A headphone amp and the cold war

The cold war

In my previous post I mentioned a subminiature rod pentode, the 1sh24b. These tiny tubes were developed in sovjet russia, during the cold war. The point being that a vacuum tube would withstand an EMP, which would fry solid state circuits. Besides, communism didn’t exactly encourage factories to invest in expensive equipment to produce transistors and integrated circuits. As a result, the Russian army continued using vacuum tube technology well into the 90’s. Their remaining stock is widely available on many web shops, typically for a few euros each.

Choosing a project

Enough history, back to my own projects.
After finishing my dual EF86 preamp, I wanted to do another tube based project. The biggest issue with the EF86 is its price combined with the wildly varying quality of modern production tubes; it’s a bit of a waste to buy 6 tubes at 20 euros each, only to find that none of them are of sufficient quality. I therefore decided to look for a cheap alternative, with good characteristics. Of all the options, the 1sh24b seemed the best, by far: they’re the size of a pencil, have a lifetime of ~5000 hours, are intended for military use (meaning that they can take a fair amount of abuse), can be used at low plate voltages and are extremely efficient. At 1 euro a piece, they certainly won’t break the bank either.

Experimenting

Information and datasheet are hard to find, and mostly in Russian, but with some trail and error I had it amplifying sound without too many issues. Ok, so now I had a tube amplifying sound, now what… . This particular tube is not the best choice for an overdrive/distortion, as it is very linear. I tried to add buffers and a tone stack to make a nice boost, but that didn’t work out the way I wanted either. When digging through some components, I came across a small audio transformer. Although I didn’t have the tools to easily determine the output impedance of the tube (or a proper datasheet for that matter), a 7K primary and 4 ohm secondary seemed to be about right.

A working headphone amp!

After connecting the transformer to the plate, I connected my in ear headphones to the output and guitar to the input. At first there was no sound, loose connection. Then, nice warm, clean guitar sound! The volume was just high enough, so no level control needed. What it did need however, was some sort of low pass filter, in case I wanted to use a distortion pedal with it. In the end I  settled with a combination of negative feedback and a full pi filter.
Currently the whole circuit is still on the breadboard, as I haven’t had the time to build a proper power supply (I’m currently using a lab supply), or work on it at all. I’ll update this post when/if I’ve made any progress. No sound samples unfortunately, as all my recording gear is in my own apartment, and all my electronics stuff at my parents house.

To conclude, the 1sh24b is great to experiment with, and can definitely be used for a guitar effect. I am amazed by its efficiency (the whole circuit draws just ~15 mA @35V, including heater supply!) and specs. For a single euro, I highly recommend playing around with this piece of sovjet history.

Looking ahead

Of course, this is not my last project. When going through the same web shop from which I bought the 1sh24b, I also ordered a hand  full of 6sh2p-ev’s, which also originate from Russian military stocks. This is the same tube that is used in the ‘culture vulture’ distortion, so at 1 euro each I thought it should be worth a try. On the digital side of things, I have been trying to get several effects working in pure data, with varying success. Moreover, I intend to build an Arduino based midi controller in the near future, to control my HOG2 and pure data effects. The main advantage of creating effects in pure data, is that I don’t need any other test equipment or components, just a laptop. While I do have a small stock of components and some test equipment at my parents house, I do not have enough space in my own apartment for anything that isn’t already there. Therefore, working on analogue effects is currently limited to about two weekends per month, during which I also want to do other things. I am therefore looking forward to working on digital guitar effects and new ways to create noise through DSP.

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A previous rendition of the headphone amp. A few days after this picture I condensed the  circuit to a quarter of a breadboard. The tube is in the middle of the right breadboard.

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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.