This is the tone stack for the red channel of a Mesa Dual Rectifier. (I redrew it in Illustrator for more clarity)
On the presence and bass pots, do they have no connection on the #2 lugs or are they supposed to be connected somewhere?



Thanks!
- Buck
www.bucksears.com

As I always understood it connects to the lug that the tail end of the arrow is pointing towards.

Buck,

In a case like this the pot is being used as a variable resistor -- so you would connect the #2 lug to ground, in the case of the Presence pot and to the Mid pot in the case of the  Bass pot.   For the sake of noise reduction then, the unconnected lug is then usually connected to the #2 lug.

Presence pot: connect the central lug to ground.
Bass pot: conect the central lug to the mid pot.

Just to clarify, the "noise reduction" referred to concerns steps taken to avoid a situation where the wiper makes intermittent contact with the resistive element.

When using a physical potentiometer as a variable resistor, you really only need to use two adjacent lugs to accomplish that *electronic* task.  If the pot is in less than ideal shape, however, any intermittent contact between wiper and resistive element is akin to someone inserting a switch and flicking it on and off.  Snap, crackle pop, .... and it ain't Rice Crispies.  Simply tieing the wiper and one of the outside lugs together assures that, even if the wiper decides to take a "contact holiday" from the resistive element, the signal remains connected to the rest of the crcuit via the lug at the other end of the resistive element.

This connection is not *absolutely* necessary, but it is considered insurance against such sources of noise, and is also considered a good practice.  I might point out that some folks will also replace that wire link between wiper and "extra" lug with a fixed resistor to achieve different sorts of tapers.  See the article on "The Secret Life of POts" at Geofex for more on that subject.  Film at 11:00.

Thanks guys.

On that schematic.....

Hmmm..... That's the first time I've seen a tone stack used "backwards". Interesting application. Is there any difference in what comes out if the output impedance gets set to 1M? I wonder if the larger treble cap could be to compensate for the effects of the presence cap?

Interesting....

Take care,
-Peter

Peter, have you got my PM ?

>>That's the first time I've seen a tone stack used "backwards". Interesting application.

My bad. It may be that the IN and OUT should be reversed. (That would make more sense, wouldn't it?)
The .pdf file on the DualRecto was a little hard to read and I probably should have gone right to left instead of vice versa.

Thanks, Pete!