Resistors and capacitors

[dprozzo]

Can anyone tell me the size of the treble bypass cap that is on the volume pot of my Santana SE? If I put a 100K resistor on a 500K pot, what is the resulting resistance? What's the formula for calculating this? (Sorry, it's been too long since Physics.)

[Jim Collins]

The value PRS uses for the bypass cap is 180pF. (This is the value used in the US made guitars. I'm assuming they use the same value in the offshore guitars.)

The value for total resistance, with two resistances in parallel is:

1/R = 1/R1 + 1/R2. If R1 is 8 ohms, and R2 is 8 ohms, two 8 ohm resistors in parallel is 4 ohms, since 1/4 = 1/8 + 1/8. In your case, R1 is 100K, and R2 is 500K. The result is approximately 83K.

[dprozzo]

I'm sure that I am missing something. That's the formula that I used and came up with the same thing.

On a Stewmac site they indicate that a 500K pot and 150K resistor means that the pickups "see" 300K. But, using this formula I get approx. 115K.

http://www.stewmac.com/wiring101/resistors_capacitors.html

[Jim Collins]

Where are they putting this resistor?

[dprozzo]

On the input and output (lugs 1 & 2) of the volume pot.

[Morning View]

1/R = 1/R1 + 1/R2. If R1 is 8 ohms, and R2 is 8 ohms, two 8 ohm resistors in parallel is 4 ohms, since 1/4 = 1/8 + 1/8. In your case, R1 is 100K, and R2 is 500K. The result is approximately 83K.
wow i just learned about that stuff in physics... i never thought i'd ever see it being used in real life... wierd...

[Jim Collins]

Have you heard about the right hand rule? It comes in handy when discussing such pickup issues as in-phase, out-of-phase, magnetic polarity, etc.

[dprozzo]

That information (on StewMac) is completely wrong.

...when the pot is up full the total resistance is whatever the pot is - both the added resistor and the cap are bypassed.
What do you mean by 'up full'? I have a 500K pot that is out of the guitar. When I measure R across the left and middle lugs (as viewed from the back), I get 0 R when the knob is turned fully clockwise (as viewed from the front - '10' position).

With the pot in the '0' position the R is 480K.

When I add a 100K resistor across these lugs, the range is 0 to 80K. Does this mean that I now have an 80K pot?

[dprozzo]

I guess my main problem was not knowing how to measure the pot's resistance, and not fully understanding how they work. Took this one apart and looked it over while reading your post and gradually it made sense. Thanks for your patience.

[Jim Collins]

No problem, dprozzo!

There's nothing like physically taking something apart to find out how it works. That's how I spent most of my teenage years - sometimes with more reversible consequences than others...
Me too. My family had an old, upright player piano. I had nearly completely dismantled, in the basement, just to see if I could make it work. It turned out the leather bellows had cracked at the folds, and there wasn't anything I could do about it. But I got the piano back together, just fine. I even tuned it -- or part of it, anyway. (Do you have any idea how hard it is to tune a piano when all you have is a pliers?)

[dprozzo]

OK, I tried something different.

I put a pair of Lace D150/D100s, with a coil split, in my guitar and lived with them for a while. I like them, but they were overly bright, and I wasn't impressed with the split sound. Sooooo, I wired a 470K and a 220K resistor in series, and then added it in parallel to lugs 1 & 3 of the volume pot. The resulting resistance of the pot was approx 300K. I like the sound better. The pickups sound just a tad darker, and the split sound is a little thicker.

That's all the experimentation for a while. :rolleyes:

[dprozzo]

OK, so what I ended up doing was adding a 1 meg resistor in parallel to lugs 1 & 3 on a new 500K pot. I forgot to measure the resistance before installing, but it should be 333K now. I like the sound better...a little less bright.

I damaged the treble bleed cap and couldn't reuse it. PRS Customer service told me that it is a 180pf cap on the Santana SEs. The closest I could find was a 100pf cap at Radio Shack. I added it, but couldn't really tell if it made any difference over no cap at all.

[richedie]

I don't believe the McCarty has the 180pf cap, does it??? I find the McCarty bridge pickup nice but the neck muddy. I think the Duncan C-5 or Pearly Gates might be a better match for the McCarty bridge.

What is the deal with this cap? Will it only effect tone when rolling off the tone/volume?

[Jim Collins]

The McCarty does not have the 180pF cap. It only has an effect when the volume is being rolled off. Normally, backing off on the volume has a side effect -- a loss of treble. This little cap compensates for that by always passing some small amount of highs.

[richedie]

Hi JIm, it's probably best if I do nothing with this guitar till I've gone through a few rehearsals and gigs. RS guitar works sells a kit to add some kick and bite to the McCarty pickups.

I am liking them fine as is though......