Cap and resistor/pot order question

[Govmnt_Lacky]

In a situation where a pot is set up as a variable resistor with Lugs 2 & 3 going to ground and Lug 1 going to the circuit.....

If I also have a cap (100nF) in that circuit... does it matter if the cap goes between ground and the Lug 2/3 junction -OR- can I install it between Lug 1 and ground?

Does it matter??

[Kevin Mitchell]

I'm a bit confused by your question.
The former: cap between ground and ground?
The latter: cap between the circuit and ground?

What's going on here?

edit: oh wait. Are you describing a zobel network? I don't think it matters which comes first - so long as only one end of the network goes to ground.

[ElectricDruid]

Yes, it matters - at least to some extent.

In one case the cap is across a portion of the resistance (depending on the pot setting), and in the other case, it's across all of it.

*How much* it matters is going to depend entirely on the value of that unknown pot and what else is going on around it, and the answer may be "not at all". But on principle, we absolutely can't say they're the same until we know the rest of the story.

So...where's the schematic?

<edit>Thinking a little bit more about it, it's going to finish up being a variable shelving RC filter, depending on both the series impedance going into it, and the output impedance too. So whether it matters depends on both of those factors.
Looking at it another way: What's the pot doing there if it *doesn't* make any difference?!? So there must be some point to it, right? What's it intended for?

[antonis]

Schematic plz..??

[GGBB]

I think you are asking if these are the same:



A:

the cap goes between ground and the Lug 2/3 junction

B:

between Lug 1 and ground

They are always different.

[Kevin Mitchell]

I think you are asking if these are the same:

Ah. Now that makes sense - my brain refused to rationalize the description 
A = zobel
B = bypass

[Govmnt_Lacky]

I think you are asking if these are the same:

Yes. And, forgive me for not uploading pics or links but....

Specifically, regarding the Presence control on the JCM 800 2204. The turret board layouts have the 100nF cap implemented like Figure A but....

The schematic had the 100nF cap differently. It is not like Figure A or B. Instead, it shows it directly between Pin 1 of the pot and the Circuit.

https://www.drtube.com/schematics/marshall/jcm800pr.gif

Here, we see the Presence control with Lug 3 going to GND. Lugs 1 & 2 are tied together with the 100nF in series to the circuit. (Presence pot is in lower right corner)

https://ceriatoneforum.com/jcm-800-2203-2204-2550-2555/biasing-2204/

Here, in this board layout, we can see that the 100nF cap on the Presence control is wired like Figure A above.

[GGBB]

The schematic had the 100nF cap differently. It is not like Figure A or B. Instead, it shows it directly between Pin 1 of the pot and the Circuit.

https://www.drtube.com/schematics/marshall/jcm800pr.gif

Here, we see the Presence control with Lug 3 going to GND. Lugs 1 & 2 are tied together with the 100nF in series to the circuit. (Presence pot is in lower right corner)

They are both like A - cap and resistance in series. The main difference is that the order of the cap and pot are reversed:

schematic: circuit -> cap -> pot -> ground
layout:    circuit -> pot -> cap -> ground

Also, the pot itself is reversed which will only affect the behaviour of the control (forward vs backward).

Electrically these are identical.