Resistor as dummy load

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by CubanB, Feb 15, 2015.

  1. CubanB

    CubanB Member

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    If you want to simulate an 8 or 16 ohm dummy load with one large power resistor, what should it's ohm rating be?

    Thanks
     
  2. xtian

    xtian Member

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    It must be able to dissipate whatever the amp is dishing out. Double the rating to be safe, especially if you're going to run it long and hard. I use a 200 watt dummy load when testing amps with a sine wave. The resistors get hot pretty quickly at full output of a 100 watt amp.

    What's your intention for the dummy load?
     
  3. RussB

    RussB low rent hobbyist Silver Supporting Member

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    Is that a trick question?


    8ohm load = 8 ohm power resistor
     
  4. xtian

    xtian Member

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    Thanks, Russ! Obviously, I read the question as I thought it should have been written!
     
  5. Jeff Gehring

    Jeff Gehring Silver Supporting Member

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    My thought exactly, tricky in at least 3 ways:

    (1) If you want to simulate an 8 or 16 ohm dummy load... If I wanted to simulate a dummy load, I could use toilet paper tubes and string.

    (2) an 8 or 16 ohm dummy load with one large power resistor... I get one resistor to provide two different resistances? Tricky...

    (3) If you want to simulate an 8 or 16 ohm... what should it's ohm rating be? Wait, what?

    All good, though, I think xtian got him fixed up with the first response!
     
  6. drbob1

    drbob1 Silver Supporting Member

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    The problem depends on how long you want to wait for parts and how much you want to spend. A 200w Wirewound resistor at 8 ohms is available... done! Or you could build a network of 20w Radio Shack resistors (building them in a 4 resistor series/parallel network gives you the same rating, say 10 ohms, as each resistor. Two 20 ohm, 4 resistor networks in parallel would give you 8 ohms, at 160w. Two 10 ohm networks in series would give you 20 ohms at 160w.
     
  7. CubanB

    CubanB Member

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    No trick question intended, but I did read something a while back about using higher loads (like 15-20 ohms for 8ohms) because it would a mimick a speaker more accurately across it's audio range because with a speaker the impedance varies at different frequencies, unlike a resistor that would be more constant. Having gone back and looking at it, I think this was more for tone purposes, like when running the attenuator in parallel with a speaker. Apparently Weber Airbrake does something like this, but I'm not exactly sure.

    There seems to be a complex relationship between all of the factors, that is way beyond my understanding.

    Yeah, I'll look for an 8 or 16 ohm 200W resistor. I guess there's a lot of options in terms of using smaller ones, having them switchable or reactive loads but for now simplest option would be best I think. I just want to be able to do some tests without having to worry about the volume a speaker gives.
     
  8. Tone_Terrific

    Tone_Terrific Member

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    For bench work a passive load, as required, is fine.
    However, if you are analyzing complex wave forms the reactance of the load becomes a factor. Then it all gets extremely messy.
     
  9. Trout

    Trout Member

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    You might want to try this?
    Oil Cooled DIY dummy load.



     
  10. CubanB

    CubanB Member

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  11. Shiny_Beast

    Shiny_Beast Member

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    Most of what I've read mirrors your statements, ideally a higher load seems to be better. A quick look at the impedance curve of a typical speaker sort of backs that up. Tube amps can usually manage a bit of a low mismatch, if you just want to run some quick tests you'd have to think you'd be fine with an 8ohm resistor on the 8 ohm tap.
     
  12. Blix

    Blix Supporting Member

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    Look at my resistor, called the Tone Shaft™ :D

    [​IMG]
     
  13. OilsFan

    OilsFan Supporting Member

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    ^^LOOK^^AT^^HIS^^RESISTOR!^^
     
  14. Blix

    Blix Supporting Member

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  15. J M Fahey

    J M Fahey Member

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    So it's a resistor envy situation?

    I thought only girls had that problem.
     
  16. Crash-VR

    Crash-VR Supporting Member

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    I was reading a thread earlier about re-amping a dummy load for playing at home. Hogy from Komet amps, said to use a 25ohm resister for a 16ohm output.
     
  17. matt1969

    matt1969 Member

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    I have a 100 watt marshall superlead. I have one 16 ohm 4X12 & a 16 ohm / 100 watt power resistor in parallel as a dummy load. This knocks the volume down quite a bit. I set the amp to 8 ohms. two 16 ohm loads in parallel = 8 ohm total. 100 watt power rating works for this because it is seeing 1/2 of the total power.
     
  18. TimmyP

    TimmyP Member

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