Tube Timbre Trasher, Not Done, But It Works

DiWhy QUESTION! why start building something that you don't completely finish? 

Answer: Because you can!!

it works!

This time I used two 1J24B low voltage pentodes, a tube I first learned about from Ken Stone's tube VCA, to create a timbre modifier. The result is a "waveform folder" common to the West Coast synthesizer sound.  Two 1J24B pentodes a few op amps and caps are almost all you need.

You may want to read previous posts here (introduction to 1J24B), here (experimenter PCB for this concept), and here (the 2x tube breakout board) before running with this project. 

We begin with 3 PCBs from my generous and always enthusiastic sponsor PCBway, check them out here. 

Clockwise from the right: primary signal processing and conditioning, a dual tube breakout board, and I/O like jacks and pots.

First I built up the 2x tube breakout board:

 

Then on the the main PCB. 

There was a trace error in the main main PCB, which is fixed in the project's post at github (here). I was sending too much current through D1 and D2 on the dual pentode breakout board and I kept blowing up the diodes. Took me a while to figure that one out, but I got it sorted.

Bodges R Us! Can you spot the trace fix?

Otherwise, other than calibrating the 2 trimmers, it worked first time. It was tricky to set it up. 

If I over-biased the CV's and audio signals it distorted the incoming signal so much that I ended up getting nothing at the output except an op amp slammed against its supply rail.  

Best way to calibrate: put a 0-5V triangle wave the CV input and a 10V ramp audio in then look at the corresponding testpoints with a scope. Turn the CV and audio input pots full up. You should see the grid signal modulating from about -1V to about -6 and the screen between about +12 and +15V.  If not, you may have a mistake somewhere, trace the signal to figure it out....If the incoming signals are making it to the tubes, put a 10V ramp signal at audio input use a 0 to 5 control voltage at CV in.  Adjust the trimpots until it sounds the way you want. The CV trim will have to be adjusted a fair amount, but you will hear the modulation effect come and go when you start to hit the sweet spot.

Overall this design is not finished--I never fabricated a front panel for it, for instance, but as it is, it's OK. For the complexity of this design, it makes more sense to use a simpler approach, perhaps use a Norton 3900 instead of 2 tubes--the two different approaches sound very similar at in terms of how they fold a ramp wave.  

You can hear what the Tube Trasher sounds like here.  I intentionally made the audio clip simple, just a single ramp or triangle audio wave modulated by a single VCO, into my DAW. No effects, no other modules....we hear a 10V P/P ramp, sine triangle from a VCO, modulated by a 0-5v triangle CV from an LFO Prime. 

For normal audio (say, vocals, a string section, a Solina, whatever) the distortion you end up with sounds pretty--well, really, bad. I didn't bother recording that....

And of course as per this sad post, 1J24B's are hard to get right now, and maybe will from here on, due to world politics. 

Power used for the prototype is +/- 15V, 20mA for V+, 40mA for V-.

For euro fans, aka doepfer smokers: the design works with +/- 12V power but I had to recalibrate the trimpots. I got it to work to +/- 10V as well.

If anyone wants to improve and refine this tube based wave folder, have at it, but for me it's time to move on. 

thanks to PCBWay for their support and encouragement while I was building the Tube timbre trasher. 

So what could be next?  

• The tubes can be flipped 180 degrees to not stick off the side of the main PCB. I put the tubes off the side as you see above to help with design and troubleshooting, but flipping them to be less obtrusive is trivial--mount the BOB with 90 degree edge connectors on the other side of the main PCB.  
• You may want to 3D print something to fit between the board and the tubes, and drill some holes to tie wrap the tubes down. As it is, they are pretty fragile.
• I think building a VCA or some sort of high-gain amplification device ahead of the trasher would be beneficial to its performance.  Things sound pretty good with very hot input waveforms, say 15V P/P vs. 10V. The incoming amplitude at input very much influences the wave folding you hear at output.
• Can this fold "normal audio" vs. waveforms? It should be able to. What would it take to do that? I  think I'd have to redesign the way the audio inputs are biased, which might be a lot of work.
• What does a quad version of this sound like, or 2 of these 2x tube designs in parallel? I was pretty excited about this design at first, but lost a lot of steam when 1J24Bs got hard to acquire due to the war in the Ukraine. That alone, at least for now, makes me not want to work on this project too much more; it is so sad..... 

After all that encouragement: Get all the gerbers, pdfs, BOMs, wiring guides, etc., at my github here. Nevermind the spooning--I encourage forkers.

Onwards: It's time to conjure a new C/C++ toolchain and MCU for the next batch of projects. I am thinking RP2040 or STM32, both popular, inexpensive, and fast, and each more capable (albeit complex) than the beloved Atmel 328.  

I won't be posting as much in the next few weeks while I experiment with toolkits and these processors. Oh and return to my day job after 2+ years under pandemic house arrest. After that, I'll be back with a vengeance. Stay tuned.