Pot or Pickup?!

In my new Partsquire I have a Mike Turk tapped pickup and 2 CTS 1Meg pots...

Normally, when I play my guitars, I turn my volume knob to about 7 and it sounds great.. Full sound specturm etc... However, when I have this one at 7.. the sound is very dull and lifeless(honestly, crappy tone and low volume level)... but when I turn it to 10... there's the full sound and tone... This happens on BOTH leads and in both positions.. with and without tone pots...

So.. anyone have any idea what gives? Bad pots? Typical behavior with 1 Meg pots? (I have only used 250k until now) Pickup resistance too high (dunno if this is even a factor)? Ideas?

does the volume tone thing happen on the neck pickup too or is this an Esquire(1 pickup) wiring scheme?

It is a 1 pickup esquire..

4 way blade

1 - Hot Out no tone
2 - Hot Out tone
3 - Low Out tone
4 - Low Out no tone

Effects are the same in ALL positions...

In my new Partsquire I have a Mike Turk tapped pickup and 2 CTS 1Meg pots...

Normally, when I play my guitars, I turn my volume knob to about 7 and it sounds great.. Full sound specturm etc... However, when I have this one at 7.. the sound is very dull and lifeless(honestly, crappy tone and low volume level)... but when I turn it to 10... there's the full sound and tone... This happens on BOTH leads and in both positions.. with and without tone pots...

So.. anyone have any idea what gives? Bad pots? Typical behavior with 1 Meg pots? (I have only used 250k until now) Pickup resistance too high (dunno if this is even a factor)? Ideas?

Chris, it may be that 1meg vol pot. Do you have the treble bleed cap on it?

Maybe try a 250k volume, and a 1 Meg tone- It's what I use- sounds great.

In my new Partsquire I have a Mike Turk tapped pickup and 2 CTS 1Meg pots...

Normally, when I play my guitars, I turn my volume knob to about 7 and it sounds great.. Full sound specturm etc... However, when I have this one at 7.. the sound is very dull and lifeless(honestly, crappy tone and low volume level)... but when I turn it to 10... there's the full sound and tone... This happens on BOTH leads and in both positions.. with and without tone pots...

So.. anyone have any idea what gives? Bad pots? Typical behavior with 1 Meg pots? (I have only used 250k until now) Pickup resistance too high (dunno if this is even a factor)? Ideas?

Chris, it may be that 1meg vol pot. Do you have the treble bleed cap on it?

No... I don't.. hrm... Good thought Brad...

It is absolutely the 1Meg volume pot doing this. When a guitar's volume pot is full on, you might think the pot is out of the circuit, but it is not. Because the low lug is connected to ground, when the pot is full on, the full value of the pot (in this case, 1Meg) is in parallel with the pickup. Putting a 1Meg resistor in parallel with a single coil pickup will make that pickup sound quite a bit brighter than putting a 250K resistor in parallel with that same pickup.

As you turn the volume pot down, an increasing portion of the pot is added, in series, with the pickup, while a decreasing portion of the pot is still in parallel with the pickup. This parallel/series balancing act has a rather significant effect on the sound. The larger the value of the pot, the faster will be the treble rolloff, as you back off on the volume. This effect is offset, somewhat, when the DC resistance value of the pickup is increased. That is why you usually see 250K volume pots with single coils, and 500K volume pots with humbuckers.

You have a couple of choices. You could replace your volume pot with a 250K pot. Or, you could add a treble bypass cap. The value of the bypass cap is somewhat critical. If it is too large, you will experience an apparent treble increase as you back off on the volume. You can either use a lower value bypass cap, or you can add a resistor to the bypass cap (either in parallel or series with the cap). Keep in mind that when you add this resistor, what you are really doing is, effectively, altering the value of the volume pot. Because the resistor is in parallel with the portion of the pot that is between the high lug and the wiper, you are decreasing the resistance between those two points. This is an attempt to bring series and parallel portions of the volume circuit into better harmony -- something that can also be accomplished by using a lower valued volume pot.

Try cheapest first. Cap's, Pot's, Pick ups.

It's the best way.

which taper are your pots ?

It is absolutely the 1Meg volume pot doing this.

Wow.. thanks for that.. while I don't have a true understanding of electronics.. I appreciate the explination.. You said that the value of the trebble bypass cap is important.. do you have a suggested value? I have quite a few (thanks Brad!) laying around and could easily add this to the wiring... As a matter of fact, after Brad (BB) suggested it, I was going to add it.. but your thoughts to the load would be appreciated...

Thanks for the info...

Try cheapest first. Cap's, Pot's, Pick ups.

It's the best way.

I agree.. but I do have all of these lying around unused...

which taper are your pots ?

