Philosophical Q on Tone Controls

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by JingleJungle, May 14, 2006.


  1. JingleJungle

    JingleJungle Member

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    Been posed a good question:
    Since most of our amps' tone controls are passive - so in a certain sense they operate in a "subtractive mode" - why do most of us consider the "neutral" setting being "halfway thru" (i.e. the controls on about "5" on a 10 scale)?

    Wouldn't the real tone of the amp be with all the tone controls fully dimed? Isn't that the point where we truly hear how the various gains tages are amplifiying the signal?

    When I got home I turned on my Carr Rambler - dimed the controls to full max and plugged a HB equipped guitar. With the amp's volume control on 8 o'clock I heard almost sonic bliss (incipit: the Rambler only outputs 28 watts - but boy are they LOUD!).

    BTW am I in some way "burning" the output tubes any faster with this kind of setting? Wouldn't think so... (but I *live* on the learning curve so...).

    Best,

    JJ
     
  2. Shea

    Shea Member

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    What he said. To expand on that, most tone stacks will always be cutting some frequencies, even with all the controls maxed. That cut is usually in the midrange area -- even if the midrange knob is dimed, there will still be some dip in the midrange in the area that is least affected by the midrange control. So in order for there to be flat response, all the other frequencies would have to be cut by the same amount as that dip in the midrange, which would entail turning the knobs down by some extent.

    A good way to see this in action is to download the tone stack calculator at www.duncanamps.com. Play around with the controls in the Fender, Vox, Marshall, and James tonestacks -- see what they do to the eq curve, and see what it takes to get closest to a flat response in each one. The Marshall tonestack is pretty close to flat with all the knobs dimed, but the Fender and Vox ones have a pronounced midrange cut. The James tonestack produces an eq curve that looks like a V with both controls maxed, an upside-down U with both controls at 0, and a wavy line that is fairly close to flat when both controls are set at 50%.

    Shea
     
  3. marlouv

    marlouv Member

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    http://users.chariot.net.au/~gmarts/ampbasic.htm
    This site has a heap of useful information about how the tone controls work.

    The usage of the tubes would probably be more with the treble and mid knobs turned down, if you compensated with the volume to have the at the same level anyway. But i dont know a whole heap about the relationships between preamps and power amps. I just use logic, which isnt the best thing on a guitar forum.
     
  4. blackjack

    blackjack Member

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    The passive tone stacks were designed that way partly due to economy in older times, familiarity in newer times, but also to compensate for the eq of the tubes themselves. That tone stack calculator is showing the response of the tone stack alone, not the final output of the amp using those settings. Are you feeding that to 6L6s or EL84s or...in other words remember that each gain and power stage and speaker etc, contributes to the eq. So, don't think of how much the tone stack is cutting, instead do this: set it the way you like it best and if you feel deprived of that extra bit of gain, just put in a higher gain preamp tube. Some brands like the EH 12ax7 will do allright in non-high-gain amps (Fenders, etc), but I seriously dislike the aggressive gain and noise that comes with it, when placed in the first stage of an amp with higher gain capability (it's actually a bad sound). Furthermore the treble knob in the typical passive tone stack is just a mixer between treble and the rest of the eq. So if you like all controls maxed, then you happen to like treble. But that's your setting for that speaker and that guitar, etc. Those are the parts of the system most easily changed, and the tone controls come in handy to tailor the sound for them. (I mean, you're probably not going to like the all-max setting for all situations). What's not so easy to change are the signal caps, and note that some types/brands of caps can roll-off plenty of high-end, which would be missed by, for example, Les Paul players.
     
  5. JingleJungle

    JingleJungle Member

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    Whoa...
    Deep stuff indeed.
    I don't necessarily *like* treble, as I can get more treble oriented sound with the controls set in a different manner, of course.
    What I found challenging from the question my friend posed me was to find a setting where nothing was subtracted from the "real" signal / tone.
    The experiment I did w/ my Carr just opened up a new window of tone knowledge - but also added more questions :)
    Could we say, for example, that opening the tone controls all the way is a manner to tickle the power amp tubes harder? Especially if one takes into consideration the fact that the bass and treble controls could be shelving (I need to ask the Carr guys) - i.e. they will be boosting something lke 3 Db/octave.

    Ahhh... more Q's than R's :)

    Guess I will do a test whereby I keep my tones "flat" (halfway up) and boost the volume until pre-crunch tone appears vs. another setting where I keep the volume lower but goose the tones to the max.

    Just my ideas - feel free to school and correct me...

    JJ
     
  6. blackjack

    blackjack Member

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    I was mainly addressing your question as to "the real tone of the amp", and that's a great question to make us realize that flat eq is not the intended or most useful tone-shape for the amp w.r.t. some other setting like 5-5-5. Imagine doing this (and I have), you pull the chassis out and attach a wire to bypass the tone stack, or float the tone stack away from ground by detaching the mid-range resistor/pot. You'll then get essentially no effect from the tone stack and that makes it's contribution flat. The power tubes and your guitar will then deliver midrange with a capital M and you'll see why the tone stacks have a midrange dip for most settings you can dial. Doing this experiment will usually deliver enough signal to overdrive the very next gain stage, so you might drop the input volume to counter that. You also asked about "tickling the power tubes more", and it's the same effect, regardless of how you increase the signal out of the tone stack (but it's only useful if you like the eq setting, right?). Supplying additional signal by using higher eq settings is not enough extra signal to age the tubes noticeably faster. Hey I don't mean to crowd your thread, I was hoping to add something useful.
     
  7. Steve Dallas

    Steve Dallas Supporting Member

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    Also, you need to take any tone shaping before and after the tone stack into account. Many amps roll off bass right at the 1st triode by using a small cathode cap and/or small coupling cap. This process continues to some degree throughout the amplifier. Any time you see a coupling cap less than .047uF, bass is being attenuated. A lot of amps like Voxes use mostly .01uF caps, so bass is attenuated all the way through the PI. Other amps like Fenders use mostly .022uF before the PI, then switch to .1uF at the PI so the PI and power amp are more or less full range, but bass is being rolled off at each coupling cap before the PI.

    All of this impacts the way your EQ is shaped even before and after the tone stack in addition to the frequency responses of your tubes and OT.
     
  8. Junior

    Junior Member

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    Because "5" IS neutral. Amp designers don't build an amp with great tone, and then add EQ that screws it up. They design the controls to deliver the sound they want on "5", and allow the user to vary that up or down to accomodate different sounding guitars.

    Again, no, for reasons stated above. And no again, as the gain stages are truly amplifying the signal at any level. That is, gain stages don't add or subtract the original signal, they replace it completely with one of a different size.

    I'll be interested in the results of the test you describe in a later post. If you try the tonestack calculator mentioned above, you'll see that setting all the tone controls to "10" doesn't change the shape of the EQ curve much (especially on a Marshall), it just raises the level. With your volume on "8", there's a good chance the tone controls are just boosting the grit, as "8" ought to exhibit pretty good compression which would reduce any effects of the tone stack.

    Philosophically, the problem seems to stem from the fallacy that "more is better". ;)
     
  9. Tone_Terrific

    Tone_Terrific Member

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    Guitar amps are "universally" (at least generally) voiced before and after the main tone stack. See: bright switches, presence controls, variations in cathode caps, treble bypass/bass rolloff caps, treble bleed etc. There is no flat in hi-fi terms. This is a good thing.
     

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