How do I get more harmonics?

Discussion in 'Amps and Cabs' started by eicca, Mar 27, 2015.

  1. eicca

    eicca Member

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    My cheap crap DigiTech modeller has more and way better sounding harmonics than my real tube amp.

    Should I just change the tubes? Do they lose their ability to bring out harmonics when they get old?
     
  2. Geetarman74

    Geetarman74 Member

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    I think it really depends on gain. A high-gain modeler could easily produce more harmonics than a low-gain or clean tube amp.
     
  3. A-Mags

    A-Mags Supporting Member

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    I think I read somewhere that class A/B amps have more harmonics.
     
  4. eicca

    eicca Member

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    My gain is insane... I'm actually thinking of swapping tubes just to get some headroom above 3. Yeesh.

    What are class A/B amps?
     
  5. Mejis

    Mejis Member

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    Someone with a stronger electrical background would do a better job of explaining Class A vs. A/B technically, but I tend to think class A circuits have more harmonic content - think Vox AC30 and the like.
     
  6. DaveKS

    DaveKS Member

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    Are you referring to the MosValve amp in your sig as a real tube amp?

    It's got one preamp tube in it I think.
     
  7. NewDr.P

    NewDr.P Member

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    i think you may be referring to sounds that you happen to like as having more harmonics. my guess is youre enjoying the treble and upper mid range frequencies that you find your amp's tone lacking.
     
  8. eicca

    eicca Member

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    It's weird, it's got plenty of treble, but it sounds very plain, like a chainsaw. None of those shimmering overtones or pick noises. Like in Long Time by Boston, the guitar has it constantly; it almost sounds "squeezed."
     
  9. The Funk

    The Funk Member

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    I would imagine the MosValve (if thats what you are using) is a hybrid amp, meaning not all tube.

    A real tube amp may sound more harmonically rich. But it also the way its tuned.

    A single ended class A amp will have more harmonic content than a Push Pull Class A or Class AB amp. This is because the 2nd order harmonics are not cancelled out by the push/pull circuitry.

    Class A, and Class AB amps are virtually identical up to the clipping point. Class A won't allow full cutoff, class AB will. There is some "musicality" that is lost when things go into full cutoff, along with an increase in distortion.

    Much of the harmonic content lies in the upper midrange, or more accurately, harmonics of the upper midrange. Harmonics over the highs will be too high to reproduce. So, you need to have enough upper mids in the sound to generate the harmonics. If you scoop out your mids, those harmonics will disappear.

    Different amps also have different signatures. My Lonestar Special is much more harmonically rich than my bad cat cougar, partly due to a midrange that will not be tamed. On the other hand, that same midrange makes the amp sound somewhat boxier in isolation than the cougar, which has a wider sound. In a mix....they can be set to sound very similar.

    The Boston sound is all about a fixed wah which will boost the mids by a lot, and a compressor which will keep those harmonics loud.
     
  10. guitarjazz

    guitarjazz Member

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    The overdrive excites more harmonics, the clean sound is richer? Waddaya mean?
     
  11. teemuk

    teemuk Member

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    Funny that you say that because AC30 is a class-AB amp.

    ...Anyway, this is one of those things that really depends on overall design, not all amps are the "same".

    In class A, by definition, a device conducts full 360 degrees, effectively amplifying both half waves completely. Because full wave is amplified by each device class-A amps can also operate single-ended in addition to push-pull. Due to required bias point a class A amplifier "idles" at very high quiescent current. This makes class-A amps very inefficient in power amplification. Average current draw is also quite near the idle current draw and remains rather steady under normal operation.
    In class B, by definition, a device conducts for 180 degrees, effectively amplifying only a half wave. Device idles in cutoff drawing no current, which makes power amplification decently efficient. Due to bias point, current draw under operation is much more "dynamic" than with class-A. However, we need at least two devices, and a push-pull circuit or alike, to combine the signals and effectively amplify a full wave. Additionally, right before each device enters and leaves from cutoff stage the amplification is highly non-linear resulting to "crossover distortion".
    In class AB, by definition, a device conducts an arbitrary value more than 180 degrees but similarly less than 360 degrees. Combining two class-AB devices to push-pull we achieve full wave amplification but with much less crossover distortion than with class-B. Efficiency naturally highly depends on "bias" (is bias point set closer to class A or B) but often so does distortion. In some cases there is a single ideal bias point at which crossover distortion is lowest and biasing higher or lower will just increase it. Naturally "hotter" bias will also decrease efficiency and henceworth headroom and overall average THD levels in signal.

    Amount of "harmonic content" could be argued. Class B and A/B amplifiers inherently have "some" crossover distortion while class-A amplifiers will not. In practice we must acknowledge that a properly introduced class-AB bias effectively makes crossover distortion entirely inaudible, if not even entirely invisible to cheapest THD meters. A class-A amplifier, on the other hand, will be so inefficient that we can expect it to generate plenty of distortion due to its poor headroom. In this regard class-AB amplifier is usually a tremendous improvement.
    Distortion characteristics of a single-ended amplifier will be highly different from those of a push-pull amplifier. Differences in that regard between push-pull class-A and class-B amplifiers are not so remarkable. Class-A/B amps will have vastly more dynamic current draw characterstics so some might display effects of power supply sag under continuos overdrive. Some class-A and A/B push-pull amplifiers may drift in bias under sustained overdrive and begin to introduce significant amounts of crossover distortion. Recovery from this "blocking distortion" is also not instantenious because bias needs to drift back closer to proper level.
    Negative feedback - if employed - may further "linearize" operation, even extending the linear area much closer to clipping by either cutoff or saturation. With price of few dB's of lost gain NFB lowers overall distortion and output impedance, which usually has very distinct audible effects. However, clipping distortion also becomes vastly more abrupt.

