• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

5V AC maining for filaments

Schottky diodes are necessary as they have a very low voltage drop, around 0.3-0.5V per diode. If you use a diode bridge at less than half it's rated current you can hold the voltage drop to around 0.8V. So a 5 V transformer would provide (5*1.414)-0.8=6.27V to the heaters.

If you run the heaters at less than nominal-5%, the tubes don't work well.
Ah,yes...ok, but the question was not how to power DC voltage, but if is possible to power with lower AC voltage...
Yes you can, at least with preamp valves, but the characteristics of the valves will not match the data sheet. (lower gm, higher internal resistance).

Reduced heater voltage is not recommended for rectifier valves or power valves.
I did something similar when I designed a microphone preamp. I used a transformer with a 6.3 volt filament winding. Fed that into a bridge. Filtered it with 20,000 microfarads and, because the filament voltage was a bit high, added a series resistor.
The resistor acted as a filament soft-start. I built several dozen of these preamps. They're still operating after 20+ years. Haven't lost a single tube.
From past experience.
I used a 5V @ 2 Amp spare winding on a power tranny.
4 off 1N5822 (40V 3Amp) Schottky diodes in a bridge.
1 off 10,000uF/10V capacitor.
Gives 5.95V DC when driving 0.6A heater current.
That is just on the lower limit of the 6.3 +/- 5% heater voltage specification.
Worked well for me in a HiFi Amp (a Baby Huey).

I would NOT run a 6.3V heater from 5V AC.

Steve Bench had an article on runing tubes at redused heater voltages. I don't have a link to that anymore. But I seem to rember him measuring some 5V directly heated triodes using voltages as low as 3.2V or lower. It seems the distortion was a lot lower at lower filiment voltages.

Here ya go: Directly Heated Triodes operated with lower voltage on the filaments.

Getting the voltage of DH types down reduces the "disappearing cathode" problem. As seen at the cathode, the effective plate voltage isn't much more than 5 -- 10V. Let that voltage drop, and parts of the DH cathode can "disappear" as the filament heating voltage overcomes the plate voltage. That converts a 1.5 law device into a 2.5 law, or even 3.5 law device, with rapidly rising distortion.

It's one possible reason why types like the 45 and 2A3 are said to sound better than 300Bs -- the lower filament voltages. Possibly why the Chinese also make a 2.5V "300B" as well.

Of course, deliberately cutting down the filament voltage below spec means derating the plate current as well, otherwise, cathode stripping is likely to compromise service lifetime.

It's of no consequence for IDH types, so run 'em at heater voltages that're within spec.