SS TT FF and DDS for dummies code article

[idiot savant]

Super Simple Tube Tremolo For Fun

Before you read this:

This is a HIGH-VOLTAGE project. If you don't know what you're doing, or have any SLIGHT doubt about working with DEADLY voltages, do NOT attempt it. Your life is NOT worth risking for a dumb guitar pedal. YOU HAVE BEEN WARNED!


the SS-TT-FF :

This is a  super simple tube tremolo built with junkbox parts utilising a very simple digital DDS LFO.
I'm no whiz at AVR programming, and some of the people here are truly brilliant when it comes to design and coding. Control functions were easy to pick up for me, but slightly more complicated subjects like DDS are more difficult for my mind to abstract.

I put together some sample code for the "simplest DDS wavetable LFO" that I could think of. This came about because the myriad of AMAZING projects utilizing processors has skyrocketed recently. Unfortunately, most of the free code posted is difficult to follow, or poorly commented, so there isn't a simple step-by-step walkthrough for dumb guys like me.

I chose C as the language because it is more readable to the analytical mind than ASM(for me).

This code should drop right into AVR Studio, and compile correctly for the ATTINY85. Please read the code and remember to disable the CLKDIV8 fuse.


This is a (hopefully) readable, understandable example code "article" that may help others get into embedded development, and ancient tube technology, because they are perfectly happy coexisting.

As I said before, this is a DANGEROUS project, only those CONFIDENT in maintaining a SAFE work environment should partake.


Nuts, and Bolts:

It is well documented that a pentode tube's screen voltage controls it's overall gain. I also read several articles at CGS SYNTH, and TUBE TOWN written by Eric Barbour of Metasonix fame. I basically copied the "tube VCA" circuit and modulated it with a AVR based DDS LFO. I have a bunch of pentodes in my junk bin, and ultimately decided to use the 12BE6. This tube was made (in)famous for it's inclusion in the AA5 radio of the post-WWII era. 12CS6, or 12BA6 may also work well, but are untested by myself at the moment. Other traditional pentodes "work" but have too much gain for our dinky 5V signal to modulate fully.

The circuit is simple, just the pentagrid gain stage modulated by the DDS LFO. Surprisingly, the 5V swing of the AVR is plenty to modulate this tube.

Power supply is also relatively simple, I used iron, but a SMPS would work just fine as well, there is minimal current drawn(excepting the 12BE6 filament). The transformer used is a Tamura PL10-12-130B wired backwards($8-12 @ mouser or digikey). This means that I insert 12VAC from a wall wart INTO the SECONDARY and recieve 120 VAC OUT from the primary. You could also use the Tamura PL5-12-130B, or the tiny PL2.5-12-130B. Both are smaller, but have enough juice for our gain stage.

My pictured version has many more options, multiple waveshapes, MCU controlled bypassing, Expression etc. but, the project files will work, and are shown to be "as simple as possible".

Please read the many excellent sources linked in the code for ideas, especially RRING(circuit salad) and HAROLD SABRO(sabrotone).

This isn't the most amazing trem in the world, but it's quite a fun little gadget. It will get pretty deep, and sounds quite nice, not pristine clean, but round and fat. Turning the depth to minimum will allow you to use it as a preamp as well, though with such a low screen voltage, and low gain tube, there isn't a ton of headroom. Output impedance is high, so it will be sensitive to loading as well, but again, simplicity was the goal here. Modulating the screen grid does have a few drawbacks, so it seems to be uncommon nowadays. I for one enjoy the sound of pentodes , and if I have time, I'll post some other fun circuits.


I've been making stupid noise thingys for years and years, but hopefully some really smart people will come in and post ways to make this project even better.



The code function here destroys text formatting, so I've compiled this post, the code, and schematic into a RAR archive for download. Check it out, and feel free to comment and suggest improvements.

https://drive.google.com/file/d/0BzdyH8_QSjbXb3hrM011eTcxenM/view?usp=sharing

edit:fixed a few typos and clarified a few examples

Thanks,
-Morgan

[italianguy63]

Gut shot?!

MC

[idiot savant]

Gut shot?!

MC

Sorry, I don't have any at the moment.

