Resonance Control for Fender Amplifiers
I got this from a post at from the old Ampage forum. I installed it in my Bassman 100 rebuild, and it works great.  Basically, it is the opposite of a presence control. It changes the negative feedback in the low end of the spectrum. One extreme gives you tight, clean Fendery tone, the other extreme gives you awesome crunchy overdrive. If you have a seventies Fender with one of those crappy master volume controls you can use the master volume pot in this circuit to give you some great tone. Remove the MV circuit and replace it with this one.

Here's how you do it:

  1. Looking at the back of the 1 meg master volume audio pot, left to right, numbering pins one, two, and three.
  2. Disconnect your feedback wire from the output jack and solder to pin three.
  3. Solder a new wire from the output to pins one and two.
  4. Solder a .0047 cap from pin three to pins one and two.
That's it!

As I said, you can use the master volume control on Silverface amps, which is a 1 meg audio taper.
Fender Resonance Control Schematic
Fender Resonance Control Diagram