Lets take a look at 2A3 tube. And how to make good quality filament transformer for AC power supply. On the old RCA 2A3 data sheet manual I found that the 2A3 can be supplied form AC and DC source. The important is the filament voltage and current. At the image below you can see capture of the original datasheet of RCA 2A3. With red color are marked the important parameters for the filament. The criteria for making a high quality filament transformer are listed too.
2a3 has a filament of 2.5V and 2.5A. So first I want to try to wind a secondary winding with center tap . That will be something like 1.25V_CT_1.25V (2.5A). In the bifilar configuration there will be two copper wires coiled together in parallel. The idea of AC power source for the filament, from my point is that the sound of the DHT is better when the valve is powered by AC. But the AC filament mode is not so easy to make it silent. The hum induced by the power source goes directly through the filament to the signal path. Different designs can be applied to reduce noise. The most classic is put center tapped winding. Look at the image below, pls.