The Mesa 5U4GB is actually an Electro Harmonix 5U4GB tube (they look the same to me as I have both of those in my box of replacement tubes). You may also try the TAD RT504, it is a 5U4GB tube. RT504 is their part number.
Even some NOS 5U4GB tubes may be suitable. I never had any issues to date with either of the two dual recs (Roadster and MWDR) with the Mesa branded EH 5U4GB tube.
I have, however, had plenty of issues with the Mark V90 killing the 5U4GB tube. When the first one failed, I tried other variants to get the same result. It would work for a while and then blow the fuse when powering up again. Tried a few NOS versions only to get shorted tubes from the vendor. plate was shorted to cathode which is not supposed to happen as there is no grid or anything in the tube except for the cathode that could shed material. I finally got a replacement which worked fine but removed it and installed a current production EH 5U4GB tube and has been in the amp for a few years.
Not all rectifier tubes are created equal even if they have the same base part number. They have their limits based on voltage or temperature. Even when using the silicon diode rectification, the tube is still in the power supply circuit so if it fails due to poor quality or end of life, it will blow the fuse. The Mark V90 I always used at 90W so it does not use the Rectifier tube. Since it was always arcing in the tube when I shut it down, that will destroy any tube. Also had overheating issues with that amp to start with.
I personally have not tried the JJ 5U4GB tube. It is bad enough that Mesa is using the JJ 6L6GC tube in the Mark VII. I have not had much luck with the JJ power tubes in the past so I try to avoid them as much as possible. Perhaps the STR445 is an exception.
If the tube has a hard short, you can measure resistance from the plate to the cathode.
If you can measure continuity between pin 4 and pin 6, that would be a major failure, the plates should be isolated. Also if there is continuity measured from pin 4 to either 2 or 8 (heater/cathode) or from pin 6 to either 2 or 8 (heater/cathode) the tube has an internal short, cathode may have decomposed (coating separated from the element). It should be an open circuit since there is no physical connection between the plates and cathode/heater. There should be some measurable resistance across pins 2 and 8 as that is the cathode/heater element in the tube. If that is open circuit, it is a dead tube.
