If your 5U4G is new or NOS, measure the B+ voltage at the HV first power supply cap. I assume you have read the usual safety precautions about dealing with high voltages.
Make a note of B+ DC V.
If in the future your amp sounds wimpy, check the B+ voltage again. A drop of more than 20 Volts as compared to new specs means that your rectifier is weak (though I have never seen it happen).
If you suspect your rectifier to be weak, compare it to a new tube.
You can probably expect a 20 year lifetime from major-brand NOS 5U4Gs.
