Variable Negative Feedback Control.

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Red Planet, Feb 4, 2006.


  1. Red Planet

    Red Planet Member

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    Anyone fool with one of these? Thinking of putting one in my JCM 800 2203 to tweak it to sing a little more during solos.
     
  2. rastaman

    rastaman Member

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  3. Blue Strat

    Blue Strat Member

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    Doesn't that amp have a presence control? Not sure if the two can coexist or whether there'd be any benefit. Could be an interesting experiment though.

    I've done the var feedback thing on lots of Fenders and other amps and the results have been great.
     
  4. conundrum

    conundrum Member

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    Isn't that essentially what a presence control its?
     
  5. Red Planet

    Red Planet Member

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    It does have a Presence Control. Not sure how it would work out but this amp is a reissue and the NFB Circuit is different from the origionals.

    I need to tweek this amp a little though. It needs just a touch of sing added into the mix to get it where it needs to be. Now this can be done just by increasing it but I thought a variable would be easier to tweak to prference.
     
  6. KLB

    KLB Member

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    Variable NFB controls are very useful, imo.

    On a vintage Marshall/Germino/Etc., I like to make the NFB resistor a 27K fixed on the 16-ohm tap, which is the stock setup on a JTM-45, then put a 100K linear pot in series with it. I use one of the two speaker jack holes for the pot. When set to 1/4-1/3, the NFB value will be approximately stock, assuming an original 47k-56k resistor. Set the control at minimum (zero ohms) for stock NFB if it already had a 27k resistor. As it is turned up, the NFB is reduced, which makes the amp have more apparent gain, mids/highs, but looser bass. The presence control has less effect the more it is turned up.

    The MOOD control on the Heritage Colonial is one of my favorite features.
    It is similar to the above arrangement.
     
  7. Red Planet

    Red Planet Member

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    A fella has got to play with his toys. :D :AOK

    Thanx KLB I'm gonna give it a try.
     
  8. rastaman

    rastaman Member

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    A fella can never have too many toys....:BEER
     
  9. itsLars

    itsLars Member

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    Hi,

    I have one in my amp which is a custom version of the Matamp C7 head. The knob is called 'attack' and is indeed very useful.

    Bye,
    Lars
     
  10. doctorx

    doctorx Member

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    I put a circuit I saw in an Ampage post called a Resonance control in my modified Bassman 100. I used the 1 meg pot that used to be the master volume. It makes a big difference, like others in this thread have discribed. On one extreme, the amp is real clean and smooth, on the other end of the pot you get all kinds of crunch and overdrive. It is not a presence control, it doesnt do the same thing.
     
  11. Red Planet

    Red Planet Member

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    What I'm trying to achieve is getting it to sing a little more than it does now. I think I wont be going way out the direction of major dirt just enuff to get the amp to sing more with my Tele without a pedal.

    Any way This Mod is going in at lunch time today and I'm taking it to the Atlanta Ampfest tonight so those guys can post what they think about it.

    Its good to have an Amptech like Jeff Andrews that will do these things on the fly for me. if any of you guys in the Atlanta area need work done please go to www.andrewsamplab.com He is a great guy with very reasonable prices and pays extra atention to details. I've had some guys around here treat you like a criminal if you want your amp worked on.
     
  12. chuby galoso

    chuby galoso Member

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    or you can try a larger cap on the presence comtrol, 1.5 or .2 this will take you neg feedback out of more than just the high end.
     
  13. doctorx

    doctorx Member

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    I'm good friends with Jeff and his brother Kevin, who is a great bass player. I saw them both last friday at the California Guitar Trio show in L5P. Jeff does do great work, he's helped me a lot at various times.
     
  14. hasserl

    hasserl Member

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    I'm not aware of the circuit you used, but a Resonance control is a little different. A Resonance control circuit should have a filter in it, much like a Presence circuit does, only with the opposite effect. A Presence circuit filters high frequencies from the nfb so that those frequencies are not cut as much. A Resonance circuit filters the low frequencies out of the nfb so that they are not cut as much, which results in a boost in the lows.

    A variable nfb does not contain filters, it just varies the amount of nfb from the output back to the pi, resulting in an increase or decrease in the amplification of all frequencies contained in the output. I've heard this type of control labeled a Soul control by some. I've called it an Attitude control when I've installed it on amps, because it really does change the attitude of an amp; if the variance is large enough the control can change the attitude from mellow and smooth to mean and aggressive. Plus, I just like saying I'm giving it an attitude adjustment. :D
     
  15. Red Planet

    Red Planet Member

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    Well the majic fairy dust is now in the amp. He hooked up a pot and I picked the magic number out and he installed the Magic Fairy Dust. It works real good. Opted not to do the pot. Gonna give it try out tonight. :RoCkIn
     
  16. ccoker

    ccoker Supporting Member

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    I have a Rivera R30 head, no prescense control, can this be done?
     
  17. KLB

    KLB Member

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    I'll be dang. I used to hang out and jam with Kevin Andrews in the mid 80's. I've recently heard about Jeff, but didn't realize they were brothers.
    Small world...

    Cheers,
    Ken
     

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