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How the Tone Stack Works, or 'I Do Not Think it Means What You Think it Means.'

Discussion in 'Amp Central Station' started by Veeseaczar, Nov 29, 2017.

  1. Veeseaczar

    Veeseaczar TDPRI Member

    Age:
    26
    38
    Oct 28, 2016
    Chattanooga, TN
    My other post about the AC15C1's tone yielded some discussions about the tone circuits in Vox and Fenders and their interactivity, so what was a quick response became much longer. I figured I'd dedicate a whole post to it. I think you'll find the info as enlightening as I did.

    Also, I'm incredibly keen to understand this subject more even more clearly, so Amp / Electrical people, if I say anything flagrantly incorrect, please inform me. That's info I definitely want to hear!

    ------------------------------------------------------------------

    So in the quest to understand why the Vox tone controls work the way they do, I stumbled across this total gem of an article:
    http://pickroar.com/1003/the-tone-stack-explained-in-english-for-humans/

    This is an important read for understanding the tone stack of most go-to valve amps: Fender, Vox, and Marshall. The author admits himself a layman, and endeavors to translate the relative arcane language of electronic engineering into simpler English. And as an even further removed layman myself, I will now attempt to distill it moreso:

    The controls of most old-school tone stacks do not independently add nor subtract the amplitude of their namesake frequencies.
    They are a highly interactive series of capacitors and resistors that together form a giant filter as the signal flows through the sequence. The fact that it's a sequence was news to me. For some reason I had always thought of these controls as working simultaneously, but that simply isn't the case. The electricity has a definite path, and each subunit is wired differently, and importantly, the output of one flows into the input of the following one. So understanding the sequential nature of the tone stack is probably the most important conceptual element to understanding how it's actually affecting your signal, which is to say, much more complexly than you've likely been assuming.

    Speaking of, that sequential order is roughly:

    IN >> Treble Circuit >> Bass Circuit >> Mid Circuit >> OUT

    The signal always hits the Treble first, which basically determines if your sound will be biased towards the treble frequencies or toward the rounder sounds. Diming it says "I want to listen to all the Treble, AND Mostly The Treble. The other frequencies can slag off." Cutting it says "Treble cannot come to the party. We're hanging out with Mids and Bass today."

    Treble Summary: Treble is a balance control *for the entire signal*--biasing the signal towards or away from the highs.

    Then the signal hits the Bass. The boundaries of what you'll hear as "Bass" are partially dictated by the rules set by the Treble pot. Other than that, the Bass is a simple Low-Pass filter. Turning it up widens the response range of what Bass frequencies will be listened to by the circuit, turning it down narrows that range.

    Bass Summary: Determines the scope or width of Bass Frequencies in the signal-- or how many bass frequencies can come to the party.


    The final control in the circuit is the Mid control, which is the only potentiometer that effectively controls amplitude. The rest can vaguely be understood as character-tweaking. The Mid is the most straightforward: The amp has a natural Mid profile--you only get to turn those frequencies up or down. The one caveat is that because the other two potentiometers are ultimately wired to the Mid knob, turning it down causes a chain reaction which dumps those frequencies to ground, lowering the amplitude of the whole signal in reverse sequence--as you dial back, Mids get killed first, then Bass, and then Treble. The author argues that if you're scooping the mids really hard, you probably just don't like the way your amp sounds. Interesting consideration given the mechanics, as it would mean you're creating a large gap in the amplifier's natural voice, and then accentuating the extremes

    Mid Summary: Controls the amplitude (volume) of the amplifier's predetermined Mid-range, but pulls the signals of the other two circuits up and down with it...
    This --I think-- will ultimately effect how the signal slams into the power section, so its setting will have a lot of sway with regards to how the amp behaves as you turn the master volume up.**

    ** That very last bit is entirely inductive reasoning. I am inferring based off other things I've read.


    This is what I've been able to glean from my reading of both this article and a few others. I recommend you read the whole contents of it and attempt to understand it. The author even includes his source material as a PDF: An Introduction to Tube Amplifier Theory by Sorlien and Keller, which he recommends reading.​

    You know, this leads me to an interesting question:
    What happens when there is no Mid control? How do the Bass and Treble interact in a two-tone amplifier?

    My hypothesis is that on amps like this--given that the mid voice is likely already set--all you can do is control how much of the middle frequencies are overlapped by the controls, and therefore how much make it to the output.

    This would explain why when you dime both the Bass and Treble on an AC15C1, you get such an aggressively scooped and spikey sound... you're driving the amp to literally only listen to the most extreme portions of the signal.The treble is biasing the amp toward the highest it can read, and then the bass control is as wide as it can get given that parameter... the parametric EQ curve would look like a looong,oomy bass section, and then a hole, and then a very loud treble curve that peaks the most at the highest end it can read. Like a turtle seen from the side, walking to the right, with its head perked up.

    HERE IS AN IMAGE I MADE OF THAT VERY METAPHOR FOR EDUCATIONAL PURPOSES:
    upload_2017-11-29_2-44-37.png

    Whereas setting the controls both closer to 12:00 brings out the mid voicing much clearer. It would then make sense to give the treble a bit of a push for articulation and overdrive character, and narrow the bass response a bit to tighten the signal up while still keeping the most vital low frequencies in the signal mix. Putting that good-good at 10 and 2, my d00d.

