Dual control pentodes and YOU

 My very first attempt at working with valve circuits (I prefer the term ‘valve’ over ‘tube,’ despite being American) was trying to recreate the Metasonix RK2 VCA. This was in June of 2021. I didn’t want to just buy one, for several reasons.

• They’re expensive, and I’m broke.
• I have an illogical and unjustifiable hatred for eurorack, with it’s teeny tiny little twiddly baby knobs and little baby jacks.
• I would rather build something and learn something in the process.

At that point, I had never messed with valves, ever. The only thing I knew about valves was that they were ancient, operated in an alien way, and made alien sounds unlike anything else. So, after thoroughly researching and learning about how tubes operate, and how they could be used on low-voltage circuits, I figured that the best way to learn was to just do it. Just get the damn tubes, and start building, and seeing what happens. Too many people get stuck in the research phase.

So, I read the manual for the RK2 and learned about the tube in it - the 15LE8. It is a type of valve called a ‘dual control pentode,’ and there are a variety of them. A lot of dual control pentodes, such as the 6AS6, are made in such a way as to allow the suppressor grid (number 3 grid) act as a control element, in addition to the normal control grid (number 1 grid) and screen grid (number 2 grid), but on a schematic, they look identical to any other pentode. 

The LE8 series of tubes (6LE8, 10LE8, and 15LE8 - all the same besides the heater requirements) are unique, however - they have two separate plates, and two separate number 3 grids, while sharing a number 1 and number 2 grid and cathode. There are a few other tubes that are like this, including the 6HS8 and the 6BU8. 

Anyway, to make a long story longer - I took a crack at trying to figure out what the RK2 was doing, and then trying to replicate it on my own. After a bunch of tinkering, I came up with this (at the time I was thinking I would call my 'synth brand' or whatever 'asphodel meadows'):

(click through for full size)

At the time that I made this, I didn't yet have access to any kind of power supply bigger than 12V, so I decided to just... try it on 12V and see what happened. 

This is what happened:

I posted these results to the Look Mum No Computer forum along with a stripboard layout, available here.

This is a 12V circuit. It is safe to use for the vast majority of people who are already into synth DIY. Also, with the exception of overvolting the heaters, tubes are VERY forgiving. 

This is just the first version of this, though. I have, since then, developed and put together a standalone version - I'll detail that in a later post and I'll also try to make it more informative so that anybody could potentially copy it and build it themselves.