Wiring the pickup to the pot center lug

[Down Time]

I have a customer requesting a two pickup four pot setup, two vol, two tone with a three way switch in a Kay hollow body. He wanted the pickups to work independently in the middle position. I noticed a slight intermittent crackling in the volume controls. I tried two sets of pots. Could this be a common issue when using the center lug for the pickup input. I do not recall ever having this problem before. Or is it I have a bad batch of CTS 500K's.

[Mark Hammer]

Crackling should not occur as a result of using any particular lug for anything in a passive situation. Is it possible you stressed the lug in the process?

One thing I find myself doing more and more (albeit with lesser quality pots than you describe) is to pop the back off when I get them new, apply a bit of Stabilant (HOME PAGE -- STABILANT) and gently pinch the rivets of each lug with my needle-nose pliers, prior to re-assembly. I find this avoids a lot of headaches.

Keep in mind that the only reason pots can function at all is because of the application of tension in several specific locations. The resistive strip is laminated on the wafer and the solder lugs only make contact with it because they are squeezed tight enough against it. The wiper only makes contact with the resistive strip because it is a piece of leaf-spring material that is pressed against the strip. Any insufficient pressure in any of those locations results inmomentary discontinuities, which show up as crackling.

Of course, later on in a pot's life, dirt gets in, and the rubbing of the wiper against the resistive strip erodes the strip and creates little buildups of residue, both of which can create momentary discontinuities in wiper/strip contact, and also producing crackling.

[Sock Puppet]

As you've tried two sets of pots, is it possible that you have a small amount of D.C. coming from the amp?

i.e. have you tried the guitar through a different one?

S.

[David Schwab]

I've had "brand new" pots get scratchy right away, and had to throw them out. I think the issue is sometimes how long they were sitting around before you bought them, and in what conditions.

As a last ditch effort, I have breathed new life into scratchy pots with WD-40. Certainly not a task it was made for, but it works well when other cleaners wont.

[Down Time]

Thanks for all your replies, I think I have a batch of bad pots. Yes David thanks, I did try the WD-40 and it helped for about a day. I have ordered another batch of pots from a different supplier. The current pots are CTS from Ernie Ball. They are made in Twain if that makes any difference. I never had a problem with them before and always preferred them because they have a low torque range, easy to use for volume swells. The Allparts and other suppliers pots seem a little stiff. I guess I will have to fish the harness out and try it again

[Mark Hammer]

If you take the back off a pot, you'll see that wipers can vary considerably in their structure. In particular, they can vary as to:
a) how many "fingers" they have, (the wiper is split and slightly feathered at the end with a number of parallel "fingers"
b) whether the fingers scrape or drag as you rotate clockwise. (some wipers are like the cowcatcher on a train, faciong in the direction of clockwise rotation, while others are like a dress train, dragging behind)
c) whether the wiper is at the end of a piece of spring metal, or is a dip in a larger continuous piece, like a dimple.

All, of course, depend on maintaining tension in the wiper so that it makes good contact with the strip.

In general, I find that larger diameter pots have both broader resistive strips, and matching wider wipers, often (though not always) with more "fingers".

Do pots with "scraping" as opposed to "dragging" wipers get noisy sooner? I have no idea. I'm just telling you about the substantial variation in the mechanics underlying pot functioning. I leave it to others to supply their own experiences, and also associate specific brands/models with specific prefered mechanisms.

[Joe Gwinn]

In general, I find that larger diameter pots have both broader resistive strips, and matching wider wipers, often (though not always) with more "fingers".

Bigger is usually better, especially in the lifetime department. One can look for the rotational life in the datasheet. If no lifetime is specified, assume that the pot is intended for occasional adjustment only.

The pots on a guitar can get an amazing amount of use. Assume someone who practices for 10 hours a week. Each song is a few minutes long, and say half of the songs require some adjustment, so we get 10 adjustments per hour, or 100 per week. In a year, that will be 52*100= 5,200 adjustments, so a pot rated for a lifetime of 10,000 adjustments could wear out in two years and be within the datasheet guarantee.

Do pots with "scraping" as opposed to "dragging" wipers get noisy sooner? I have no idea.

In a properly designed and built pot, this makes no difference. What does matter is choice of materials, the number of fingers, and how well tension is maintained. All matters of mechanical design and manufacture.

[Down Time]

Thanks again for all your replies. I did order some of the Allparts CTS pots. The customer did not mind the stiffer torque range of the Allparts pots. Maybe the stiffer pots will be quiet and last longer in this application.

[Down Time]

I received an order of 500k pots from Allparts and replaced the Ernie Ball CTS pots, the problem is gone, no more crackling. Must have been a bad batch of Ernie Ball CTS pots.