Mesa Boogie Mark V 35 “Xtreme” mode - negative feedback removed?

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RandallFlaggRandallFlagg Frets: 13929
edited June 2019 in Amps
On my Mesa Boogie Mark V 35 the Xtreme mode on channel has most of the negative feedback removed according to the manual. It's louder, fuller sounding and has a much more immediate attack with no compression, the presence control doesn't do much in this mode. I like it and I've read that Walter Trout uses this channel on his Mark V for his live tone.

With the gain dialled back you can get some nice lightly overdriven tones.

What is negative feedback and what does removing it do? Do all amps have it or do some amps have zero negative feedback as well?


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Comments

  • ICBMICBM Frets: 71960
    Yes, if it’s like the other Mesa amps with the ‘Extreme’ mode it turns off the power amp negative feedback.

    Negative feedback (NFB) is where a small amount of the output of an amplifier (or just a gain stage) is fed back into the input, in reverse -  a sort of self-regulation. This reduces the gain, and tends to flatten out the non-linear parts of the response of the amp, so it produces a more even tone and more controlled dynamics. Hence removing it makes the amp more aggressive and spiky.

    Not all amps have it - a couple of classic examples that don’t are the Vox AC series and the Fender 5E3 Deluxe. Lack of it tends to be associated with cathode-biased amps, which are more inherently self-limiting since cathode bias is itself a form of NFB, although it’s not usually referred to as such.

    "Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski

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  • exocetexocet Frets: 1948
    On my Mesa Boogie Mark V 35 the Xtreme mode on channel has most of the negative feedback removed according to the manual. It's louder, fuller sounding and has a much more immediate attack with no compression, the presence control doesn't do much in this mode. I like it and I've read that Walter Trout uses this channel on his Mark V for his live tone.

    With the gain dialled back you can get some nice lightly overdriven tones.

    What is negative feedback and what does removing it do? Do all amps have it or do some amps have zero negative feedback as well?
    Negative Feedback is used in many power amp sections to flatten the frequency response and reduce distortion. It originated from the audio world where these are desirable “goals”.
    A small amount of the output stage is “inverted” and fed back into the input of the power amp.
    Most amps with a Presence control have negative feedback,
    The most famous example of an amp without negative feedback is the AC30.
    Negative feedback has a pronounced effect on the transition from clean to dirty. Amps without it have a more gradual transition compared to those that do.
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  • ecc83ecc83 Frets: 1589

    Worth mentioning also I think that the triode has inherent NFB when used in its conventional mode (the cathode follower, often used to drive tone stacks has 100% NFB and thus a voltage gain just less than one.

    Triodes can be configured as a "cascode" stage which defeats the feedback but the circuit is rarely seen in guitar amplifiers.

    Dave.

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  • richardhomerrichardhomer Frets: 24793
    Drivel!

    How’s that for negative feedback?
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  • RandallFlaggRandallFlagg Frets: 13929
    Drivel!

    How’s that for negative feedback?
    Perfect!


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  • RandallFlaggRandallFlagg Frets: 13929
    Thanks for the explanations above.


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  • jpfampsjpfamps Frets: 2723
    ecc83 said:

    Worth mentioning also I think that the triode has inherent NFB when used in its conventional mode (the cathode follower, often used to drive tone stacks has 100% NFB and thus a voltage gain just less than one.

    Triodes can be configured as a "cascode" stage which defeats the feedback but the circuit is rarely seen in guitar amplifiers.

    Dave.

    The main application of Cascoding triodes is to reduce the Miller capacitance and hence enhance bandwidth. Useful for RF, not amazingly useful at audio frequencies. It also uses 2 triodes, which may seem wasteful in a guitar amp.

    Degenerate feedback in triodes has a different effect to global negative feedback around the power stage in that it increases output impedance. 

    Presence controls generally decouple the negative feedback at higher frequencies, thus increasing gain. Of course this method requires that there is negative feedback applied, and explains why the presence control doesn't work in Xtreme mode.

    For what it's worth, resonance controls do the same but at low frequencies.
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  • KeefyKeefy Frets: 2272
    I've just realised I prefer little or no NFB in my amps:

    Matchless Lightning Reverb - based on AC15, so no NFB
    5E3 clone - no NFB
    Micro 5E3 - no NFB
    Princetorn Reverb 'Custom '68 RI - some NFB, but less than in the original  design

    The Revival Drive lets you set the amount of NFB - surprise surprise, I generally prefer it off!
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  • Modulus_AmpsModulus_Amps Frets: 2569
    tFB Trader
    You can make the negative feedback variable to to find your sweet spot with a simple pot, but that setting will depend on how the pre-amp is voiced and visa versa. I do this on two of my Gryphin amp models and it works really well.

    I generally prefer turning the negative feedback lower for bedroom playing and turning it up a bit when playing louder to control any harshness, you can hear the amp "open up" as the feedback is reduced.

    Most fixed biased amps have negative feedback, some amps like Dumbles and clones can have feedback at multiple places in the circuit. .... it is good to have variety




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