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Dummy load for amps

Discussion in 'Hobby Electronics' started by gtrdude, Jun 26, 2004.

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  1. gtrdude

    gtrdude Guest

    I'm building a cabinet switcher for my guitar rig. It will be relay based,
    and I need to provide a dummy load for amps that are not in use. The
    biggest amp is 120W, but I figure that a 50W resistance ought to be enough
    just for safety's sake. There are 4 amps to consider, plus the fact that
    all of the circuitry has to fit into a single rack space.

    Any comments?

    Paul
     
  2. Ross Herbert

    Ross Herbert Guest

    On Sat, 26 Jun 2004 13:27:18 +1000, "gtrdude"

    |I'm building a cabinet switcher for my guitar rig. It will be relay based,
    |and I need to provide a dummy load for amps that are not in use. The
    |biggest amp is 120W, but I figure that a 50W resistance ought to be enough
    |just for safety's sake. There are 4 amps to consider, plus the fact that
    |all of the circuitry has to fit into a single rack space.
    |
    |Any comments?
    |
    |Paul
    |

    I would use Arcol 50W or 75W (they also make 100W thru 600W versions)
    resistors available from RS Components for about $8 ea. Depending upon
    how much heat is going to be generated these may need to be mounted on
    a heatsink. I would mount the resistors in a rack case positioned at
    the top of the cabinet (not below other electronic equipment) and fit
    one or two 40mm fans in it as well.
     
  3. Phil Allison

    Phil Allison Guest

    "gtrdude" <

    ** No need for that if the amps use transistor or mosfet output stages.


    ** Relay switching the output of typical valve guitar amps ( eg Marshall,
    Laney, Orange etc ) while running at full power is a dodgy idea - there
    will be a brief moment with no load connected and /or a back emf spike from
    the cabinet as it disconnects.

    As a result, valve bases may arc across between pins 2 and 3 ( anode to
    heater ) or the output tranny can suffer insulation failure.





    ............ Phil
     
  4. **You don't strictly require a dummy load for MOST SS amps. If you're using
    a transformer coupled SS amp, then a dummy load might be a good idea,
    though. You should ALWAYS use dummy loads for valve amps. The cheapest and
    easiest, is to find an old (or new) jug element. Measure it. It should be
    pretty close to 20 Ohms. Take a few turns off the element and dunk it in
    some water. It should cope with lots of power.
     
  5. gtrdude

    gtrdude Guest


    Good point Phil. I already use a commercial cabinet switcher with great
    success. I'm simply building my own to handle more amps and cabs. Maybe a
    make before break scheme is needed? What do you think?

    Paul
     
  6. gtrdude

    gtrdude Guest

    Nice suggestion Trevor, but completely unusable inside a rackmount case!!
    : )

    Paul
     
  7. gtrdude

    gtrdude Guest

    Cool. I'll have a look at those on the website. Thanks for the good
    advice!

    Paul
     
  8. Phil Allison

    Phil Allison Guest

    "gtrdude"


    ** Make before break relays are scarce items.



    .......... Phil
     
  9. Hello Paul,
    think a bit more about about what Trevor has just said.
    Do you remember the famous Heathkit Cantenna?
    A 50 ohm resistor in a one gallon paint tin filled with oil,
    used as a dummy load for radio transmitters.
    Here is a link. Scroll to bottom of the page to see the Cantenna.
    http://www.surplussales.com/RF/RFDummy-3.html

    You could easily adapt Trevor's suggestion to the oil filled
    paint tin and it would be usable in a rack for sure.

    The resistors or resistive element as Trevor suggested
    is mounted on the lid only, the can is filled with oil.

    Solder some metal strips to the can or strap the can to
    a bracket with big utilux hose clips. You can buy empty
    new paint tins for a few dollars or use a fancier container
    and fancier resistors. It is up to you how much you want
    to spend. Trevor's suggestion was to get you thinking. :)

    Regards,
    John Crighton
    Hornsby
     
  10. Somehow the wire jug element in the water trick sounds somewhat safer to me.
    A gallon of hot flammable oil seems a tad risky to me. But hey, maybe I'm
    just being a bit cautious.

    Cheers,
    Alan
     
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