    Single-ended amplifiers tend to clip asymmetrically. Amplitude of even order harmonic components is higher with asymmetric distortion. Push-pull amplifiers tend to clip more symmetrically. Amplitude of odd order harmonics is higher with symmetric distortion. Inherently to operation, each half of the push-pull circuit also corrects asymmetric distortion created in the other half; push-pull circuit effectively "cancels" even order distortion created in it.

    What comes to preamps, in tube amps each gain stage is usually a single-ended amplifier. Bias point is often arbitrary between class-A and class-AB. Within certain area the operation is like class-A, however, clipping will tend to be more asymmetric. Often guitar preamp stage deliberately enhances asymmetric distortion characteristics. Employing push-pull circuits in tube preamps is rare.

    In most guitar amplifiers, distortion characteristics are mostly created by both pre and power amp simultaneously. Modern higher power amps may operate on a design goal of minimising power amp distortion. Pure power amp distortion is rare in guitar amps.

    MOST IMPORTANTLY.... Distortion does not create amplifier's tone alone. Guitar amplifiers also deliberately emphasize or de-emphasize certain frequencies as well as limit bandwidth, radically. This EQ'ing is also highly interactive in how distortion overall sounds. Also, it's highly interactive in two different manners: pre and post distortion.

    Vox AC-30 employes a class-AB push-pull output stage with slightly higher bias than average. More notable is that the power amplifier does not employ any global negative feedback and that the output tubes are true pentodes, which have slightly "rounder" characteristic curves than beam tetrodes and less abrupt clipping. Also, without additional pedals this amp's clipping distortion is largely power amp -based, as signal comes fairly clean from the preamp and its gain staging allows very little clipping distortion to occur.

    AC30 is from the era when instrument amplifiers were designed to be "clean" and therefore the stage to clip first was usually the power amplifier. AC30, with its low headroom, most certainly is not "clean" but the preamp signal is fairly bright (no farty clipping of bass notes) and the loudspeakers roll down quickly before most obnoxious treble frequencies. The distortion simply isn't too apparent or "harsh".
    If we take a modern high-gain amp we might find a different approach: Now certain preamp stages are deliberately overdriven to distort. Often in very asymmetric manner. Output stage has plenty of power reserve for headroom and ample level of NFB keeps things "tight". The power amp is largely designed to reproduce the preamp signal without too distinctive additional coloration.
    ...And then there are thousands and thousands of amps between such extremes. Trying to generalize something from that is simply a huge disservice, IMO.
     
  12. eicca

    eicca Member

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    The overdrive on the DigiTech has a lot more harmonics. I couldn't ask for more from my MosValve's clean channel, best thing I've ever heard :)

    This is all fascinating to me. I put my EQ pedal pre distortion and jacked the 800hz band all the way up and that sure brought out the harmonics.

    The tubes that are in there now are really treble-y. I imagine swapping for more midrange-y tubes would bring out the harmonics, based on what I've learned?
     
  13. eicca

    eicca Member

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    Also, I believe my amp may be class A based on how it's biased--it puts out the same heat regardless of how loud I'm playing.
     
  14. Mejis

    Mejis Member

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    Thanks for clarifying re. the ac30. There seems to be conflicting info about whether it operates in class A or A/B (ie that it runs in "class A" until the onset of clipping and then switches to A/B). I always thought of it as one of the quintessential class A amps, so my mistake! :bonk
     
  15. logdrum

    logdrum Member

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    My Mesa Blue Angel had been debunked here in TGP as not class A at all but a Cathode Bias AB despite Mesa saying so. But when I checked the current draw and watts consumed using a Watt Meter, on idle it draws more than my Tremoverb, Mark III or any bigger wattage AB, despite it being a sub 30 watt amp. And when I play it the watt consumed and current drawn difference between idle and playing is not that much. Based on this alone it is an A leaning AB amp probably.
     
  16. michael.e

    michael.e Supporting Member

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    Are we talking harmonics with the use of gain? Or clean tones?

    Here is what I get in terms of harmonics with regard to clean tones.

    I am standing here working while I have my guitar [Esquire] around my neck. Thought I would record what is on, which is a Bogner Metropolis set to a Voxy setting. I know it is not exactly in tune, but I just threw this down to illustrate harmonics, or what I think of them in a clean tone.
    http://www.mediafire.com/listen/pwecx3tmnwvro74/Esquire-Metropolis.wav
     
    Last edited: Mar 27, 2015
  17. DeaconBlues

    DeaconBlues Member

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    Are these even tube amps? Tube preamp with an ss power amp? What's the real story on the Real Tube thing?
     
  18. NewDr.P

    NewDr.P Member

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    this is why amplifier manufacturers should list how many harmonics are in they amps.

    someone should invent a supplemental supply of harmonics to put in your amp holes.
     
  19. eicca

    eicca Member

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    After reading all of this, the more I play, the more I feel like the tubes aren't clipping the midranges. It's all treble....

    I'd hope new tubes will do the trick, maybe new capacitors?
     

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