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The newer 5U4GB's are a little more stout in design over the G's. However if your set was designed to use the G types then there should be no issue with either.
As for myself, I would use what I would have most of in my stock considering that the GB's are more plentiful to buy. You should get just as good of a service life out of either as well.
The 5U4 family of tubes were so commonly used and a popular choice that they are still abundant today. (however prices seem to be going up!)
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