Low voltage tube preamps

Like most guitarists I love the sound of a good tube amplifier. I first got into tubes when my uncle brought a broken hifi tube amp for us to work on. He had been building radios since he was a small child, while I couldn’t tell the difference between a resistor and a capacitor at the time. We slowly worked on the amp in the weekends (although he did most of the work and I was just watching and asking questions). When some of the tubes had to be replaced, he tried to find some spares in the basement at his job (he worked at a university). When he came back for a few more, they proposed that he would simply take all of them, since no one would ever use them again. As a result, I now have a decent stock of tubes.

Rather then just leaving them collecting dust, I wanted to do something with them. The most logical step would be to build my own tube amplifier. However, the complexity and cost were a bit too much.  After some online research I found the valve caster, a ECC83 (12AX7) preamp, with 12 Volts on the plates. To my surprise, the schematic was dead simple and I had it running on the breadboard without too much trouble. Although it sounds fine, it didn’t give me the sound I wanted.

When looking through the stock of tubes I came across a number of EF86’s. On the internet I also found a schematic of a starved plate EF86 boost/overdrive. To keep the heater supply simple I just added a second EF86, running the whole circuit on 12 Volts, without the need of any regulators or large resistors. The result was a very touch sensitive, warm boost or overdrive, exactly what I was looking for! I did have some issues with ‘ghost noting’, although I’m not entirely sure if that’s the proper term. It appears that at certain settings there was a 120Hz amplitude modulation. Although some hum is not a huge issue, too much ghost noting is, basically you hear both the fundamental, as well as the fundamental plus 120Hz, which is out of tune. When used less extremely, it can actually be used as an effect by it self, adding some eerie ambiance (giving the feeling something’s wrong, but you can’t really tell what). The song below illustrate this, although it’s very subtle.

After found what caused the issue (a few missing capacitors), I fixed it and made the ghost noting switchable. The pedal has been on my board ever since and is always on when I’m playing, regardless of the genre.

I can’t find the schematic I used, but here is a layout of a similar project, although this one only uses a single EF86. There are few problems with this project though. Mainly because the EF86 is not intended to work at such a low plate voltage. Most good new old stock (NOS) tubes will work, but only the best will give a good result. I’ve tried several new production EF86’s, but they didn’t give any audible output at all. So unless you happen to have a few EF86’s laying around, this project is probably not worth it (unless you don’t mind spending a ton of money finding the right tube).  A more realistic consideration would be to use the special quality version (E80F) of the tube instead. The downside is that they’re even more expensive and quite a bit larger too.

In my next post I will explore the incredible 1sh24b subminiature rod pentode as the only active active component in a headphone amplifier.