How can I get power sag without using a tube rectifier?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by somedude, Jun 21, 2004.

  1. somedude

    somedude Member

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  2. saros141

    saros141 Guest

    Series resistor.

    Hey - is Canada Heaven, or is Canada South of Heaven?
     
  3. somedude

    somedude Member

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    South of heaven.

    So, something like an attenuator, but meant for household current?

    Is that what a variac is?
     
  4. saros141

    saros141 Guest

    Not really... I'm talking about inside the amp, after the wall voltage has been stepped up by the transformer and turned into direct current by the rectifier. A mod that really should be done by a tech, and would of course affect the originality of the amp.

    Heh heh, I get it, in that case this whole world is South of Heaven. I guess you got dibs on that as a band/album/song name.

    :D
     
  5. somedude

    somedude Member

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    If I get it done, I'll get a tech to do it. I'm just getting my knowledge up so I can communicate my idea better.

    So, after the tranny you'd install a series resistor?

    If that's true, would it be possible to some kind of variable rate resistor in there? And, if I can do that, would it continue to effect the sound of the amp even when bypassed (by how much if any)?


    The amp I'm looking to do this to is a Mesa 2:100 power amp. It's supposed to have the same circuit as the Dual Rectifier, except that it lacks the tube rectifier, so I can't get the sag sound I could get out of a Recto.

    Here's a pic if it helps any
     
  6. somedude

    somedude Member

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    Oh, and the band Slayer made an album called South of Heaven back in '88 or something. It's where I got that from.
     
  7. rjmmusic

    rjmmusic Member

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    The mod I've seen puts a 100 ohm, high wattage resistor in between the rectifier output and the first filter cap. To bypass it, install a switch so that when the switch is closed it shorts out the resistor. It wouldn't affect the sound of the amp at all when bypassed.
     
  8. Jeff Flowerday

    Jeff Flowerday Member

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    WeberVST also has a copper cap module rectifier that needs to be installed inside your amp. You can purchase them to emulate a multitude of rectifiers.

    http://www.webervst.com/ccap.html
     
  9. somedude

    somedude Member

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    I thought those were only for amps that already had a tube rectifier that you wanted to convert to solid state?

    I already have a solid state rectifier, and there's no tube socket for me to plug one of those in.
     
  10. Jeff Flowerday

    Jeff Flowerday Member

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    I'll repeat myself, there are module versions of his rectifier that need to be installed inside the amp. They replace the solid state internal rectifier, no tube socket required.
     
  11. somedude

    somedude Member

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    Thanks for the clarification, I'll send them an email and see what info they can give me on it.
     
  12. vladorg

    vladorg Supporting Member

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    Hmm,
    that's quite interesting. Pretty much I can swap my Mesa's soid state rectifier with one of these bad boys and have it sag like a tube rectifier. Is this mod possible to be done by everyone or just a tech? Thanks for the tip Jeff,

    Vlad
     
  13. somedude

    somedude Member

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    FYI

    I sent another email asking if it was possible to wire both rectifiers in the amp, with a swich to choose between the two. Just waiting on an answer.
     
  14. Jeff Flowerday

    Jeff Flowerday Member

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    The mod should be done by a tech. If you made a mistake you could easily take out your power transformer.


    Jeff
     
  15. Jeff Flowerday

    Jeff Flowerday Member

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    Yes it could be done. How easily depends on how the inside of your amp is layed out.
     
  16. Swarty

    Swarty Member

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    Switching rectifier schemes will impact the bias. The original bias circuit will have higher B+, and thus draw more plate current than the sag inducing rectifier thingy.
     
  17. somedude

    somedude Member

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    I'd get a tech to do it. I like to do my own work, but it's not something I'm interested in doing myself as I'm an amature when it comes to electronics.


    [​IMG]
     
  18. Jeff Flowerday

    Jeff Flowerday Member

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    Swarty is right. If you wanted to switch and use the 5U4 version of copper cap the B+ change between the two would probably be a little too large to have the tubes biased up properly in both settings. A 5AR4 version of the copper cap would work better. You could find a bias setting that would work with both B+.

    The thing you have to think about are you looking for the SAG effect of a tube or the voltage drop of a 5U4. The voltage drop from the 5U4 copper cap will definately change the feel/tone of your amp along with the simulated SAG you are looking for. Lower voltage usually means slightly warmer/darker and less head room.
     
  19. somedude

    somedude Member

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    Just to clarify, does the sag effect that I'd normally associate with a tube rectifier come from a reduction in voltage, current, or both? Would it be possible to get a description, or a link to a description, of the basic physics at work in tube sag and how it relates to the rest of the amp?

    If I read you're last post correctly, the 5U4 version of the CCap will both reduce the voltage, and produce the simulated sag?

    I'm basing wanting the 5U4 CCap based on the fact that a Dual Recto uses a 5U4 in it's rectifier, that's my only reason for looking in that direction. I am open to other options however.

    Do you suppose the Dual Recto has some circuit to compensate for the change in B+ and keep the tubes in correct bias, or do you suppose the change in B+ is part of the sound?
     
  20. Jeff Flowerday

    Jeff Flowerday Member

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    SAG is what happens when the rectifier isn't able to keep up with demand.

    So what you are looking for might be either, the amp running at a lower B+ or the effect of SAG. You have to push the amp a little to get the rectifier to SAG.
     

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