I this world, there are lot of tubes, with different filament supply. In most of my tube rectification application, I stand with two choice of rectifier, the one with 5 Volt filament (most Octal and UX type) and the others with 4 Volt filament (RGN or AZ families).
The best way to use two type of rectifier with different filament voltage is to use dedicated switch to select the exact filament voltage. You can also use Resistor to drop the filament voltage (up to 1V drop and 1A), but this way is not recommended due to a lot of heat must be dissipated (also not too good in terms of sound quality to).
I just finish the switch installation this afternoon. Now I can choose to use 4 Volt or 5 Volt filamen voltage. You can see the picture below.
The idea is quite simple. All you need is a dedicated transformer with 4 Volt and 5 Volt AC supply. Then, you can buy a good SPDT switch and arrange as below schematic. So now you can choose to use either 4 or 5 Volt filament voltage.
Oh, don’t forget, you must create a DIY socket converter to. On below example, you can see an Octal-to-B4 socket converter. Not a neat design, but it just simply works!
Enjoy the flexibility of 4 and 5 Volt type rectifier. Enjoy the good sound to!
PS: I would recommend 83 Mercury Vapor Rectifier (just pay extra attention when handling this one, since Mercury is deadly!) and GZ37 for 5 Volt rectifier (some 5U4C of Russian also not a bad choice for under US$ 10). For 4 Volt rectifier, the RGN and AZ families are most recommended.