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FDP Forum / Guitar Mods, Repairs, and Projects / Fixed resistor vs pot and effect on tone

Tinkerer
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San Diego, CA USA

Apr 17th, 2019 01:28 PM   Edit   Profile  

Does a truly accurate 250k volume pot set on 10, so fully wide open, have the same effect on the tone of a pickup as would a 250K fixed resistor across the pickups hot and ground leads and no volume pot?

I would think it does, but what I understand about electronic theory would fit into a very, very small container, so I would very much appreciate confirmation or correction.

Thanks!!

Peegoo
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Just beyond Mars

there's a world of fools
Apr 17th, 2019 02:13 PM   Edit   Profile  

It's not really the resistor (variable resistor or pot) that shapes the tone; it's the capacitor in tandem with the resistor.

The cap serves as a frequency filter. The cap rating is what determines the frequency roll-off point.

All the variable resistor does is progressively drain high frequency content in the signal to ground (out of the signal to the amp) as the tone knob is slowly rotated from 10 to 0.

Peegoo
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Just beyond Mars

there's a world of fools
Apr 17th, 2019 02:19 PM   Edit   Profile  

The article linked below contains a table that shows frequency cutoff point for several popular tone caps used in passive guitar circuits.

Really good primer on the topic.

Tinkerer
Contributing Member
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San Diego, CA USA

Apr 17th, 2019 02:40 PM   Edit   Profile  

Thanks Peegoo! What I was wondering is about the effect of the volume pot itself, without additional treble by pass r/c components, on the tone of a pickup. My understanding is that that the volume pot, even if it is on ten, will still decrease some of the "brightness" and signal level from a pickup, as even with 250k or 500k of resistance, the are still some frequencies that get shunted to ground.

I'm continuing to monkey around with different control setups and wondered if I could replicate the tone effect of a pot set wide open with just a fixed resistor? My volume controls are always fully open- I just never use them, so I'd like to lose the volume pot, but don't want to lose its tonal effect on the pickup.

Thanks again!

Leftee
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VA

Knuckleheading
Apr 17th, 2019 02:52 PM   Edit   Profile  

Geno is correct.

Also...

Yes, the volume pot does affect the tone. Even with the volume turned all the way up, some of the signal shunts to ground. The higher value the pot, the less signal that goes to ground.

I have a H/H Strat with a volume control for each pickup and no tone controls. It rocks hard.

Peegoo
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Just beyond Mars

there's a world of fools
Apr 17th, 2019 03:54 PM   Edit   Profile  

A pot does add a bit of capacitance to a passive control circuit strictly by being there, even if it's wide open on 10.

Everything in the circuit operates in a subtractive manner where frequency content is concerned...even the wire you use.

Why would you want that subtractive effect in the circuit when there's no volume pot to begin with? You are taking away the sound and tone of the signal. There's no way to know precisely how a pot affects the tone and how to replicate that with a resistor, because all components have a tolerance factor; it's "wiggle room" built into the spec of the parts.

If you have a good scope, you could test all your components and get a close approximation, but the ears are not that good at determining such small variations in spec. Combine this with the fact that you are sending your signal through a cable to your amp, and that cable has a capacitance that affects the tone of the guitar. Unless you use that exact same cable all the time, all bets are off.

By all means you can experiment and have all kinds of fun with this stuff; it's cheap to do--these are jellybean parts and they cost pennies.

But sonically, it's a wash because of the imprecision introduced into the equation by all the stuff (and more) I mention above.

Tinkerer
Contributing Member
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San Diego, CA USA

Apr 17th, 2019 04:30 PM   Edit   Profile  

Thank you both!

Peegoo, the reason I'm thinking about adding the resistor, is that the pickup without a volume pot is a tad too bright to my liking, and I thought adding the resistor would tame it and sound like a wide open pot sounds, without needing to have a pot. I'll run the experiment, but I wondered if the idea/theory made sense.

Leftee
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VA

Knuckleheading
Apr 17th, 2019 04:34 PM   Edit   Profile  

Give it a try and let us know!

Cal-Woody

USA/California

Why do I keep fixing things that work?
Apr 17th, 2019 05:31 PM   Edit   Profile  

From this conversation I wonder if going to a lower resistance pot 100k ohms, then adding the resistor would be a better option. He stated that his current 250k pot made the pickup sound bright and with the added resistance, would you recover to the desired level of treble and still have the volume control for level adjustments?

