weird behaviour of tone control circuit

[JazzNote]

A while ago i installed a new floater on my 1998 LeGrand. It's the same PU as used on the LeeRitenour L5. Eventually i noticed that after some time of playing, the volume of the guitar changed by itself, getting lower and then might suddenly increased again (that happened on 2 different amps).

Question to the experts: could a faulty tone control capacitor lead to the described behavior of the circuit?


The PU was wired to a 250kohm volume control and a 250kohm treble control (actually 256kohm) operating with a 0.047 capacitor. When trying to find the faulty part i found out that it's neither the pickup nor the volume pot (without the treble control it all worked fine).

[KirkP]

When this happens, does the treble drop as if you had rolled back the tone control, or only the volume?

How much does the volume drop?—e.g., slightly vs. a lot?

Do you hear any other noises like pops or crackles?

In theory I guess it could be an intermittent current leak across the capacitor, but that’s something I’d only expect on electrolytic capacitors, which no knowledgeable tech would use in a guitar.

[sgosnell]

My first thought is that there could be a cold solder joint somewhere in the wiring. It's difficult to solder to the back of potentiometers, because they act as heat sinks, and cold joints are not uncommon. This can lead to your symptoms, as the connection quality changes. Not a certainty, but something I would definitely check.

[lawson-stone]

Does it happen on other amps? Could be an amp issue.

[GNAPPI]

Check for cold solder, and if the cap is metalized polyester (like orange drop) look for cracks in cap ends.

[JazzNote]

When this happens, does the treble drop as if you had rolled back the tone control, or only the volume?

How much does the volume drop?—e.g., slightly vs. a lot?

Do you hear any other noises like pops or crackles?

In theory I guess it could be an intermittent current leak across the capacitor, but that’s something I’d only expect on electrolytic capacitors, which no knowledgeable tech would use in a guitar.

The volume drop was significant enough to irritate me while playing, i would say a lot .... no other noises.

My first thought is that there could be a cold solder joint somewhere in the wiring. It's difficult to solder to the back of potentiometers, because they act as heat sinks, and cold joints are not uncommon. This can lead to your symptoms, as the connection quality changes. Not a certainty, but something I would definitely check.

i already fixed the circuit because this is my currently most played guitar.

Does it happen on other amps? Could be an amp issue.

The same happened on 2 different amps .....

Check for cold solder, and if the cap is metalized polyester (like orange drop) look for cracks in cap ends.

I have changed the treble pot and the capacitor but am not too happy with the result as the replacement components are of different values and sound different from the old. The new pot is only *220 kohm, the old was 257kohm, the new capacitor is 0.033 the old was 0.047 and the new circuit is not as pleasing tonewise as the old one was. But it's working perfect otherwise.
* i bought it as 250kohm value

[GNAPPI]

Remember most / many pots are +/- 20% unless you can find close tolerance selected part$ :-)

[JazzNote]

Remember most / many pots are +/- 20% unless you can find close tolerance selected part$ :-)

The tone pot is a thumbwheel, i guess they don't come tolerance selected. So i ordered three new ones and will use the one which comes closest ....

[sgosnell]

Using a .047 in place of a .033 cap will give different tone. Whether better or worse is a matter of personal taste, but certainly different. The actual resistance of the pot only makes a difference when it's wide open. Having a lower resistance means it will be slightly less trebly when full open, like a higher resistance pot rolled off very slightly, not a lot of real difference. I must say that I have thought that I fixed circuits, but in reality didn't quite get it done correctly. I'd still recheck everything, but it's your problem, not mine.

[KirkP]

The tone pot is a thumbwheel, i guess they don't come tolerance selected. So i ordered three new ones and will use the one which comes closest ....

Sgosnel is correct that there’s no need for close tolerances on tone or volume pots. Since you have already ordered three, you may as well choose the highest resistance of the three, since that will give you a slightly wider tonal range (on the treble end). But you only need to do that if your current tone pot isn’t bright enough when set to 10. If you always roll the tone back, don’t bother changing the pot.

I think the .033 cap will give you a brighter tone than the .047. If you now find your pickup seems too bright, you should probably go back to .047 before going to the trouble of changing the pot.

[JazzNote]

Using a .047 in place of a .033 cap will give different tone. Whether better or worse is a matter of personal taste, but certainly different. The actual resistance of the pot only makes a difference when it's wide open. Having a lower resistance means it will be slightly less trebly when full open, like a higher resistance pot rolled off very slightly, not a lot of real difference. I must say that I have thought that I fixed circuits, but in reality didn't quite get it done correctly. I'd still recheck everything, but it's your problem, not mine.

Sgosnel is correct that there’s no need for close tolerances on tone or volume pots. Since you have already ordered three, you may as well choose the highest resistance of the three, since that will give you a slightly wider tonal range (on the treble end). But you only need to do that if your current tone pot isn’t bright enough when set to 10. If you always roll the tone back, don’t bother changing the pot.

I think the .033 cap will give you a brighter tone than the .047. If you now find your pickup seems too bright, you should probably go back to .047 before going to the trouble of changing the pot.

Thanks guys!

I changed back to the .047 which i prefer and also wired it slightly different. I also find that the Bartolini wiring doesn't suit my needs as well as the Bravo wiring, so i changed that too (see schematics below, Bartolini takes the tone control wire after the volume pot, Bravo directly from the PU)

What i still dislike is that about 80% of the treble cut happens in the area of 8 to 10 of the pot setting. What could i do to spread this more even over the whole range?

Bartolini wiring:


Bravo wiring:

[Franz 1997]

In my case, sudden volume drops have been due to either dirty pots, or in one case a faulty pot which had to be replaced. A faulty tone control pot can cause the same problem.
A dirty output jack can also cause the problem, but in this case the volume drops down to near zero..Deoxit usually cures the problem.

[sgosnell]

IME that sudden volume drop is caused by linear taper pots. I prefer audio taper because it makes the volume change more gradual. I also prefer the Benedetto wiring scheme. There are many ways to wire two pots, and many of them work, but IMO Benedetto found the ideal scheme.

[KirkP]

I changed back to the .047 which i prefer and also wired it slightly different. I also find that the Bartolini wiring doesn't suit my needs as well as the Bravo wiring, so i changed that too (see schematics below, Bartolini takes the tone control wire after the volume pot, Bravo directly from the PU)

What i still dislike is that about 80% of the treble cut happens in the area of 8 to 10 of the pot setting. What could i do to spread this more even over the whole range?

The Bravo circuit with tone control connected to the pickup is how most modern Gibsons and Fenders do it. The Bartolini circuit with the tone control connected to the output jack is what’s known as “vintage” or “50’s” wiring. They should sound identical when the volume and tone controls are set to 10, but behave differently when rolled back.

Edit: Here’s an excellent explanation and demo of various pot tapers:

Based on that and other stuff I’ve read, I suspect you do have an audio taper pot, but it’s not a good approximation to a log curve, as its curve is too steep near maximum rotation. So look for an audio taper pot with a better curve.

I do have one guitar with a linear volume pot. As demonstrated in that video, there’s not much volume cut until I dial down to around 3, then it becomes very sensitive in the low range. I usually set that guitar to 3 or 4 so it will be easier to make quick volume adjustments.

Another of my guitars has Schatten thumbwheels with an audio taper. They have the opposite behavior where most of the volume change is in the upper half of rotation, so I usually set that around 8 to be in the sweet spot. I go back and forth between the two guitars without much trouble.

More info about audio pots:
Fralin - Volume & Tone Pots