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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).

Matsumins valvecaster

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.

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and R3 is 270k (works a lot better for some reason).

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.

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  • \$\begingroup\$ Thanks, that was quite informative :) The main point is using a battery to make it portable! But it's far from a 9v battery - it's a fully loaded 5AH (5000mAh) 12v battery. With the current design it should last no less than 10 hours and if you're cranking it like crazy then no less than 6 hours. That's plenty of battery time :) Also I went quite crazy and managed to fit a whole 1-2A charger inside the casing. It's really really messy as it was my first project on which I have a fetish to keep improving it. Everything fits perfectly and it's a quite small box in comparison. \$\endgroup\$
    – Giedrius
    Commented Mar 1, 2016 at 11:05
  • \$\begingroup\$ @Giedrius, In addition to what I was talking about then, you'll want a buck boost with good (maybe even great) filtering to keep your anode voltage steady throughout the battery's life. Changes in anode voltage will definitely effect sound (more so than heater current). Keep tubing. \$\endgroup\$
    – Dave
    Commented Mar 2, 2016 at 12:17
  • \$\begingroup\$ Thanks :) I was thinking of a simple DC DC boost but so far I have not come up with simple and effective solutions without using chips (I'd like to avoid them, just for the sake of simplicity) but the other problem is efficiency. Maybe you have some simple suggestions as I believe you have much more experience in tubing? \$\endgroup\$
    – Giedrius
    Commented Mar 3, 2016 at 9:57
  • \$\begingroup\$ @Giedrius, That's a good enough query to warrant another proper question. There are plenty of folks on here with tube/valve experience. Also, there are chips that are quite robust and forgiving, so don't be too scared. \$\endgroup\$
    – Dave
    Commented Mar 3, 2016 at 12:55
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Although I haven't tried it, I don't think a 12 volt heater would get hot enough on 5 volts to allow the tube to work. However, the 12AU7 has a center-tapped heater, allowing it to officially be used on either 12.6 volts, with the two sections in series, or on 6.3 volts with the sections in parallel. The 6.3 volt connection would probably work as well on 5 volts as the 12.6 volt connection does on 9 volts.

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  • \$\begingroup\$ I'm using an ECC85 tube not a 12AU7 so it needs 6.3v for heater filaments. It's very similar to 12AT7 but cannot be run on 12v. \$\endgroup\$
    – Giedrius
    Commented Feb 29, 2016 at 23:40
  • \$\begingroup\$ The low voltage tubes that are sometimes called " Space charge Tubes " run more heater current than a normal tube .At a guess you should give it the required voltage. \$\endgroup\$
    – Autistic
    Commented Mar 1, 2016 at 2:17
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Thank you all for your help! I've decided to stop playing and wasting energy with 6.3v heaters. Going to go for a 12AT7 which is a very very similar tube to the current ECC85 inside the amplifier. Found a very good deal on NOS Telefunken ECC81.

ECC81/12AT7 can be heated directly from 12v power supply so no more hassle with energy wasting LM317 I believe this is a far better solution than going through a LM317. Especially in efficiency! If I use LM317 to heat ECC85 I'm using more than 300mA to heat the tube BUT if I am using ECC81 it drops in half - only 150mA.

I don't think that using LM317 is a bad idea in a whole but when it comes to portable designs and efficiency.. well it really is quite terrible at that

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