XX/Judgement - Twin Pentode Waveshaper

 I originally designed this circuit back in 2021. It was built from the schematic and I verified its function on 27 June 2021. I originally posted this to the Look Mum No Computer forums. That diagram was a fucking disaster. It is seriously the worst. It is a jumbled fuck-fest of nonsense and I can not believe that I made that and thought, 'yeah, this is legible! this makes sense! i will share this!'

Anyway, I redid the diagram and posted it in that thread and I am also posting it here.

Click through for full size.

How does it work? 

I don't have a clue, honestly, but I will try to lay out my thought process.

I originally wanted to make something using this tube because the Metasonix RK2 XS-VCA uses it. Since it uses the single tube, I figured it would be an easy point to start experimenting. I also wanted to make something that would run on a standard Eurorack power supply, because this was going to be a Kosmo-format module (For more information on Kosmo format, click here to read about it on the Look Mum No Computer website. In short, the format was created by Sam Battle and it is superior to Eurorack. Eurorack sucks. Eurorack is for babies. I hate Eurorack.).

Anyway - the RK2 uses a 15LE8 but at the time I decided to get the 10-volt heater version instead, the 10LE8. Why? It is inexplicable. I have no idea, it was four years ago. Any of them will work for this so long as you accommodate the heater requirements.

The 15/10/6LE8 is called a 'twin pentode' in the General Electric tube manual. I will quote the (frankly, far superior General Electric) datasheet verbatim:

    "The 6LE8 is a miniature twin pentode having separate plates and number-3 grids for the two sections with a common screen, number-1 grid, and cathode. It is primarily designed for use as a color demodulator in color television receivers.
    The high current of the 6LE8 provides enough voltage output to drive the grids of color picture tubes without further amplification. The common screen grid provides internal matrixing of the red and blue information to provide sufficient drive to the green gun.
    The characteristics of the 6LE8 are such that it may be used in either of two modes; (1) the chroma information to grid number 1 and two phases of reference signal to the two number-3 grids, or (2) reference signal to grid number 1 and two different phases of chroma information to the two number-3 grids."

Reading this, I latched onto the third paragraph pretty hard. That sounded interesting to me. Like something I could exploit or fuck with. In my head, I substituted 'chroma information' for 'CV signals' and 'reference signal' for 'audio signal.' With that in mind I concocted this monstrosity. I do not have any explanation as to why or how I picked the component values I did. I think it was literally just trial and error, and once it sounded suitably fucking evil I said 'good enough for rock and roll' and that was that.

There is an old Youtube video I made of it here.

There is also a stripboard layout I made for it, before I learned that DC-DC boost/buck converters were a thing - it has an LM350 on it for the heater on a 10LE8. Stupid. Just use a boost or buck converter module. If you need the stripboard layout you can find it in the forum post I linked above.

The best part is that I can point to this circuit as a good starting point for anyone wanting to get into tubes for synths and modules. It runs on 12 volts and uses a tube that is fairly easy to get for a decent price, at least in the States. 

One of the most frequent questions I got regarding this module was whether or not another tube could be used - the answer is no. The xLE8 is a unique beast. I am not aware of any other tube like it.