Adjusting the tone stack - Sunn 2000S

[BobL]

I recently, on recommendations from sluckey, built the Sunn 2000S preamp into the Weber 6S100 circuit, and it is working pretty well.


However, I would like to make some adjustments in the mid control, and I'm not really sure how to do it.


My impression is that the mid control (I guess listed as the 'Contour' on the schematic) is either:


 - Not cutting enough when reduced, or;
 - Cutting a mid that is lower than the one I need to adjust.


It's possible that the changes I need to make are in the treble control as well.


2000S: https://el34world.com/charts/Schematics/files/Sunn/Sunn_2000s.pdf


What I'm wondering is what parts of the preamp I should be adjusting, and hopefully get some recommendations on where to start, value wise, if I want to make those sorts of adjustments.


As always, the help is very much appreciated!

[TheKT88KilledJFK]

I've got a sunn sceptre that I converted the 6an8 PI to 12ax7 LTP PI.  That created a lot of mid range "clarity" and reduced the "twang" of the amp.  That would be a fundamental way to change the mid range character of the amp.  You could also raise the feedback by reducing the resistor value but I am not really a fan of that sound when I tried it. 

As far as simple changes,  removing the contour bypass cap (circled in red) and increasing the mid and bass caps will increase the range of control of the mid and low dials.  Also reducing the two resistors that feed them helps return the frequency back to the original 600Hz. 

I came up with 70n for the caps and 30k for the resistors of the tone stack.  This puts the mid range adjustment at the original 600hz but gives you 7db more range with the same relative Q.  If you want to move this adjustment to 700hz, then 50nF caps can be used instead. 

I just like removing the contour bypass because it only offers a slight bump in range of control but shifts the frequency.

Edit:  also, if you want the tone stack to behave like a traditional guitar tone stack, just change the 56k resistor above the bass control out for a 250k resistor.

[sluckey]

You are tweaking this for a bass, right?

[BobL]

You are tweaking this for a bass, right?

Yes!