What would happen if i use 5VDC to power the heater for a EL84 (or equivalent) tube? I mean i understand it would run colder than normal, but how would that affect the tube's performance? What about 12AX7?
How well you do depends on the current rating of the 5 VAC winding. As has already been indicated, you should be within 10% of nominal. For 6.3 V. heater tubes, that means not less than 5.7 VAC. How can a 5 VAC winding yield 5.7 VAC? Regulation effects, that's how. You should be just fine running a 300 mA. 'X7 off a 3 A. rated 5 VAC winding. AAMOF, a pair of 'X7s, which draw a total of 600 mA., rate to be OK. OTOH, the 6BQ5/EL84 draws 760 mA. of heater current. You might be OK running a single tube off a 3 A. rated 5 VAC winding, but a pair will bring the voltage down below the "magic" 5.7 V.
If you rectify it using Shottky diode bridge with 15,000 microfarad capacitor you are right in ballpack for 6.3V tubes.
Lower voltage heaters do work. I routinely use my variable voltage DC bench power supply to power bread board projects. So it is easy to adjust the DC heater voltage with a knob. I find that 5V is right at the edge where tube start to work. I've tried looking on the scope at an amp's output while messing with the heater voltage and 5V is right on the edge. At 5.5V you are safe. But you want your amp to comtinue to run even if the AC mains drops to 117V. If the mains voltage is 125 at your bench and you adjust the heater volts to 5.0 it might not work if you plug the amp into some other outlet. The building code allows a voltage drop up to 5% (in long runs of wire) and you might actually see that one day. I'd say 5.5V is a practical minimum
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