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Dummy load

Discussion in 'Shock Brother's DIY Amps' started by RollingBender, Jul 28, 2020.

  1. RollingBender

    RollingBender Tele-Afflicted Vendor Member

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    For an 18 watter with an 8 ohm OT, would a 12.8 ohm resistor cause any issues to use as a dummy load? I’m getting ready to feed it a signal and do some scope probing.
     
  2. koolaide

    koolaide Tele-Afflicted

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    I would want an 8ohm resistive load with a wattage rating of minimum 25 watts. But that's just me. Speaker load is frequency dependent it is not a static thing. I will admit there are others more knowledgeable than me. I just would not want to damage the OT.

    Edit-
    I checked a reliable source and an 8ohm speaker is about equal to about a 6 ohm resistive load.
     
    Last edited: Jul 28, 2020
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  3. regularslinky

    regularslinky Tele-Afflicted

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    Not another band name thread. :)
     
  4. corliss1

    corliss1 Friend of Leo's Platinum Supporter

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    Resistors are cheap. Transformers aren't. Get the right match :D
     
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  5. tubegeek

    tubegeek Friend of Leo's

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    What's your goal with the scope? To signal trace and see if everything is working more or less right or to conduct a power test for full output? If it's the latter, you'll want an 8Ω load of somewhat more than 18W. 25 OK, 30 better.

    If you're just doing signal tracing and not planning on driving the power amp section to its limit, then that resistor load would do just fine just to have it working into SOMETHING. Just don't crank it up.

    I think I'm using a 10Ω ceramic wirewound, maybe only 15W, in the Hammond conversion I'm working on (at a snail's pace.) Again, just to make sure it's always working into something.

    You could parallel 3 25Ω/10W and have a dandy load for testing.


    The philosophy "transformers aren't cheap, resistors are," is definitely something to keep in mind as you work though. Although an 8Ω/30W isn't exactly CHEAP.
     
    Last edited: Jul 28, 2020
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  6. Nickfl

    Nickfl Friend of Leo's

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    If you have a scope, it's worth getting a selection of high power resistors in the common speaker values. I got some of these 50W resistors from Weber in 4,8, and 16r, they are cheap and even have spade terminals so you can swap them in easily on amps that use crimp connectors.
     
  7. awasson

    awasson Poster Extraordinaire Silver Supporter

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  8. printer2

    printer2 Poster Extraordinaire

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    It should not be a problem.
     
  9. elpico

    elpico Tele-Afflicted

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    A speaker listed as "8 ohms" is almost anything but.

    What they mean is the speaker has an approximately 8 ohm minimum, but that minimum value only happens when you play certain notes. When you play most notes the speaker impedance rises to as much as 200ohms.

    Easy to see then that 12.8 ohms is a perfectly fine dummy load. If anything it's better than the speaker.

    Here's a chart from eminence that shows how a guitar speaker's real impedance changes with frequency. (It's the lighter line and ohms is on the right)

    [​IMG]
     
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