Changing frequency on a contour control

[Blackboarcult]

So I'm in the process of designing a circuit that requires a Contour control for a distortion/overdrive.

I tried to implement the one from the Marshall 8100 Valvestate, but it worked more as an A/B switch than a pot with a steady, smooth turn, so I checked out other schems in search of ideas.
I found that one, from a Carvin BX1500 bass amplifier, and managed to adapt it to run at 9V (creating Vref point etc).
It works wonderfully well, but it operates on too low frequencies for my taste.

Carvin's own words:

When set to the leftmost position, the mids are unchanged. Turn the knob to the right and you will incrementally scoop your mids out at about the 250Hz range- this is the range where a lot of the body and low midrange heft of your bass sound is. Too much sound in this range can be detrimental and really muddy up a mix, but the contour knob really lets you find the sweet spot. At full clockwise the midrange content at 300Hz is reduced by 9dB.

My question would be:

How do I calculate values for it to operate in the 400-to-800Hz range?

The circuit is deliciously simple and I'd like to be able to adapt it for use with guitar.

Any advice would be greatly appreciated, thanks in advance folks!

[Frank_NH]

You would probably want to simulate this to get the whole story, but it looks to me that modifying C134 and C135 would allow you to control the center frequency for the mid scoop.  If you have this on a breadboard, try reducing C134 (e.g. try 22 nF) since that cap plus P2 form a low pass filter.

There's also a contour control on the Marshall Shredmaster - you could examine that circuit for some ideas.  Good luck!

[Blackboarcult]

Thanks Frank!
I finally opted to rip the freq + boost/cut circuit from the Pearl OD-5, it's an additional pot but gives way more versatility, however, I'll save your info for future needs ;-)