[morzh]

Rod

No. That is yes, I know this technique, but there are cons:

1. If I were to use DC on all tubes, we haved 6 tubes at 1.75A each. This makes it about 10A RMS. This said, I will need a bridge rectifier as the winding is not centertapped double voltage. 4 diodes, each dissipating roughly half that current times Vf which (Schottky could be used as the voltage is low) is about 0.4 - 0.6V. Lots of heat.
2. The voltage will be dropped by 2xVf. That is about 1V. Considering the filament voltage is 2.5V it is too much.
3. Yes I could bring DC to 47 tube only. Less current, but still a lot of reduction of the Filament voltage.
4. In either case a decent size capacitor is needed to filter the DC as the current is large.
5. Last but most important: I try to stick to original circuitry, except for cases where there is little choice, like dropping the voltage in AC/DC radios and using a capacitor to do that instead of huge resistors or curtain-burnere cords.

All in all, even though possible in theory, the early radios with low voltage / large current filament tubes are not the easiest to use DC for filaments.