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.



Staying true to the title of this blog, I’ll first share the ideas behind two of my most experimental ‘songs’. The first track I’ll discuss was recorded several years ago. Back then, I didn’t have much fancy equipment, so I had to be creative. At my disposal were an electric guitar (an Epiphone LP black beauty), a Boss HM-2, a Boss FZ-2 and a 5 watt Blackstar amp. I simply put the guitar in front of my amp and let it feedback. With the controls on the guitar, effects and amp, I had some basic control over the color and pitch of the sound. It turned out to be quite an interesting exercise, as the sound responded both instantaneously, but also on a much longer timescale. Balancing between uncontrollable ringing and no sound at all proved to be difficult, but not impossible. It’s also worth noting that the distance between the guitar and the speaker are crucial if you feel like trying this.

The second track I would like to discuss was recorded much more recently, last week actually.

After buying a HOG2, one of the first things I wanted to try was to manipulate (white) noise and hum into music using additive synthesis (i.e. by adding harmonics to the fundamental). The noise mainly comes from a hyper fuzz, with no input signal and high gain settings. This method proved to relatively easy compared to my first attempt at harsh noise, as the signal is always in a stable state (it won’t change, unless you turn a knob). The down side is that it’s harder to create a slowly, organically, evolving sound. Some delay and reverb help, but it’s not the same.

Who knows, maybe I’ll combine to two methods to get the best of both worlds.