Stupidly Wonderful Tone Control/Tube Reamer - What values to use?

[theundeadelvis]

Howdy folks! I built a ROG Tube Reamer a while back, and it's always been a tad trebly for my liking. I figure it might be a good candidate for Mark Hammer's SWTC, but I'm not sure what values I should use for the components? The TR has a 100k volume pot. Any suggestions? I'm not clear on how to do the math to calculate the ideal values. Thanks in advance!

[azrael]

Well, you could always just adjust one of the caps in the build, you know? There is a treble roll off cap in each opamp stage, and there is a low pass filter right before the second opamp. increase the size of any of those caps to roll off more treble.

[theundeadelvis]

I considered doing that, but I do like the versatility of a tone control. Every once in a while I'll like to make ears bleed with a little excess treble. 

[Mark Hammer]

Take a look at the Serendipity schematic over at the Madbean site: http://www.madbeanpedals.com/projects/Serendipity/docs/Serendipity_ver.5.pdf

The SWTC was intended to provide a simple lowpass filter, inserted before the volume pot, that would not result in volume level changes.  If a standard RC lowpass filter is buffered, it can be implemented, without penalty, in other ways.  The Serendipity shows how.  The Proco Rat also provides an example of a simple variable lowpass filter, followed by a JFET buffer stage to isolate it.

If you replaced the .022uf cap with a .027uf unit, and placed a 10k variable resistor (pot) in series with the 1k resistor, you would have a variable lowpass filter whose corner frequency would range from 536hz-5.9khz, which is a nice range from bright to fairly smooth.  Using a .033uf cap would give you 438hz to 4.8khz, which is a little smoother still.

[theundeadelvis]

Thanks you Mark! You are an invaluable member of this community, and I always appreciate your thorough yet clear advice.

[theundeadelvis]

Installed the SWTC using your suggested values Mark, and it does exactly what I was hoping for! Thanks again!

[Mark Hammer]

Excellent!  I love it when standard values, a little bit of math, and some intuition about how frequency-information translates into tone, come together.

I tell ya, there are some days when I feel like a chef, sipping a taste of the broth, squinting, and thinking "Needs a little bit of bay leaf, and juuuuussst a tad more cumin". 

[theundeadelvis]

Maybe that's where I go wrong, b/c I'm always "more garlic! more Sriracha!"