A tilt tone control -- flipped Jack's SWTC2 around

[midwayfair]

One of the things that might not always be useful about Jack Orman's SWTC 2 is that the bass cut frequency is much lower than the treble cut frequency. This makes for an interesting tone control (and I've used it in one of my designs).

I like to put a stopper on the bottom of the tone control to keep it from turning into a straight low-pass filter.

Flipping it around lets us ditch R1 and even load the output less if we want. However, I still like the stopper resistor. An even better addition is a cap in series with the tone control to ground. This way, you can control the cutoff frequency of the treble cut without changing the output loading.

This version also lets you experiment with different values for the tone pot as well, as the tone pot's value is no longer the deciding factor for the cutoff frequency of the bass cut.

It's pretty easy to set it up to do, say, a tilt across 700Hz or 400Hz (to "boost" or cut at one frequency with minimal volume change) or to have the bass cut and treble cut be in different frequencies as before.

Maybe someone will find this useful.

[sonic66]

How would changing the volume pot to 1M or 500k affect the circuit? Could this then be used in a fender champ circuit in between halts of a 12ax7?

[midwayfair]

How would changing the volume pot to 1M or 500k affect the circuit? Could this then be used in a fender champ circuit in between halts of a 12ax7?

The volume pot value affects the high-pass frequency AND the attenuation of bass frequencies.

With a 1M volume pot, you'd have to x10 the resistors involved and 1/10 C1. You also have to factor in that the source z of V1 in a champ is 38K, so you'll have to rework C2||C1.

You might be interested in a somewhat similar tone control I designed for my champ build based on a BMP control. I've used it in pedals as well. It's a similar number of parts to this (plus a switch), but it's designed to allow scooped or flat mids, and the flat mids setting will also restore quite a bit more gain. [n.b. the treble is not actually a dual anymore] When the pot is cutting treble, it's a shelf that boosts the bass. You can use the Tone Stack Calculator for it to mess with the frequencies. (Just set the resistor to ground as 100M to take it out of the circuit.)

[Mark Hammer]

Think of R1 as being part of the volume pot - the part you can't adjust.  C1 provides for an "express route" around that part of the pot.  So, when calculating what you want or need R1 and C1 to be, you have to think in terms of "an express route relative to what?".

As R1's value begins to form a smaller fraction of the total resistance of R1+pot, C1 provides less of an advantage for high freqs, and adjustments of the volume pot interacts less and less with the rest of the circuit.  The treble cut function will still work, but the treble/brightness boost will be much less apparanet, if at all.

That's not a design flaw, merely a consideration in achieving the function you want it to achieve.  Note that, like all passive tone controls and filters, it will eat up a bit of signal.  E.g. if R1=100k and volume pot = 100k, in combination, they behave like a 200k volume pot that can never be turned up above halfway.  C1 will allow for much less attenuation of treble content (at max volume, it's like the volume pot isn't even there), but whatever is below that frequency range will not come through at full volume.

A similar sort of control IS used in many early Fender 2-knob amps, like this Princeton.  In the Fender version, once the Volume control is above a certain point, the tone control is unable to add any noticeable treble.  At max volume, it is essentially out-of-circuit, although the treble-cut aspect is still functional.  John's version allows for the treble-enhance aspect to work across more of the volume-pot's range, albeit at the sacrifice of some level.

But hey, you don't get something for nothing.

[Kipper4]

I'll try this in a breadboard experiment Jon
Thanks for sharing.
Rich