thank you very much. A couple more questions:
1) is it able to tell from the diagram if there other rectifier that can be used and handle the current drawn by the 220uF capacitor?
2) is it necessary or recommended to change the other two capacitors? If yes, what values should I use?
3) comparing to adding diodes, in your opinion, which way will yield better sound?
There won't be another tube rectifier that will work better. The problem is that this circuit is designed to expect the voltage drop that exists with the 5U4 at the current at which its operating.
I gave you a range of capacitor values. But to be clear:
replace the 220uF units with 47uF. Replace the 100uF units with 22uF.
Once this is done you'll find additional diodes to be moot. They may also cause commutation noise by interacting with the inductance of the power transformer winding, which can manifest as an annoying low level buzz. If it were me I wouldn't bother and instead work on correcting the problem.