    *(over)Drive* like a pro. Keep the controls at 10 and 2. *Big dumb wink*. upload_2017-11-29_2-57-58.png


    Discuss.
     
    Last edited: Nov 29, 2017
    Badger06 and JL_LI like this.

  2. mexicanyella

    mexicanyella Tele-Holic

    726
    Jan 26, 2012
    Troy, MO
    Awesome visual aids. I really hate weak-ass turtle mids. I like to drive my Dodge Caravan with some strong-ass steering wheel mids. Sometimes I even drive with my hands at 10 and 1, or even 9 and noon.

    I generally dime the mids and treble and cut the bass until it doesn't sound overbearing, flabby or hyped to me. Then I adjust the highs down a bit until they don't seem harsh or piercing, if needed. Sometimes that's not needed. I want it loud, pretty clean, with mids in attendance. I play a really bright strat-shaped instrument with a hardtail bridge and just one bridge humbucker (adjusted pretty low) and I feel like this gives me the ability to do some humbucker-sounding rockin out with the gain up, and some sort of Tele sounding twang and snap with the gain down.

    With my head, hands, guitar and rig, weak-ass turtle mids are a bummer.
     
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  3. dan40

    dan40 Tele-Holic

    933
    Aug 19, 2015
    Richmond Va
    I will add that amplifiers that only have treble and bass controls will have a resistor that sets the mid frequencies. In a Fender, this value is usually 6.8k and you will see this resistor tacked onto the bass pot. Disconnecting this resistor from ground will effectively disconnect the entire tone stack from the circuit and allow the full signal to pass on to the next stage. This is known as a "raw switch" and the extra boost it provides will give your Fender (or other amp) a very Tweed like sound.
     

  4. luckett

    luckett Friend of Leo's

    Jun 14, 2011
    .
    That's exactly why I only use amps with lizard mids.


    [​IMG]
     
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  5. bradpdx

    bradpdx Friend of Leo's

    Jul 16, 2006
    Portland, OR
    I feel like sometimes the "low tech" explanations are much more complicated that the actual engineering explanations.

    In a Fender/Marshall circuit, there is no "order" to the EQ - that is, the signal is not processed by a "treble stage" before moving to a "bass stage", etc. It's a single passive network.

    The tone circuit is completely passive, which means that it cannot add gain - it can only subtract signal AS A FUNCTION OF FREQUENCY. The "raw" signal is the sound of the guitar with no EQ - in other words, the "flat" signal. That is indeed very much like a tweed amp, as the small ones have no or little tone control at all.

    The Fender tone stack circuit subtracts primarily mid and low frequencies, because players wanted more clarity out of their amps. Yes, the controls are horribly interactive, which is really just a result of trying to do some quasi-useful EQ with a small number of parts in a primitive tube design. It's not magic, it's low-cost engineering. And it's become the de facto standard for guitar because we're all used to it.

    The #1 thing a novice might do to become better versed in matters of audio electronics is to understand "frequency response" and to be able to read it on a chart and relate it to ones ears. Really, really useful. Unfortunately, most explanations on the web try way too hard and make matters worse.

    This little software tool is pretty good for visualizing tone stack response, but if you don't know how to read a log/log chart, you may get thrown:

    http://www.duncanamps.com/tsc/

    It's unfortunately just an ancient Windows app, but you can run it on macOS using any typical emulator like Wine or Crossover.
     
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  6. RLee77

    RLee77 Friend of Leo's

    May 15, 2016
    Silicon Valley
    @bradpdx +1
    Lots of good accurate info there.
     

  7. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia

  8. uriah1

    uriah1 Doctor of Teleocity Silver Supporter

    Feb 12, 2011
    Around
    I tried to install baxandall circuit once on some amp.
    Tone eq is dark road. Too many options.
    I love amps now with only tone knob.
    lol
    wish you luck
     
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  9. mexicanyella

    mexicanyella Tele-Holic

    726
    Jan 26, 2012
    Troy, MO
    I don't know how to do helpful colored lines on images, I admit it. But that fish image could be as useful as luckett's lizard image. I do kind of like dorsal fin mids. "Dorsal fin" and "bandpass" are pretty much interchangeable in my tone vocabulary.
     

  10. markal

    markal Tele-Holic

    Age:
    43
    522
    Mar 24, 2016
    Colorado
    Is there an iPhone app that measures frequency response?
     

  11. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia
    It's the bass pot.


    Yuck yuck



    After reading "weak-ass turtle mids" I had to post something
     

  12. Anode100

    Anode100 Friend of Leo's

    May 9, 2014
    Behind my beard.
    Hmm.

    So, with a graphic EQ in the effects loop, one could conceivably mimic the profile of any and all sonic reptiles?

    The Chameleon of Tone - or, if you get it wrong, it's a Diaper Viper. Or something...
     
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  13. Veeseaczar

    Veeseaczar TDPRI Member

    Age:
    26
    38
    Oct 28, 2016
    Chattanooga, TN
    But homie.... Which way is tha bass tho? You like to smooth out your trebles?
     

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