
In any event, all of this is of course superseded by the indication from Israel and in the manual that the 5U4GB should be used in the Franks. He should update the website description of the Franks, though, which indicates 5U4G as I mentioned.
Enjoy! Regards,
-- Al