Please explain how the value of a tone pot and the capacitor interact.

Discussion in 'Tele-Technical' started by boneyguy, May 30, 2008.

  1. boneyguy

    boneyguy Doctor of Teleocity

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    I don't get the interaciton of the tone pot value (eg. 500k) and the cap value (eg. .047uF), What is the relationship and how do they interact?


    If you didn't have a cap on the tone pot would it function as a volume pot?

    Please explain.
     
  2. idoru

    idoru Tele-Meister

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    Yes. Capacitors only allow signals over a certain frequency to pass - in this case, to ground - based on their value in uFarads. Signals higher than that cutoff freq become quieter as the resistance from the pot is reduced. Without a cap, the full signal goes to ground so you hear everything getting cut. The higher the pot value, the less signal you lose to ground at full resistance.

    c-
     
  3. Telenator

    Telenator Doctor of Teleocity Vendor Member

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    OK then, if I have a guitar that sounds a bit too bright, (not much, just a bit), would changing just one of the two pots (volume or tone), take the egde off?

    I have a Tele Thinline with a single Volume and single Tone both of which are 500K pots.
     
  4. bnjp

    bnjp Tele-Meister

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    I did this on a squier. I originally had two 500k pots with an .047 cap. It was too bright. Then I changed only the tone pot to 250 and it helped just enough.
     
  5. Telenator

    Telenator Doctor of Teleocity Vendor Member

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    Thanks man.
     
  6. boneyguy

    boneyguy Doctor of Teleocity

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    So then what's the difference between changing the value of the pot as opposed to changing the value of the cap. Does it amount to the same thing?

    In other words if I wanted a brighter sound from some single coils and I changed the 250k pot to a 300k would that difference be something that could be accomplished by keeping the 250k pot but changing the cap value.

    In other words, once again, are they both (pot value and cap value) affecting the very same thing only through a different means?
     
  7. idoru

    idoru Tele-Meister

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    Changing the value of the pot effects how much signal is going to ground when you've dialled the tone to full. More resistance means less signal goes to ground, which is why 1M > 500k > 250k as far as treble is concerned.

    On a related note, a "no-load pot" presents infinite resistance when dialled to 10, thereby losing no signal to ground. This is about the same as going straight from the volume pot to the output jack.

    Changing the cap effects the frequency of the signal that goes to ground - there will always be some loss based on the pot values above, but .047 rolls off at a lower frequency than .033 or .022.

    So yes, both factors have an effect on the tone - but it's not the same thing. With the tone at 10, the pot value is more important. As you roll off the tone, the cap has a much bigger effect.

    c-
     
  8. varakeef

    varakeef Tele-Afflicted

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    So instead of replacing a pot with higher value pot (say 250 kohm to 1 meg) if tone dialled up is not trebly enough, one could have a by passswitch for the tone pot to get as bright sound as possible with that particular pickup?
     
  9. idoru

    idoru Tele-Meister

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    Right - like the Esquire wiring, or using a push-pull pot. No load pots are easy enough to make if you have the right sort - you just scrape off some of the conductive material at the end of the pot's travel. Just don't install the pot in series...

    c-
     
  10. Nick JD

    Nick JD Doctor of Teleocity Ad Free Member

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    Another way is to turn the "HIGH" pot on your amp clockwise a little bit. :D ;)
     
  11. tdowns

    tdowns Former Member

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    Ok. You just had to ask. So, I spent a little time with PSPICE today so you can see the electrical response of the tone pot. The pickup is a Tele bridge. The conditions are as follows:

    • Pickup inductance = 3.2H
    • Pickup resistance = 7.4K
    • Pickup interwinding capacitance = 160pF
    • Volume pot = 500K (set at max CW)
    • Tone Pot = 500K but 442K actual
    • Tone cap = 0.05µF
    • Cable = 20ft
    • Amp input impedance = 1MEG
    • Amp input capacitance = 100pF

    The labels on the graph are TONE KNOB SETTING (0 thru 10)/RESISTANCE. Remember it is a audio (non-linear) pot. The resistance values for the 0 through 10 positions are based on experimental data I collected on audio pots a few years ago.

    [​IMG]

    Notice the resonance peak with no tone pot connected aka the no-load pot. You can now see how the TBX would give you everything between the max CCW and almost open.

    I doubt some of you would experience the same no-load response as plotted because every amp input impedance is not as high as 1 meg. If the amp input impedance is more like 500K, that no-load response will be less pronounced. But, a Fender Twin is 1MEG input impedance.

    Look at the curve for the 500K pot set on "8". Having a 250K pot (277K on the plot) is like a 500K pot turned down to "8".

    Regarding the tone cap value, it matters most at a setting of "0". With the 0.05µF cap, the resonance is moved down to 300Hz. At other points on the dial, it is simply destroying the resonance at ~3.5kHz. If you used a smaller cap value, that resonance would be higher than 300Hz.
     
    Last edited: Jun 5, 2008
  12. idoru

    idoru Tele-Meister

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    Nice work mate - that should really help those who are struggling with the concepts.

    Of note to bass players is that little low-freq resonant bump with the tone rolled off all the way - that little guy's why my Jazz has such a wicked dub-tone in the neck position :)

    c-
     
  13. tdowns

    tdowns Former Member

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    I updated the plot. I noticed I essentially plotted it backwards, just as if the tone pot was wired backwards, i.e. highs cut at max CW. I thought it looked strange the see all the responses bunched up at the high end. The curves were correct based on the resistance shown, but the taper of the pot was wrong. I hope this helps.
     
  14. telecaster1987

    telecaster1987 Tele-Meister

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    Thanks TDowns!

    As an old R&D tech I've been looking for a response curve like the one you've posted!
     
  15. tdowns

    tdowns Former Member

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    Thanks. I'm glad somebody appreciates the plot. I put quite a bit of work into that.
     
  16. kp8

    kp8 Friend of Leo's

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    Terry Downs does more by 9 am than most tele pickers do all day.

    You da man!

    That is awesome! I am gonna get that as a tattoo.
     
  17. Nick JD

    Nick JD Doctor of Teleocity Ad Free Member

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    Terry, why is the voltage in the low frequencies higher at the "0" position than at "1"?
     
  18. tdowns

    tdowns Former Member

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    Thanks kp8. I feel the love. Please post a pic of that tattoo when you get it done. :eek: I'll supply you a PDF file for it instead if the picture if you are going for a hi-res tattoo. :)

    As the tone pot is turned down (CCW), the tone network is mostly resistive. So it only serves a damper to the existing resonance. When the tone pot completely connects the cap to the circuit, it goes back to a resonant condition. The resonance now is around 200Hz.
     
  19. Montana_Dawg

    Montana_Dawg Tele-Holic

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    Nice job on the graph, TD! (As always)

    It also illustrates the reason why I prefer linear pots for tone rather than audio taper.
     
  20. tdowns

    tdowns Former Member

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    Thanks. So you prefer linear so you can get more of a wah effect down near the CCW end with the pinky?
     
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