The 5U4GB valve is a full-wave direct-heating(no indirect filament Soft Start) diode, designed to operate as rectifier in power supply for CRT TVs, amplifiers and other equipments with high direct current consumption. The 5U4GB valve has a standard T-12 bottle with straight sides, which allows it to be used in place of the 5U4G and 5U4GA types.
Currently 5U4GB is used in guitar amplifiers as the Mesa-Boogie Dual Rectifier and Mesa Boogie Triple Rectifier or Fender EC Twinolux.
Amplifiers that use the 5U4GB tube or another vacuum rectifier tend to have fewer problems due to over-voltage and power spikes. Because it has considerable thermal inertia, the 5U4GB takes a while to come into operation, giving the equipment a "soft-start" whenever the power is turned on.
The 5U4GB valve can supply currents of up to 300mA with peaks of up to 1A and starting transients of up to 4.6A. However its possible use 5U4GB with filter capacitors with much higher values than other rectifier valves.
I have seen 5U4G and GB, but not GT.
5U4GB are suited to currents up to 275mA at 450V.
Usually the G refers to the Glass Bottle.
GB has a bit more max current capability.
Said it I would prefer NOS 5U4G 1940/50, but no Sylvanias 1970s.