There isn't really anything special going on inside though. Power supply is on a piece of 4"x1.5" perfboard on the right hand side of the box. Tube socket is on a small piece of perf with a terminal strip wired point to point. the pots and processor are on another large piece of perf on the left side.

It was all junkbox stuff I had lying around. I've been using the whole thing as a testbed for tube projects, so it's a little less than tidy.

[rj.Najera]

 Hi sorry for bringing back the topic but I found one 12be6 tube and I really want to make a project with it , as I'm a novice with electronics I wanted to ask you if... is there any other project with this tube?, the one you suggest is great I think but, by now it's out of my possibilities.

Hope you can help me
Thanks

[idiot savant]

Hi sorry for bringing back the topic but I found one 12be6 tube and I really want to make a project with it , as I'm a novice with electronics I wanted to ask you if... is there any other project with this tube?, the one you suggest is great I think but, by now it's out of my possibilities.

Hope you can help me
Thanks

You -could- just make a standard gain stage with it. Really it was designed for cheap radios---the All American 5 radio of the 60's. It's not particularly useful for much. That's partially why I used it, there's tons of them out there, for cheap-to-free. The tremolo was fun to make, as I just threw it together in a day with junk parts that I had sitting around. It could easily be redesigned for other tubes, but you'd have to tweak it some. The other fun thing is that the screen-grid modulated tremolos are really quite rare. Aside from a few bargain amps from the golden age, nobody has done it in years.

Assuming you can get 150V safely, for simple gain stages, you could run it like a pentode. Set it up as I've shown, instead of connecting the LFO, tie the screen to B+ through a 470K-1M resistor, and add a cap (100nf)from screen to ground. You could also triode strap it. Screen to anode, 68K from there to B+, suppressor to cathode, and a 2K from there to ground. Bypass that with an electrolytic, say 1uf or bigger depending on your desired frequency response.

If you're new to tubes, I highly recommend building something simpler with low voltages first. It takes experience to work safely with deadly voltages. Burns, fire, death... It's not worth risking it for guitar pedals.

Rick Holt(frequencycentral) has lots of fun low volt tube stuff that is safe for beginners and sounds great. The valvecaster is an excellent and safe project too.

-M

[amptramp]

The 12BE6 is a pentagrid converter tube and it seems the first grid (next to the cathode) should be where the LFO signal comes in and the incoming audio should be on the third grid.  Grids 2 and 4 are connected internally and go to B+ and grid 5 is connected to the cathode internally.  The other way of looking at it is that grid 1 is the linear grid and grid 3 is a remote-cutoff grid, which is made non-linear for automatic volume control reasons.  It is not clear from the drawing what grids you are using for what.  Since grid 3 is non-linear, maybe you used it for the tremolo and grid 1 for the audio.  BTW grid three on its own can give you tremolo if you bias the grid from the tremolo source.

[idiot savant]

The 12BE6 is a pentagrid converter tube and it seems the first grid (next to the cathode) should be where the LFO signal comes in and the incoming audio should be on the third grid.  Grids 2 and 4 are connected internally and go to B+ and grid 5 is connected to the cathode internally.  The other way of looking at it is that grid 1 is the linear grid and grid 3 is a remote-cutoff grid, which is made non-linear for automatic volume control reasons.  It is not clear from the drawing what grids you are using for what.  Since grid 3 is non-linear, maybe you used it for the tremolo and grid 1 for the audio.  BTW grid three on its own can give you tremolo if you bias the grid from the tremolo source.

Thanks for the input on... inputs!

In the case of the tremolo, I'm just using it like a regular pentode. Signal on grid 1. LFO to grid 2(and 4). Grid 3 is tied to cathode. As you say grid 3 can be used as an alternate input with a remote-cutoff characteristic.

I don't remember if I ever tried the tremolo that way or not. I might have been thinking (probably incorrectly) that I would have to level shift the LFO to get a symmetrical swing there. I tried making a compressor years back modulating grid 3, but I never got it working to a satisfactory level.

I did make a little ac4-ish amp maybe 10 years ago using both grid 1 and grid 3 as inputs. There's some pretty fun things you can do with these weird old tubes.

I also considered trying a tremolo with dual control pentodes or beam modulators, but it looked like level shifting of the LFO signal would be needed there as well.

Thanks for the comments, I'll try using grid 3 again when I have some time!

-M