My recent build is a little stiff...

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by guiltless, Jan 29, 2012.

  1. guiltless

    guiltless Member

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    So I have an amp build that I just finished. For all intents and purposes it is an Allen modded bassman with 100w Iron (UL OT with the UL taps disconnected) and B+ in the 450v range with 2 5v4's or 470ish with SS rec. 2 6L6's and an 8ohm out. The filtering is stock for an AB763 circuit. It was built from a bassman 135 that I got for dirt cheap. And yes it is heavy as hell.

    I am liking the sound so far. It cuts really well and has ridiculously quick response. The tone is big, raw, and open with not a ton of compression so it is very unforgiving when you play. The problem is that the notes seem to decay pretty quickly. I know this is a function of the huge OT, the filtering and the huge power supply.

    So what can I do to help with this issue? Switch to a 50w OT? Keep a 100w OT but change it out for a non UL type?

    I was thinking about dropping the b+ but where I am at is right in super reverb territory. Same with the filtering. My biggest thought is the OT at the moment.

    Any insight would be much appreciated!
     
  2. LarryN

    LarryN Member

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    50 watt or less OT would be where I would start. You could drop the B+ even more after that.
     
  3. diagrammatiks

    diagrammatiks Member

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    The problem is that the notes seem to decay pretty quickly. I know this is a function of the huge OT, the filtering and the huge power supply.

    this has nothing to do with filtering or beefy power supplies.
     
  4. guiltless

    guiltless Member

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    With what does it have to do?
     
  5. LarryN

    LarryN Member

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    Without being there to play it, I'm guessing you mean the stiffness associated with a big clean transformer, all other things being right. Less compression means quicker notes which I relate to transformers, filtering, voltages, etc.
     
  6. diagrammatiks

    diagrammatiks Member

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    stiffness and a note dying out aren't related.

    An amp can be stiff and still have plenty of sustain.
     
  7. TimmyP

    TimmyP Member

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    Fast decay - at least at loud volume - can be owing to a signal polarity inversion (the sound vibration the instrument causes - feedback instead of +). (I once made a speaker-level polarity-reverse box just to see if it would make any difference - the change in sustain was big.)
     
  8. guiltless

    guiltless Member

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    I guess I am looking in the wrong place then. How would I go about helping with fast note decay?
     
  9. guiltless

    guiltless Member

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    Interesting thought, I'll give that a try. I did find it interesting today that I dimed the amp with a 2 x 12 cabinet and I got very little feedback even though I was standing pretty close to the amp.
     
  10. diagrammatiks

    diagrammatiks Member

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    It's usually a sign that there's some signal blockage somewhere. Signal is getting clamped down before it naturally fades.
     
  11. guiltless

    guiltless Member

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    Hmm... This is when I need a scope
     
  12. LarryN

    LarryN Member

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    Oh, you mean real stiff.
     
  13. guiltless

    guiltless Member

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    I guess I need to research clamping...
     
  14. diagrammatiks

    diagrammatiks Member

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    well you need to determine if the notes are being choked out or not.

    stiffness will relate to the initial attack of the amp. As the filtering gets lower the relationship between pick attack and amp response will start to level off to the point where if you hit the note harder the amp will just sag and compress a little.

    if your notes are choked that means that the amp is no longer amplifying the strings even when they are still ringing. That's a simple of something wrong in the amplification stages.
     
  15. guiltless

    guiltless Member

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    The attack is good. If I dig in the amp responds. If I cascade drive pedals, the amp just gets bigger and louder. There is not a ton of compression given the 100watt iron and the size of the power supply. But when I give it a good strum, the notes just don't hang around very long.
     
  16. guiltless

    guiltless Member

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    I guess now that I have closed it up and played it, it would be good for me to open it up again and re-trace the schematic to make sure everything is as it should be.

    As a side note... should I look into upping the grid resistors on the output tubes from 1.5k to something higher like a 5.6k?
     
  17. guitarcapo

    guitarcapo Senior Member

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    What's the plate voltage at all the tubes? Cathode voltages?
     
  18. TweeDLX

    TweeDLX Member

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    You might want to try for a lower plate voltage in the pre-amp by using a larger value dropping resistor in the power supply at that node. What component values are in your PI? Plate resistors, tail resistor, cathode resistor, etc. I've found I really like the Brown era values of 6K8 (tail), and 680 ohm (cathode) for adding some juice to the tone.
     
  19. phsyconoodler

    phsyconoodler Member

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    Lift the negative feedback wire off the output jack and see what happens.You may have too much negative feedback.
     
  20. LarryN

    LarryN Member

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    If the amp doesn't actually sound like something is wrong with it, I still think it could be the OT. It's huge, if it's the one I'm thinking of. No saturation there. It just sounds to me like it might be a big clean amp, like a Twin, only more so (OT). Did you reverse the speaker yet?
     

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