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.
Best Answer
This speaks volumes.
Filaments: Using 5V on 6.3Vrms heaters isn't impossible. You'll produce less heat, which means your cathode is more likely to pit and degrade. However, it shouldn't be noticeable given the low anode voltage. Thus your tube will likely live as long as a normal tube would. You may find that is sounds more distorted than if you heated it properly.
Power: USB 5V charger for filaments - sound like a bad idea, mostly because you're much more likely to get ground loops from using two discrete chargers. So using a linear regulator would be the easiest solution. The only other solution being to use a buck circuit to take 9V down to 6.3Vdc -> 5Vdc (which ever you happen to want to use). This is about the time where someone points out that they're using a 9V battery and that means that you won't get ground loops. Except that every foot pedal design I've seen (and I've seen many) have a 9V wall Jack and a switch so that they don't need to have a battery. So when you have the wall wart plugged in, you'll likely hear ground loop hum. When you're on battery, you wouldn't hear it (assuming that you were in fact using the 5V USB charger).
If you want my honest opinion, which you're getting anyways, don't limit this design to a battery. There's a reason we don't use these tubes with batteries (they do have battery tubes, these are not those) because they take so much power to heat properly. Take the time to find a good high power 9V wall-wart (think 1.5A-2A). Create a buck circuit to drive the heaters at 6.3Vdc (heck, go nuts and drive them at 6.3Vrms some people swear that AC sine wave heating sounds best). And while you're at it, go read up on tube amp designs like from this UK website.