....but they did invent or at least used first the Big Muff Tone control.
The 1954 Gibson GA-40 uses the same circuit but calls it a "voicing" control.
it uses a 470k/1nF pair in conjunction with a 2M pot to roll off bass or treble and the neat thing is it's flat at a 12:00 rotation and little insertion loss....
Another very cool thing is you can play with the values in the Duncan Tone Stack Calculator. I'm playing with the parts values using a 1M pot, 38k Z-in and 1M load....
If you note the effect that changing each component has you can really make this one knob TC do just about whatever tone sculpting your heart desires....
Hopefully this is useful and not simply some mojo secret I've been once again slow-on-the-uptake to discover.....
I believe I have seen that EQ on a Supro schematic too. Who knows where it originated from. In principle it is simple- just panning between the outputs of RC low-pass and high-pass filters.
Cool. I usually refer to it as a Supro circuit as that pre dated the Muff. So this is earlier than the Thunderbolt? Wonder where the "Tilt" circuit as Merlin calls it originated from.
Correct me if I'm wrong, but I believe R.C. Ballard actually worked for RCA. So yes, the circuit is probably featured in their books etc.
...Oh, and pickups for electronic guitars were practically invented around the same time this circuit was already in use (probably in TV and radio sets) so there will be great difficulty to attribute first employment of this tone control to anything related to amplification of electric guitars.
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As said, it's simply a crossfader between hi and low-pass signal paths so several variations revolving around that concept exist: For example, for more range the filters can be other than just plain 1st order, and I've seen some circuits where the filters are actually active instead of passive. Sometimes gain stages are incorporated to the signal paths. The idea can be expanded even further: Instead of crossfading between the two signal paths make each path feature its individual volume control; now the circuit turns from from "tilting" to introducing individual bass and treble controls.
Tone controls of vintage Gretsch amps, for example, feature some of these alternative embodiments. Another famous example that uses these ideas is Marshall's original model "200", known as the "Pig".
Another tone control idea that revolves around the very basic concept is introducing -anykind- of frequency response within those two signal paths and then panning between those two frequency responses.
Component values of the basic circuit...? Well, they are usually dimensioned so that loading of the driving stage is minimized. Depending on cut off frequencies of the filters the frequency responses will either interweave to generate "flat" response in total when the potentiometer is dialed to "center" position, or they will not interweave, which produces a band-stop "notch" filter at frequency in between cutoff frequencies of the filters (as in traditional "Big Muff" tone control).
