Mesa 400+ AMP-MOD.???

Discussion in 'Amps and Cabs [BG]' started by anderbass, Feb 27, 2006.

  1. anderbass


    Dec 20, 2005
    Phoenix. Az.
    There's probably a good reason, why this won't work but, could a set of 5 toggle switches be tied into some point of this circuit, to shut down pairs of power tubes? So that with one switch activated, 2 power tubes would be shut down, and total output would be 250w. and with 2 switches=200w./ 3=150w. /4=100w. and with all 5 switches activated we would have a 50w.amp? It sure would be cool to have real power tube saturation, at any volume.
  2. That's actually pretty easily doable. All you need are switches that lift the cathodes of the power tubes from ground, effectively turning the tube "off" without screwing up the primary impedance of the output transformer, which is what would happen if you just pulled output tubes.
  3. Plain Old Me

    Plain Old Me

    Dec 14, 2004
    Hmm, I am intrigued by this idea. It would be nice to get some of the great tube OD when just practicing.
  4. Eric Cioe

    Eric Cioe

    Jun 4, 2001
    Missoula, MT
    I think they made a Cadillac like this in the 20s-30s. I think it had 16 cylinders, and you could cut it in half with a switch.
  5. Plain Old Me

    Plain Old Me

    Dec 14, 2004
  6. 5 Switches > Attenuater. Period.

  7. BurningSkies

    BurningSkies CRAZY BALDHEAD

    Feb 20, 2005
    Seweracuse, NY
  8. Plus

  9. It's mostly the second one though. ;)
  10. anderbass


    Dec 20, 2005
    Phoenix. Az.

    I've been standing by, waiting to hear from somone else, that has, or wants to try this mod. How many pairs of power tubes could I pull, for a short term test, (without hurting anything) To hear what this mod might sound like, before taking the plunge? And would these ground lift switch leeds, support a illuminated toggle or rocker switch? Im pitcturing a blank, (1) rack space filler, with a row of 5 Iluminated switches hovering over my 400+ " mmmmmmmm blue lights " (Homer Simpson)
  11. Plain Old Me

    Plain Old Me

    Dec 14, 2004
    I think I need to take my 400+ into a tech sometime to get the bias adjustable and now to get this done too. Not that I don't like screwing with electronics, but something about voltages that can kill and not really knowing what I am doing dictate that I take it to a tech. I think the switches could go on the front. And with purple lights, to match the power light. :)
  12. anderbass


    Dec 20, 2005
    Phoenix. Az.
    mmmmmmmmm purple lights.
  13. I am on a thread resurrection jag. Besides it's always better to revive an old thread than start a new one right?
    Anyway, on a realted topic, a tech told me once you can just yank pairs of tubes by hand to get lower output. Not sure how this affects the impedance the OPT sees and whether it would be potentially dangerous (punintentionally) to drive the amp hard in this case.
  14. Tim1


    Sep 9, 2005
    New Zealand
    Regrettably PsychoBassGuy no longer is allowed to appear on this forum - although I have at times suspected that he might be here under a new name.......
    I miss him, provided heaps of spark and considerable knowledge, and the name seemed to suit.
  15. lorenk


    Apr 8, 2008
    Bend, Oregon
    I was under the impression that you can remove sets of power tubes from the 400/400+ amps and not have a problem with impedance. Am I wrong?
  16. it changes the impedence the primary sees and thus the secondary it's not a wonderful way of doing it. Get a smaller amp if you want power amp distortion
  17. BassmanPaul

    BassmanPaul Gold Supporting Member

    Aug 25, 2007
    Toronto Ontario Canada

    If you disable one or more pairs of output tubes either by pulling them or switching out their cathodes you change the impedance seen by the primary of the OPT. That means that the impedance at the secondary also changes. you can gat away with this in a guitar amp because of the frequency range in use. In a bass amp it's not such a good idea.

  18. KramerBassFan


    Jan 3, 2009
    I'm curious, if people want "power tube distortion" then why do they buy these big wattage bass amps???

    You REALLY have to crank a 300w amp to get it to distort, and why not just get a 50 or 100w amp FROM THE BEGINNING instead of trying to find ways to hack up old and no longer made amps such as Mesa Bass 400's and 400+'s?