Tinkerer
Contributing Member
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San Diego, CA USA

Apr 17th, 2019 05:48 PM   Edit   Profile  

Hi Cal- Woody, just aclarification- it's not that it sounds bright with a 250k pot- that's the sound I want, it's that the pickup sounds bright without the pot, and I'm hoping to replicate the effect of a wide open 250k pot with a 250k fixed resistor.

Peegoo
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Just beyond Mars

there's a world of fools
Apr 17th, 2019 08:54 PM   Edit   Profile  

Ah...now I understand.

The thing to keep in mind is that in a pot, the resistive track's length is what adds up to the pot's rating--in your case, 250K.

But with the pot wide open, the resistance in the circuit will be very very small because the pot is cranked to 10 and very little signal is passing through the resistive track. It's virtually a dead short between the wiper (tab 2) and tab 3. Very little signal is bled to ground.

Considering this, it may be better to dial a 250K pot to 10 and then use a DMM to measure Ohms across tabs 2 and 3 and see what you get.

I'm not an electronics engineer, but I'm guessing the effect you're describing (attenuated highs with the pot in the circuit) is due to capacitance of the pot in the circuit and not necessarily resistance.

But ya never know until ya try! Many smart people, including Samuel Langley (Chief Scientist at the Smithsonian Institution) laughed at the Wright Brothers :o)

DrKev

Paris, France

It's just a guitar, not rocket science.
Apr 19th, 2019 04:27 AM   Edit   Profile  

Yes, you can use a 250kΩ resistor in place of a pot and it will sound virtually identical.

Let's assume the pickup never changes and that it's in a passive circuit, and then ignore it and ask what *else* in the circuit affects tone. The answer is...

1) the combined parallel resistance to ground through the volume and tone pots, the input impedence of the amplifier or first pedal input buffer, and

2) the capacitance of the cables connecting the guitar to the amp/pedal buffer.

Of course pot values can vary +/- 20% around the nominal value, but even in the worst case scenario of comparing two 230kΩ pots to two 270kΩ pots, you'll not notice the difference unless you do A-B recordings to compare and even then it'll be slight. Changing to a longer or shorter cable, can have a more obvious change on your tone.

Tinkerer
Contributing Member
*****

San Diego, CA USA

Apr 20th, 2019 08:56 AM   Edit   Profile  

Well, after several more experiments I think I have come up with a configuration that works very well for me. I made this guitar almost a year ago as a single pickup (Kinman P-90). Over the months I played with the pickup location, and different ways of giving me some tonal options, and recently added a Urban humbucker, but also initially using that pickup as more of a tone shaper by wiring it with a pot and combining it only with the Kinman.

I decided this week to expand the swimming pool route and move the humbucker to a traditional neck location, adding a 3 way, and after experimenting with no vol pot, then a 250k and then a 500k fixed resistor, I decided to make a new flat control plate (replacing my modified and inverted strat output jack), and add a mini pot and have a fully functional volume pot.

All of this meant re-doing the graphics in the swimming pool route. It's a bit tough to see in the photo but the artwork is my "logo" and then above the Kinman, the word "Warning", and below the Kinman the words "High Voltage Authorized Personnel Only". For a period of time some weeks back, I also had a tube hot glued in there, but decided to forgo that for this "final" config.

I almost forgot the best part- 5lbs, 11oz!!!

Thank you all for your electrical info!!

"Final" configuration

(This message was last edited by Tinkerer at 10:57 AM, Apr 20th, 2019)

Peegoo
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Planet Peegoo

Lewd guitar player
Apr 20th, 2019 03:56 PM   Edit   Profile  

I love this stuff. Make your own guitar and then Frankenstein it to your satisfaction.

I am right there with ya on this. There is something magical about making music with an instrument you (or a personal friend) made. It adds a very cool vibe and the guitar has a life of its own.

Tinkerer
Contributing Member
*****

San Diego, CA USA

Apr 20th, 2019 10:39 PM   Edit   Profile  

Thank you Peegoo- I find the process you describe so satisfying!! I am very happy with how this one turned out!

FDP Forum / Guitar Mods, Repairs, and Projects / Fixed resistor vs pot and effect on tone




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