Nota original a R.G. Keen y su respuesta acerca
del control de Tono del TubeScreamer
Esta es la reproducción exacta de mi pregunta a R.G. Keen y su respuesta cerca del tipo de potenciómetro de Tono usado en su esquema del TS. Todo en Inglés, para quien quiera verlas sin errores de traducción.
...what is still bugging me is the response of the tone control. I've used a 20K linear pot and the control performs almost no tone change in the first half of the run, then slowly adding treble to a big increase in the very last few degrees. I've also used a Log taper but still too abrupt.
>I've read about the unresponsiveness of this control in the original units (which I cannot access), for instance, in the "Son of Screamer" article by Jack Orman, but still it seems too abrupt to me.
>The reason of this note is to ask you about the meaning of "20KG" that you put in the schematic. I know the meaning of "A", "B" and even "J" pot tapers but, what is "G"?... I have surfed the net to no avail.
>Does it have an effect in the response?
I put that "G" in there because on one unit, it was labeled that way, I took it for a taper marking. Somewhere I have a book with a description of about twenty different tapers, and I assumed it was one of them.
The problem with tone controls like the one in the tube screamer is that the abruptness of the tone control is also determined by the position of the control. If there was an inductor in series with the capacitor, it would be one bandpass section of one of the standard types of graphic equalizers. The "Q" or sharpness of the filter is controlled by the resistance in series with the cap/inductor elements when the control is at either end of the control range, but in the middle, the resistance of the control itself adds to the "Q" spoiling resistance of the capacitor/inductor string. The same thing is happening here. The 220 ohm resistor in series with the cap is what determines the maximum boost or cut in combination with the feedback resistor whenever the control is max or min. In the middle, the rest of the 20K pot is effectively in series and blunts the effect of the control.
If you really want to spread the control range out, you need a pot that transitions the center region very quickly, and gets to the sensitive ends as fast as possible. You might want to experiment with a 50K control with 10K "tapering" resistors from the outside lugs to the wiper on both ends. This creates a pot taper that is logarithmic on both ends of its travel, audio on the low end and reverse log on the high end. It's worth a try.
Another way is to dump the screamer tone control entirely and use some completely different control. I'm pretty certain that the reason the control in that pedal is like that is because the designer had to put in some kind of control, and that one was cheap. I personally think a Baxendall style control would be more like what you are looking for. That rips up a substantial part of the circuit, but it gives much more delicate shading of control than the abruptness of the existing one.
©Piso-tones Ltd. Calambres y Rubén Hernández | guitarristas.org