Is it safe and not harmful to run a preamp tube filaments at 5v? The tube is ECC85. I built a tube preamp in a DIY portable guitar amp. It's very similar to Matsumins valvecaster with a few slight changes so the anode voltage is only 12-13v depending on the battery charge level. It's a starved-cathode design but I really dig the warm slight distortions it gives to a guitar.
My schematic is very similar to matsumin, capacitors I believe are a little different and the gain pot is replaced with 50k resistor and R3 is 270k (works a lot better for some reason).
I am considering this because right now I'm using LM317 but it wastes a lot of power from the battery and also I want to put a USB charger (yes, that's quite silly) so 5v 2A would be really useful.
And if it's safe would the output level be somewhat lower or I should worry about more distortions from the tube as it is a starved-cathode design.
What's your advice and perhaps you have some recommendations on chips that have quite good efficiency of providing 5v. Again, the main part of the question is still whether this would harm tubes and how to get better efficiency for heating them.