This is a write-up of my experience in building this very nice sounding module, for anyone who might want to build or modify it.
The idea was to build a VCA that can double as a voltage controlled wavefolder and overdrive/distortion with common and low cost parts. More specifically:
- Any dual control pentode (originally made for TVs or radios) priced at EUR 2-5.
- A generic 12V 1A DC “wall wart” type of supply (which you probably already have several lying around).
- A boost (or buck) converter module found in AE for around EUR 1-2, to derive the dual power supply.
- A voltage regulator, and an assortment of common semiconductors and passive components, all for not more than EUR 2.
Of course, there are other very nice projects around that do similar things. For example, you can build the Valve distorting VCA or the 10LE8 tube wavefolder/drive, but the former calls for a double triode which is priced around EUR 25, and the latter requires a twin dual control pentode, which is a specialized and more difficult to find tube. The aim here is to build a module that can do more, for a fraction of the cost (or hassle).
For the project described here, you will need a dual control pentode, i.e. a pentode in which the suppressor grid (g3) is made available at a pin.
In this example, the suppressor is available at pin 8.
Joe Sousa has written a nice accessible piece about dual control pentodes that illustrates how they work and lists their historical uses. At the end of the article he lists many tubes of this kind, but there are many more pentodes designed for dual control, most of which can be found NOS (or at estate, garage sales) at very low prices. The Russian 6J2P which is still made in China and found in AE for around EUR 2, is also an option. You just need to consult the tube datasheets (all of which can be found online) to make sure that the tubes you are perusing are suitable.
My guess is that these tubes are sold off at low prices because they were produced in mass quantities for TV manufacturing and, later, TV repairs but sold off cheaply in bulk when the technology became obsolete way before factories ended their production. The fact that they are not typically used in audio(phile) or guitar amplifiers, makes their contemporary use very niche and keeps their price low, at EUR 2-5 a piece.
Enough with the introduction, on to the project!
I thought there was no need to reinvent the wheel, and I used the tried and tested CGS27 “tube VCA wave folder” by Ken Stone as the starting point. In fact, this circuit worked so well off the shelf so I did not bother with making substantial changes. The project described here is nearly identical, save for a few modifications to account for the dual control, the input boost (described below), and the heater/supply considerations. Note, that this circuit is different than the, more known, Ken Stone tube VCA (CGS65) which uses op amps and a specific Russian subminiature pentode (the prices for which have gone up considerably).
My changes to the Ken Stone circuit, and the implementation for my particular build, are the following:
- Added an input and a series attenuator (B100k) potentiometer followed by 1M resistor to ground to the suppressor (g3) pin, to allow for the second CV input that can be used as the typical amplitude modulation input of a VCA. This is therefore the second CV input of the module. The first CV input is the one shown in the Ken Stone circuit, which works as an offset control when used as a wavefolder after a VCO, or a bias/boost control when used as a VCA or distortion module.
- Added an op amp amplifier at the input which can by used to boost line level signals to levels that can saturate the tube input. Again, no need to reinvent the wheel, so I used the CGS60 “Stomp box adapter” by Ken Stone (“from effect … to synth” bit). The op amp circuit can be completely bypassed using a DPDT switch, when using synth level signals. A crucial thing is to keep the input attenuator “drive” control after the op amp so that the potentiometer drives the tube input and not the op amp input.
- Tube filaments draw a lot of current (typically 300mA, but more than twice that for a good few seconds when the filaments are cold). Ken Stone did not specify how this can be achieved in a modular synth in which so much current for a single module is a luxury. I went for a dedicated module supply using the ubiquitous 12V DC centre positive 1A “wall wart”. For tubes that specify filament heaters up to 9V (typically, 6.3V) one can use an LM317 regulator. For 6.3V heaters at 300mA, I found that the inrush current of the cold filaments could be a little too much for the regulator, so I came up with the following solution: I adjusted the output of the LM317 to 7.8V and dropped the remaining 1.5V through two 10Ω current limiting resistors in parallel. I found that this limited the inrush current to less than 1A which is something the LM317 and an 1A “wall wart” can handle. A better solution could be a soft start circuit using an extra transistor as shown in the LM317 datasheet. Note that dropping ~6V at 300mA will get the LM317 and the 0.5W 10Ω resistors hot. My solution was to add a small heatsink to the LM317 and use 2W ceramic resistors. Technically overkill, but I was uncomfortable building something which was too hot to touch. With the specified parts, the heatsink and resistors are still hot, but not too hot to touch momentarily.
- To get from 12V to the +/-15V required for the rest of the circuit I ended up using a “Double SEPIC” module based on the XL6007 IC (relevant discussion of the module can be found here). These modules can be found in AE for as little as EUR 1.5 (postage included). The module proved very noisy due to the inductor used therein, but the addition of a 1000μF/100Ω lowpass filter at the module’s output worked like a charm. The XL6007 regulation and internal soft start circuit dealt effectively with this RC filter’s concerns of voltage dropping or additional inrush current, although I did end up using two different “power on” switches for the heaters and the main circuit to keep the total inrush current at manageable levels when using a supply rated at 1A.
All of the above are based on my experience from building a prototype, and playing around with a few other options. They are not a “how to” recipe. You can probably come up with better ideas of how to achieve this (and your suggestions are more than welcome!). At the very least, you will need to adapt the above to your specific needs, and to the needs of the particular tube at hand. For instance, I found that tubes with odd heater needs (12.5, 15, 16, 19V…) can be effectively powered by a 19V laptop “brick” power supply, another ubiquitous device which we all probably have in a box somewhere. Actually, a single laptop “brick” can easily power 3-4 such modules. In this case, the bonus is that odd filament heater voltage tubes are generally more plentiful and/or cheaper. I also found out that the circuit works at +/-12V.
The module ready for action at a friend’s modular…
I will try to come up with some oscilloscope pictures and/or sounds from the module, but that might take a few months with the holidays and then another hectic academic year coming up. In the meanwhile, keep the comments coming!