1. Please take 30 seconds to register your free account to remove most ads, post topics, make friends, earn reward points at our store, and more!  

Please Identify This Amp Part...

Discussion in 'Amps and Cabs [BG]' started by Tweaker27, Apr 8, 2015.


  1. Tweaker27

    Tweaker27

    Aug 28, 2014
    New Haven, CT
    Hello,
    This little piece has vibrated loose many times over the years and I have-resoldered it.
    However, this time it snapped in a place that I can't solder and it needs to be replaced.
    It is part of the fan speed control on my Mesa 400+
    I'd just like to know the name of this part so I can get a new one.
    Thanks!

    2015-04-08 16.26.07.
     
  2. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    It is a flameproof resistor, 270 ohm, 10W, +/- 5% tolerance. It is a wire wound power resistor. Often called cement or ceramic block.

    How do you know that you need a new one?
     
    Last edited: Apr 9, 2015
  3. Jim Carr

    Jim Carr Dr. Jim Gold Supporting Member

    Jan 21, 2006
    Denton, TX or Kailua, HI
    fEARful Kool-Aid dispensing liberal academic card-carrying union member Musicians Local 72-147
  4. Tweaker27

    Tweaker27

    Aug 28, 2014
    New Haven, CT
    Thanks...
    It's not that I need a new one.
    It's that the lead broke off so close that I can't solder it.
     
  5. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    That can happen.

    10W127 270 Ohms Cermet Power Wirewound Resistor : Resistors | RadioShack.com

    If the resistor is mounted on a circuit board it is good practice to point the bottom, where they seal it, away from the board. Sometimes flames can erupt there when the resistor blows. Also elevate the component a millimeter off the board. Don't lie it flat against the board. This can prevent charing if it get too hot. It gives it a bit of space. It depends on the amp, some need these precautions, some don't.
     
    Last edited: Apr 8, 2015
    PortlandBass77 likes this.
  6. Raf Seibert

    Raf Seibert

    Dec 16, 2013
    Texas
    You might want to consider applying some RTV or other adhesive to hold that rather heavy resistor in place on the board. That would help keep it from vibrating loose. If you do, be sure to use an RTV that is electronics comparable.

    Raf
     
  7. Mr. Foxen

    Mr. Foxen Commercial User

    Jul 24, 2009
    Bristol, UK
    Amp tinkerer at Ampstack
    What cab are you using? If its vibrating your head enough that bits fall off, might want to brace it better.
     
    seamonkey likes this.
  8. Tweaker27

    Tweaker27

    Aug 28, 2014
    New Haven, CT
    Great tips! Thanks so much!
     
  9. agedhorse

    agedhorse SUSPENDED Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    Whoa, most (if not all hardware type) RTV silicone products are NOT thermally conductive, they are thermally insulating. In order to be thermally conductive, special fillers are used, which alter other properties such as electrical conductivity.

    Also, it's a poor idea to use any adhesive to attach an aluminum part to a resistor. The bonds between a cantilevered aluminum piece and a hot resistor is likely to be weak, resulting in bond failure and a piece of (conductive) aluminum now loose inside the amp. This would void any warranty, and also the safety agency approval on the amp as it's not an approved, safe assembly method that has been tested for suitability.

    If additional dissipation capacity is needed, the proper aluminum body resistor that is designed to be mounted to the chassis is the safer and proper solution.
     

  10. It sounded like they were talking about RTV'ing between the board and the elevated resistor to prevent vibration failure, not RTV'ing a heatsink to the resistor.
     
  11. Mr. Foxen

    Mr. Foxen Commercial User

    Jul 24, 2009
    Bristol, UK
    Amp tinkerer at Ampstack
    "use a bead of silcon the stick the aluminum there for a heat sink." part kinda gave the game away.
     
    B-string and agedhorse like this.
  12. Growlmonkee

    Growlmonkee

    Jan 30, 2013
    Florida, U.S.
    Clear GE silicone, without mold/mildew inhibitor, is a good electrical insulator, (after it cures), it's also a good thermal insulator, so do not use it to attach a heat sink to anything, the heat sink would be isolated from the heat source, and putting any on that resistor would keep it from cooling properly. It also has one property that makes it a poor choice to "glue" anything in place, it sticks to itself best, but does not bond well at all with surrounding materials. Putting a dab between 2 large caps, that see high voltage, on a circuit board is a great way to keep them from touching, and shorting, but using it to glue anything in place is a bad idea. I would steer clear of it in this application. Try to think of a mechanical way to hold it in place, (like a clip)
     
    B-string and agedhorse like this.
  13. SirMjac28

    SirMjac28 Patiently Waiting For The Next British Invasion Supporting Member

    Aug 25, 2010
    The Great Midwest
    You beat me to it I was just about to say the exact same thing. :)
     
  14. You do realize agedhorse actually designs bass amps for his living, for a large well known company? Furthermore,if it weren't for us college educated idiots, you wouldn't be posting on an Internet using a computer.

    We actually study and measure thermal conductivity using real measurement devices, not toy cars, and we can say with absolute certainty that rtv silicone is a very poor conductor of heat.
     
    B-string likes this.
  15. Raf Seibert

    Raf Seibert

    Dec 16, 2013
    Texas
    My recommendation for RTV (silicon) was addressing the problem mentioned in the fist post, that this resistor had broken from the board numerous times, and now had leads too short to reattach. That lead me to the thought that once replaced, the resistor would benefit from being restrained from moving so freely as to snap the leads. Hence, RTV. In the original post I see no need mentioned for additional heat sinks.

    Raf
     
  16. Rtv is a fair to poor glue, but terrible at thermal conductivity
     
    B-string likes this.
  17. sure buddy. You're an expert because you stuck rtv in a toy car. I now realize you know more than professional engineers and amp designers.
     
    B-string likes this.
  18. Do you even know what thermal conductivity is? It's not the same as temperature tolerance. A gasket material may withstand hundreds or thousands of degrees, but that does not mean it transfers heat.
     
    B-string likes this.
  19. The tiles on the space shuttle can withstand thousands of degrees--they are a special ceramic--but they are engineered to have extremely low thermal conductivity. Rtv, glues, and caulks typically have very low thermal conductivity. When we install transistors on heat sinks, we don't use glue or rtv silicon. We use a special thermal paste--the thinner the better--which eliminates the air voids and helps transfer heat from the hot transistor to the heat sink.
     
    wcriley likes this.
  20. Mr. Foxen

    Mr. Foxen Commercial User

    Jul 24, 2009
    Bristol, UK
    Amp tinkerer at Ampstack
    "You got an education so your actually knowing what words mean is useless against my ignorance."
     
    wcriley likes this.

Share This Page

  1. This site uses cookies to help personalise content, tailor your experience and to keep you logged in if you register.
    By continuing to use this site, you are consenting to our use of cookies.