Dummy load.

Discussion in 'Amps and Cabs' started by modestcargo, Jan 31, 2012.

  1. modestcargo

    modestcargo Member

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    Alright. I want to run my 50w traynor bass master at a level that approaches full output. I would like to do this by running the speaker out into a 100w dummy load - perhaps four simple 25w resistors and a crude heat sink - and by hooking a speaker level di box into the parallel speaker out.

    I then would like to run this into my delay pedal, then into the left channel of a good solid state pa amp and then in to one driver in a 4x12 cab for a band practice to small gig type of situation.

    I plan to have my guitar see an a/b box, with the b side running into a roland jazz chorus sixty (sans speaker for easy load out, maybe even build a head cab for it if this sticks) then from its line out in to the right side of the power amp and to the second driver in the 4x12 - the transplanted Roland or possibly a weber 12f150. The bottom two slots in the 4x12 would be left empty.

    Upon doing some research I have found that I am not the first one to have this idea.. :D I guess he used a variac to keep his amps from blowing up.

    My question is.. Were the amps blowing because of being dimed for hours on end, or as a direct result of the dummy load not being a "proper" thing? Or hm.. Was the variac just to extend tube life? Was he blowing too many tubes mid set?

    Thanks for any help. Please don't tell me to "just buy a hot plate man".
     
  2. modestcargo

    modestcargo Member

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  3. jwnc

    jwnc Member

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    Man.. you are much braver then I am. Honestly if you want to do this you need to buy a proper dummy load/attenuator. I have the Rivera Rock crusher and its the best one I have ever used.

    Hope this helps.
     
  4. Frank Speak

    Frank Speak Member

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    I can't help you with what you propose, but I have a Rivera RockCrusher that I can use as a load box for DI purposes if I need/want to do it. I *think* some of the other attenuators can be used in that capacity as well. If you went that route, you would have a box that's good for more than just a dummy load.

    Edit: The last feller beat me to the punch. Lol
     
  5. Bruce Clement

    Bruce Clement Gold Supporting Member

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    You can safely do a resistor-only dummy load, as long as you understand impedance. Use good quality aluminum-housed power resistors. IMHO, the reputation of amp failures w/ attenuators is a result of 1) amps run full-out will burn tubes sooner, and 2) the cheap sand resistors used in early attenuators are prone to breakage due to physical shock (load in/load out, being thrown in the back of the band van, etc.).

    Good:


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    Bad:
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  6. modestcargo

    modestcargo Member

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    Perfect. Thanks man I just needed confirmation.

    I have been told that I need to double the impedance of the transformers output to make it easier on the amp. However would that not be harder on the transformer? I was originally just going to match it to 8 ohm.
     
  7. aflynt

    aflynt Member

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    I tried something similar with my Pro Jr. a few months back, and wound up frying the power tubes after about 2 sets. I'm not quite sure why, honestly as I've run the amp at the same volume many other times and always got several months to a year out of new JJ EL84s. I used a 100w power resistor from Parts Express (this one) and a home made line out box. It didn't really sound as great as I thought it would and was kind of a PITA to set up so I just went back to using pedals for overdrive.

    -Aaron
     
  8. Bruce Clement

    Bruce Clement Gold Supporting Member

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    The failure could be coincedence. I have used resistive dummy loads for years worth of gigs, never had a failure.

    A speaker's impedance does go up w/ frequency and power, so you can increase your load impedance a bit. I personally wouldn't double it, maybe +50%. It's not rocket surgery, so just experiment and listen... Having said that, the THD HotPlate gives the exact rated ohms at the "load" setting. And I trust Andy Marshall's judgement on these things.
     

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