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Brainstorm a multi featured dummy load for testing.

Discussion in 'Shock Brother's DIY Amps' started by Outlaws, Jan 21, 2018.

  1. Outlaws

    Outlaws Tele-Meister

    242
    Jan 16, 2007
    None
    I want to build into a rack chassis (unless a reason to comes up or I see a neat enclosure) a multi featured dummy load for testing guitar amps.

    I am looking for spitballing ideas for things I should include.

    Things I know I want to ideally include:

    1) single input from amp with a switch to loads

    2) 200 watts of power handling for 2/4/8/16 ohms (maybe more for 2/4 but I don’t see messing with bass amps ever)

    3) test points DMM leads

    4) test point for scope probe

    5) possibly a watt meter

    6) speaker out bypass (hard toggle not as part of rotary)


    So I guess I want to hear what people would change or add to make a neat test device for a bench.
     
    Last edited: Jan 21, 2018

  2. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    Install a BNC female connector for connecting to a scope so you can use a standard BNC male-to-male cable to connect the scope.

    Use the rack chassis as part of the heat sink.
     
    Outlaws likes this.

  3. Outlaws

    Outlaws Tele-Meister

    242
    Jan 16, 2007
    None
    I am thinking instead of a watt meter I might just add an switch with a series break to bananas for inserting an external meter inline. More flexible.
     

  4. Outlaws

    Outlaws Tele-Meister

    242
    Jan 16, 2007
    None
    What Impedance BNC connector should I get for this? 50? 75? 100? Something else?
     

  5. Old Tele man

    Old Tele man Friend of Leo's

    May 10, 2017
    Tucson, AZ
    How LONG time-wise do you anticipate full-wattage being used? If lengthy, consider an oil-bath cooling container for those power resisters, because even the heaviest-duty resistors will change resistance somewhat when HOT...cooling mitigates the problem.
     
    Last edited: Jan 22, 2018

  6. Wyatt

    Wyatt Tele-Holic

    934
    Nov 3, 2004
    I have one of these around...have NEVER used it for anything but a dummy load.

    I bought it to diagnose a particularly difficult amp, but fixed the amp while the Weber was in transit. I though, "while I'll use it for sure"...nope.
     

  7. Outlaws

    Outlaws Tele-Meister

    242
    Jan 16, 2007
    None
    Not long enough to worry. I am only doing 200/250 watts so I have room to spare. I just want to be able have a 100-150 watt tube head wide open for troubleshooting or measuring output wattage clean/clipping.
    It’s fun and function but nothing for long duration use.
     

  8. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    How about a couple of PC type fans for some added airflow?

    Oh, and don't forget the thermal goop when you mount the power resistors. It serves two useful purposes IME. Better heat transfer is one, and the other is finding out how many other things it can end up on. :)
     
    Outlaws and robrob like this.

  9. sds1

    sds1 Tele-Holic

    Age:
    41
    589
    May 4, 2017
    Orlando, FL, USA
    Oh I enjoyed this comment very much.
     
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  10. Outlaws

    Outlaws Tele-Meister

    242
    Jan 16, 2007
    None
    So all the info I can find is I use a 50ohm BNC chassis jack?

    These resistors are not cheap lol. I might skip the 2 ohm.
     

  11. sds1

    sds1 Tele-Holic

    Age:
    41
    589
    May 4, 2017
    Orlando, FL, USA
    Not sure exactly what you're looking for, but those metal wound 100w monsters are $6 for a 2-pack on Amazon.
     

  12. robrob

    robrob Poster Extraordinaire Ad Free Member

    Dec 29, 2012
    United States
    Yes, I've always used standard 50 ohm BNCs for oscilloscope jacks.
     
    Outlaws likes this.

  13. Outlaws

    Outlaws Tele-Meister

    242
    Jan 16, 2007
    None
    I am building it for 200 or 250 watts. And if I did a 2 ohm I would try to make it bass friendly at 400-500 watts. These 250 watt resistors $35/ea. The 200 watt was more expensive at Mouser for some reason.
    I will look up the 100 watt ones and maybe just run two in series for each to reach 4/8/16 if it cuts my cost in half.
     
    Last edited: Jan 23, 2018
    sds1 likes this.

  14. Outlaws

    Outlaws Tele-Meister

    242
    Jan 16, 2007
    None
    thsnks. So are BNC 1x probes normally 50ohm and this is why running a straight cable requires this load on the test side? Would I need to account for the added 50 ohms on my readout on the scope?
     

  15. sds1

    sds1 Tele-Holic

    Age:
    41
    589
    May 4, 2017
    Orlando, FL, USA
    Well, correct me if I'm wrong but 4 x 8Ohm 100W in parallel would yield 400W at 2 Ohms for $12. Plus you could tap your 4 Ohm at 200W and even 8 Ohm at 100W with no additional parts/cost.
     
    Outlaws likes this.

  16. Outlaws

    Outlaws Tele-Meister

    242
    Jan 16, 2007
    None
    Ya, when I take a look at them I might do with 3 position rotary a 2/4 with a toggle, a 8/16 with a toggle, and use the third spot as the bypass out.
    Before I was thinking a bypass toggle, then 4/8/16 on the rotary with the 4 having the toggle for 2.
     

  17. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    As far as overall circuit, have you seen the schematics for these available from Aiken Amps? Mine's based on the 2/4/8 version.
     
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  18. Outlaws

    Outlaws Tele-Meister

    242
    Jan 16, 2007
    None
    I just looked at that with your suggestion, I hadn't, thanks for posting. It is basically what I am doing, except I am adding test points for a BNC, bananas for DMM probes, and series patch jack for amperage measurement.
    I have a heavy duty 3 position rotary already so I am planning on using that rather than sourcing a 4 pole.
     

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