Using a direct heater electron tube as space charge tetrode for driving hi Z headphones from the anode powered with a few volts?

At your risk. After re-reading One Tube set Works on Flashlight cell article found on junkbox.com using 49 electron tube. How about tubes starting with 4 indicating 4 volt filament? These should be less energy consuming than Pleiades Electra 2. Why on universe should we expend energy in order to listen to music? Saw today a cat warming herself up under the winter sun.