Pickup volume & tone circuitry question

[picker]

I want to replace the pots on my OLP MM3 with better made units. But, the ground connection soldered to the bak of the pot is right on top of the printing that says what value the pots are. so, three questions:

1. What brand of pot would you use to replace the ones it came with?

2. What value pots should I use?

3. If the bass sounds a little on the dark side, would using a higher value pot (like going from 250K to 500K) brighten it up a bit?

[TimR]

I think you'll find the capacitor has more of affect on the tone than the pot. This being the simplest form of filter. I'll see if I can get any better details later on.

[rumpelstiltskin.]

the most common pot value is 250k Ohm. you may prefer an audio taper pot. it's easiest to check with an ohmmeter of some sort. multimeters can be simple and ridiculously cheap, but they can also be expensive. but pots are relatively cheap. you might prefer to let stewart mcdonald do the thinking for you, but you can find a 250k audio pot at digi-key as well.

 

if you're looking to brighten the tone a bit, try the mod that 09 did on his P. check out the information here. of course, a 500k pot will brighten the sound a little, too. but be careful with that. i made a jazz bass have a little bit too much high and not enough low by putting in a 500k pot.

 

robb.

[picker]

Would changing the tone pot to a 500K and leaving the volume pots 250K be a compromise in terms of brightening the tone, do you think?

[TimR]

Found it: The capacitor affects the cutoff frequency.

 

http://guitarelectronics.zoovy.com/category/wiringresources.1basicwiringfaqs/#q5

[mro]

To find value of pot,disconnect all leads to the pot in question. Take an ohm meter and measure the 2 outside leads. This should read the full value of the pot. I agree with TimR.

[David King]

MRO is correct but it should only be necessary disconnect one outer lead, not both to get an accurate reading. Traditional tapered pots are usually stamped with an "A" before or after the value as in "a250" or "250kA". This is hardly universal but generally true for pots used in guitars.

 

Before you jump in and replace the pots on your OLP try it through a different cord and amp combination. Cords can have a significant effect the tone and an amp could be loading down the pickup with too low an input impedance and cause the pots to not work as they were intended.

[Fred TBP]

About 10 years ago I found a very cheap bass (P/J copy) that gave me exactly the sound I wanted for slapping. I tried different pickups and controls, but finally went back to the original electronics and discovered the sound of the instrument was good just the way it was.

 

I just bought a 5-string OLP from a friend and decided the only thing it needed was new strings and a quick spritz to the controls with contact cleaner. I also raised the pickup a bit and stuck some more foam underneath to keep it from shifting around. So if it ain't broke...

 

Mr. King is very right about using quality instrument cables, and it won't bust your budget either.

[jeremy c]

As others have said, changing capacitors, not pots is the way to change the sound of the bass.

 

Of course, Fred's suggestion of changing strings is an excellent one.

 

Capacitors are really, really cheap and you could just use alligator clips and have a nice science project.

 

The Varitone dial on a Gibson 355 is just a switch which engages various valuses of capacitors.

[picker]

I have tried changing the cable, with little noticeable difference. The strings probably could be changed out, I'll probably try that next.

 

But, I read this stuff on the site Tim R linked above. In talking about the cap & pot filter that is the usual pasive tone control on guitars and basses, it says...

 

Keep in mind that the capacitor value only affects the sound when the tone control is being used (pot in the bass setting) The tone capacitor value will have little to no effect on the sound when the tone pot is in the treble setting.

 

And, in talking about using a "no-load" pot, basically eliminating the pot & cap from the circuit...

 

By eliminating the path to ground thru the pot, the only load on the pickup is the volume pot. So if 250K pots are used, the load is reduced from 125K to 250K and if 500K pots are used, the load is reduced from 250K to 500K (high resistance = low load) The reduced load allows more power output from the pickup and reduces the amount of high frequencies that bleed off to ground. This gives a noticeable increase in brightness and output in the full treble setting. The no load pot can be used in place of any standard tone control on any guitar.

