Educate Me On Pots

[Strat Avenger]

Okay, I have known for decades about 500k pots for humbuckers, and 250k pots for single coil but I've been thinking about it lately and there's something that I'm not quite understanding.
When the control is rolled all the way "up", it should read zero ohms, regardless of whether it's 250k or 500k (or 1 meg-ohm), so why would there be a reduction in high frequencies with the 250k at full volume?

Here's another question; My CTS pots (250k and 500k) read 0.1 ohm when rolled wide open, but I have 4 Bourns 500k ("low friction") pots that read from 5 to 7 ohms when wide open. What's up with that? Do those few ohms of resistance affect high frequency bleed to ground?

 

[TBurst Std]

Well of course they affect it, but at a discernable level? Only your ears can tell you.

 

[mayfly]

When the control is rolled all the way "up", it should read zero ohms, regardless of whether it's 250k or 500k (or 1 meg-ohm), so why would there be a reduction in high frequencies with the 250k at full volume?

This is true, but only partly. When that volume knob is at max, the output of the pickup is indeed directly connected to the output jack. But! The third leg of the pot is always grounded. This means that there is always the resistance of the pot to ground on the output of the pickup. This puts a load on the pickup which is always there.

For humbuckers the two coils are connected in series which means it has twice as much impedance as a single coil. Twice as much impedance means that it has half as much drive capability (if you will). This is why you need a 500k pot for a humbucker otherwise you'll load it down too much and will sound muddy. Some people say that a humbucker sounds muddy anyway, but I digress....

Interestingly you can use a 500k pot (or a 1Meg pot!) with a single coil, which will dramatically decrease the load the pickup sees. You'll hear a difference here as well (increased brightness), but not nearly as dramatic as loading down a humbucker with a 250k pot.

I was just working on a strat that had very effective tone controls. Turns out there was an extra resistance tacked on to the top leg of each tone pot. This made the strat tone controls much more useful with a much wider 'sweep'. Fun games with resistance and how it interacts with the impedance of pickups...

Trevor

P.S. - you probably can't hear that 5-7 ohms you're seeing on the Bourns pots.