To be honest.. I don't know.. I just asked for 1Meg CTS pots... Didn't ask for linear or anything...

Sorry.. I've never used 1Meg pots... only 250K... Heck.. I never asked for anything special when I bought 250's either... And I am sure it makes a difference.. I just never contemplated it..

The value of the bypass cap is really up to you. Personally, I prefer to address the problem by changing the value of the volume pot, but that's just the way I do it. I have experimented with bypass caps, though.

Back in the very late 60s and very early 70s, Fender was installing 1Meg volume pots in their Teles. They used a .001uF bypass cap. I think this cap is much, much too large, though there are some who will argue with me, on that. (It's all personal taste.) Some folks think a 680pF cap is small enough, but I still think that is too large. (For reference, .001uF = 1000pF.) People still end up using a bypass resistor, along with the 680pF cap. (The cap is a compensation because the volume pot value is so large. The resistor is a compenstation because the bypass cap is too large; it is a compensation for the compensation.)

A better approach would be to start with a bypass cap down in the 100pF range, and work your way up. I have found that with a Tele equipped with a 500K volume pot, a 120pF bypass cap was as large as I wanted to go. Anything more than that, and I experienced an apparent treble increase as I rolled off the volume.)

Of course, much also depends upon your amp and, believe it or not, the guitar cord you use. Each guitar cord has its own capacitance, which makes it act as a fixed tone control. That's why a guitar will frequently sound duller or darker with a longer cord. If you normally use a 15 ft. cord, and you suddenly start using a 25 ft. cord, you'll probably hear a difference (if the cords are, otherwise, the same.)

{snip}

As you turn the volume pot down, an increasing portion of the pot is added, in series, with the pickup, while a decreasing portion of the pot is still in parallel with the pickup. This parallel/series balancing act has a rather significant effect on the sound. The larger the value of the pot, the faster will be the treble rolloff, as you back off on the volume. This effect is offset, somewhat, when the DC resistance value of the pickup is increased. That is why you usually see 250K volume pots with single coils, and 500K volume pots with humbuckers.

{snip}


If you look at a typical Tele wiring diagram, the volume control is wired as a voltage divider, which means the load it presents to the pickup(s) doesn't change when the volume knob is turned -- only the shunt resistance presented to the amp input does. If the guitar is miswired so that the pickup is connected to the volume control's wiper and the output jack is connected to its high lug, then the control will still function but the load resistance presented to the pickup(s) will vary as the volume knob is rotated, which can severely affect frequency response.

Great topic. Jim is right on as usual, and has explained it very well. I decided to play around with my PSPICE analysis tool regarding this.

One big factor not mentioned yet is that the taper of the audio pot gives you more series resistance than you think relative to its position.

Really DO click on the images for a clearer view.

Here is a plot of the voltage division ratio vs. rotation:
http://terrydownsmusic.com/Archive/audio_pot_taper.jpg

Notice that at "7" or 70% or CCW rotation of 90 degrees, the division ratio is about 0.3. This means there is 700K in series and 300K in shunt. So, at 70% of the pot's rotation, it has all but 30% of its resistance.

Here are some plots with the following conditions:

Standard Tele bridge pickup
1M volume
1M Tone pot turned max CW
20ft cable
1M amp load
100pF amp load

Here is the response with the 1M pot fully CW.
http://terrydownsmusic.com/Archive/1meg_pot_max_cw.jpg
Notice the huge resonance at ~2.5kHz. That puppy is ringing!!!

Now, with the volume at "7", with no treble bypass cap.
http://terrydownsmusic.com/Archive/1meg_pot_70.jpg

The high frequencies are destroyed. The resonance is completely damped, and the 700K of series resistance creates a low-pass filter with the cable and amp capacitance.

Here is the volume at "7" with a 0.001µF treble bypass cap.
http://terrydownsmusic.com/Archive/1meg_pot_70w_tbc.jpg

Notice the resonance is back, but the low end is lowered.

Here is the volume at "7" with a 470pF treble bypass cap.
http://terrydownsmusic.com/Archive/1meg_pot_70w_tbc479pf.jpg

This is closer to the original max CW plot. Note however, the resonance is now moved out to 4kHz.

I'm not saying that 470pF is the ideal value. I'm just using it as an example.

1Meg pots unleash that Tele resonance in a big way, all that high end that we all love (we at least I think most of us do). It has tradeoffs. It makes the treble bypass solution a bit more tricky, and it makes the high end loss due to the guitar cable more noticeable.

I did install a bypass cap and everything is great... Thanks for the help and the lessons...

Gotta love the TDPRI...

You really do. Where else are you gonna find this kind of stuff? Mr."Gearhead"? Not likely. Maybe the FDP or one of the sites dedicated more to electronics, but here it's all about our teles. I love this bar.