 

If I am reading all that correctly, it seems slightly contradictory. The first part says that if the tone pot is all the in the treble position, it has little or no effect on the tone. Then in the second part, it says the tone is gonna be brighter with the pot and cap network eliminated from the circuit, and that a 500K volume pot will pass more high frequencies than a 250K pot will because higher resistance=lower load, which allows more power and high frequencies to pass. Am I understanding that correctly?

[Big Daddy from Motown]

Changing pot values from 250k to 500k will not give you a noticeable difference. It would be very subtle. Can you tell the difference between an amp with 1M ohm input impedance and 500K ohm input impedanse? That would be more noticeable than the pot change. How about turning up the treble control on the amp? that's what's it's there for.

[rumpelstiltskin.]

Originally posted by Picker:

If I am reading all that correctly, it seems slightly contradictory. The first part says that if the tone pot is all the in the treble position, it has little or no effect on the tone. Then in the second part, it says the tone is gonna be brighter with the pot and cap network eliminated from the circuit, and that a 500K volume pot will pass more high frequencies than a 250K pot will because higher resistance=lower load, which allows more power and high frequencies to pass. Am I understanding that correctly?

so what's happening is that when you have the tone control fully open, only the pot itself and not the capacitor has an effect on the tone. by rolling off the tone knob, you are including the capactior in the circuit to a great degree. so you'll only notice capacitor changes when you roll off the tone knob.

 

however, if you take the the tone pot completely out of circuit using a no-load modification, you no longer have the volume and tone pots in parallel. thus, the resistance attached to the pickup's output increases from 125k (two 250k pots in parallel) to 250k (one 250k pot). it is as described.

 

robb.

[picker]

Originally posted by robb.:

...if you take the the tone pot completely out of circuit using a no-load modification, you no longer have the volume and tone pots in parallel. thus, the resistance attached to the pickup's output increases from 125k (two 250k pots in parallel) to 250k (one 250k pot). it is as described.

 

robb.

So, increasing the resistance from 125K to 250K has a noticeable effect, but increasing it from 250K to 500K doesn't?

[rumpelstiltskin.]

i disagree with that statement. i found it had an effect that i found undesirable in my former MIM jazz. it certainly made things brighter, but i felt it also took away low end. that may have only been relative to the high end, so it may not have lost any low end. i felt the high end wasn't very musically useful; it was not at all like turning up a treble EQ onboard or on an amp. it was like turning up the really high end "air" type contols on a graphic EQ, like 10k and above.

 

robb.

[Big Daddy from Motown]

The gain in brightness is up in the 10KHZ range. I just don't think its worth the trouble for bass. Just open up your bass and disconnect the damn pot and see for yourself.

Robbs description is spot on (as usual)I also agree that the modification is undesireable. If the pots are bad replace them.

[TimR]

They are talking about the "Fender no-load pot". When a normal pot is at "0ohms", there is still a small amount of resistance and the capacitor is still in the circuit. The "no-load" acts as a switch at 0 and removes this and further brightens the sound.

 

Try some brighter strings.

 

Or maybe a graphic preamp. I used to use a Boss G7B on my Charvill to do the opposite because it had no bottom end.

[rumpelstiltskin.]

Originally posted by TimR:

When a normal pot is at "0ohms", there is still a small amount of resistance and the capacitor is still in the circuit.

oops. i misread the diagram. you're right. there is still a small amount of resistance in the circuit, so the tone capacitor plays a small role in the sound of the instrument.

 

the tone capacitor is connected to the wiper of the pot. the wiper is the variable part that connects to the resistive element inside the pot. the funny thing about how the circuit is wired is that, while one end of the volume pot is grounded, the tone pot floats. only the tone capacitor is grounded, so the high frequency signal path goes through the resistive element of the pot to the wiper, and then through the capacitor to ground.

 

essentially what that makes it is a series RC circuit that shunts to ground (i.e. it's parallel with the pickup). the R is variable, of course, because it's a pot, which makes the corner frequency variable.

 

robb.

[TimR]

Sorry Rob, I always assumed that the pot and capacitor formed a simple RC circuit, but I see this is not the case. The tone pot and capacitor are both in series to ground, so this must mean the value of the volume pot must also have some influence. Can you explain further or give me a link?

Thanks.