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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