The 6U5 has a 6-pin base, whereas the 6E5S is an octal base. The 6U5G has an octal base, but the pin-out is different from the 6E5, so you can’t use the 6U5G as a plug-in substitute for the 6E5.
You could rewire the base to suit, and it might be OK, but you may need to tweak some components.
