The two amplifier tubes UX-201A and UX-171A both need a 5.0V filament supply.
The filament is directly heated, because the tubes come from an era where radios have been mostly battery powered. Indirectly heated tubes appeared in the mid 1920s, but I got the other ones already. I chose tubes that have been produced in the millions, so they are still available. And they have this nice pear shape and use those screw-on-board sockets.
I wanna use them, because they come from the early 1920s and look stunning.

As the filament is the cathode of the amplifier circuit, AC voltage would introduce tons of hum. There are three common solutions to that problem:

1) use a clean, rectified DC voltage
2) use a "humdinger" pot for every tube
3) use an AC filament supply with a grounded center tap

I decided to use option 3 to avoid bloating up complexity by adding additional adjustments and parts.
My transformer has two 6.3V supplies, so I made a symmetrical -6.3V - 0 - +6.3V supply from them.

As the tubes need 5.0V each, I used two equal 15 Ohm resistors for each supply leg to drop the voltage for each tube. It seems to work - let's see if it will be hum-free.