The 'Presence' control?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Cheebatone, Jan 10, 2007.

  1. Cheebatone

    Cheebatone Member

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    Sorry for re-hashing a discussion I've had before, but I can't find the thread where it was held and I need the info...

    I was always under the impression that Presence was just a super-high Treble control, but a forum member (John Phillips, I think) set me straight. Unfortunately, I can't fully remember what he told me! :eek:

    I think he said that Presence was governed by controlling something called Negative Feeback? Is this right? He also said, whatever it is, could be used to vary the other end of the sonic spectrum as well, and that that is precisely what the 'Resonance' control on the Peavey JSX amps is.

    So, anyway, could someone varify or correct me on this, with a brief summary of what I need to know? Please (and this isn't the first time I've asked this) help a dumb-ass...

    Cheers!

    Charly
     
  2. John Phillips

    John Phillips Member

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    Your memory isn't quite as bad as you think it is :).

    The negative feedback loop in an amp is a sort of 'damping' (that isn't quite the right electronics term, but it will do) and makes the power section run more linearly, or 'smoothly' - by taking some of the output signal, running it 'backwards' (that's the negative bit) and re-applying it to the input, as a sort of self-regulation.

    If you put what are essentially tone controls into the negative feedback loop, you can affect the degree of smoothing at those frequencies. Presence affects the top end, resonance the bottom end. Basically turning either of these 'up' actually turns those frequencies down in the loop, and allows those ranges to be less restricted. They don't sound the same as simple bass and treble controls because they affect the dynamics of those frequencies more than the 'amount' of them - which is why you tend not to hear them doing very much at low volume, but once the power section is really cranked they can become more effective than the normal tone controls.
     
  3. Cheebatone

    Cheebatone Member

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    Thanks John! :)

    I seemed to remember you said something about them only really working well when the amp was on full blast, so my thinking is to have Negative Feedback Loop controls, one each for the top and the bottom (and a regular Mid cut/boost tone control in the middle), in a single valve amp, designed to produce power-amp overdrive/saturation etc, for use in recording and pratice situations.

    Will this still work at low volumes, or does it have to be flat-out and very loud?
     
  4. Jon Silberman

    Jon Silberman 10Q Jerry & Dickey Silver Supporting Member

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    Only thing I would add to John's post is that he's discussing true presence controls. In fact, there are amps out there where what's labeled "presence" is actually a trad high-end treble control.
     
  5. waveman

    waveman Member

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    So is the a book with some schematics that discuss how to add a presence control to an amp?
     
  6. brad347

    brad347 Member

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    Not all amps have negative feedback. If your amp doesn't utilize negative feedback in its design, then it's impossible to add a true presence control.
     
  7. axepilot

    axepilot Member

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    Great thread and great responses! :AOK

    I've noticed that the presence control on my Germino C40 doesn't really come alive until the amp gets up to a good stride.
     
  8. marshallnoise

    marshallnoise Member

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    Ok, so when I turn up my presence control above 8 on my Laney, it sounds great and clears up on the distortion channel, but on the clean it introduces a very loud high pitched squeel and so does the resonance switch.

    What is up with my negative feed back loop?
     
  9. brad347

    brad347 Member

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    You are getting a parasitic oscillation.

    See if you can make the wires connecting to the grids of the power tubes just a little bit shorter-- sometimes even an inch shorter is enough to fix it.

    If your amp has PCB mount tube sockets, then it's a flaw in the design and you're stuck with it.
     
  10. 909one

    909one Member

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    I put a switch in the negative feedback loop of my 5E7 Bandmaster Clone. It changes the tone significantly when there is no feedback loop. It defintitely sounds more raw, midrange present and has a little more distortion. It sounds more like an AC-30, which I believe have no feedback loops... But I might be really wrong about that.
     
  11. abro163

    abro163 Member

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    The 5e7 has two feedback loops, one is the 10m resistor the other the more typical 56k at the speaker jack, which one are you talking about? (speaker one I presume)
     
  12. marshallnoise

    marshallnoise Member

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    Those are the ones going from the output transformer, right? Mine are not PCB sockets.
     
  13. brad347

    brad347 Member

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    No. Those are the plate wires. Those can be long.

    The grid wires are the ones connecting to pin 5.

    But speaking of those wires connecting the OT to the plates... you can also try reversing those (they might be brown and blue). This may cure your oscillation or it may make it worse.
     
  14. marshallnoise

    marshallnoise Member

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    Ok, the daisy chaning wires that go from tube to tube in the power section. Those look good, but they could be looked at again. What about the preamp tubes? Or were those what you were referring to in the first place?

    I recently went through the whole amp (Laney VH100R) and replaced all 6 preamp sockets and the power amp sockets too. It does have a new OT and I left those long in length for a "just in case" scenario. Although, that makes very little sense in the long run. I'll shorten those up appropriately.
     
  15. brad347

    brad347 Member

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    If it didn't do the squealing before you put the new sockets and OT in, then I'd be almost certain that you need to reverse the brown/blue (if that is their color) wires on the OT that connect to the power tube sockets.
     
  16. marshallnoise

    marshallnoise Member

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    https://www.thegearpage.net/board/index.php?threads/183750

    In that thread, I did as you described with a different OT. While I am not having the severe issues I had before when I had the wires crossed up, I think you might be on to something. This is a different OT with different specs so perhaps the differences in the actual symptoms would be similar but different.

    I'll give that a try soon and see what happens.
     
  17. marshallnoise

    marshallnoise Member

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    Hey Brad, if you see this, you were right about the grid wires. I shortened them by about 2 inches each side and it worked great. I tried swapping the OT primaries to the tubes...NOT GOOD! Darned near went deaf. But once I got it back, it was awesome. I still think I have some tube issues (quality that is). But other than that, the presence works great now on both channels.
     
  18. brad347

    brad347 Member

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    great! Glad to hear it.

    Sorry, I should've warned you about the potential violent oscillation that can occur if the OT leads are hooked up backwards! :eek:
     
  19. marshallnoise

    marshallnoise Member

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    Scared me poopless! :rotflmao

    But, it does not make those noises anymore and I am happy with that.
     
  20. Jahn

    Jahn Listens to Johnny Marr, plays like John Denver Supporting Member

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    Ancient graveyard bump for this thread because as a new Marshall owner, I found it extremely helpful in two categories- one, understanding this Presence knob and what it does at different volume levels, and two, understanding parasitic oscillations and how to remove them (the trick on mine was to shield the wire going to the first tube grid from the high sensitivity jack).

    Tech stuff all figured out on this amp, time to just rock!
     

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