Tone stack understanding for solid state Fender amp.

[Ivan Funk]

I'm familiar with the Fender Twin tone stack but I don't understand this one.
This is the tone controls in an a solid state Fender Champion 30 amp.
The controls don't do much and even at zero there's still a lot of treble and mids.
They seem to cut a very narrow Q and are not interactive like a Twin tone stack.
What components would you change for more cut and at a wider Q for each control?

[Scott Swartz]

With the unorthodox design of that circuit, you are going to need to put it in a circuit simulator to get anywhere.

At a high level here is what I see.

-The frequencies above the mid dip go through C17 560 pF.

-The highs and attenuated mids then join, and sebsequently split to the treble control pathway and the bass pathway.

-To understand the bass and treble sections it is important to see that the junction of R34 and the top of R35 bass pot / R36 form a virtual earth mixer on opamp U3B, same concept as used on every PA mixer but only 2 channels in this case (bass pathway and treble pathway).

The relative gain of the 2 pathways could be changed with R34 and R36 in the standard way to adjust virtual earth channel gains, but there may be other interactivities that you would have to analyze with a circuit sim.

And then all the cap values could also be adjusted to change how it sums together and the controls interact.

Also note how its common to do the mids in a separate stage to minimize interaction ala various Peaveys, so that could be a limitation in tweaking this circuit.

[Ivan Funk]

Thanks for the great explanation Scott.
Much appreciated.

After looking at it for a while yesterday I came to an 70% understanding and your description fills in the rest. I can see the mixer.

After some trial and error this is what I ended up with: (see below)
I don't know if re-routing the high pass C17 560pf to the top of the treble pot is a bad idea but it makes the treble pot work much better. Before, even with the treble at zero the ultra highs were still bleeding around everything. Also changed a few cap values. Works better. I'm tempted to get rid of the whole thing and change it to a more simple Fender tone stack.

What is the function of R33 47k?

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[Ivan Funk]

Revision # 2:
Works much better:

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