Dummy Load

Discussion in 'Amps and Cabs [BG]' started by i_got_a_mohawk, May 3, 2005.

  1. Is there such a thing you can get that will act like a dummy load? possibly have a tap to select what ohmage it is ?

    Just curious really, i saw a marshall powerbreak, is this the same kinda idea?
  2. Plain Old Me

    Plain Old Me

    Dec 14, 2004
    You can use a resistor as a dummy load, but I don't see why you would... Remember that resistors suck up as much power as a speaker cab rated at the same Ohms.
  3. At the place i practice there is an 8 Ohm cab, i wanna take my SVT II along to have a chance to turn it up full bore, but its only 2 or 4 ohms, and im not too sure about these peope who say its ok to run above the impedance
  4. The Marshall is for a totally different purpose. Getting the amp to overdrive to get tone without being so loud your ears bleed. So you turn the amp up to get the tone, and siphon off a lot of power so that what goes to the speaker is a reasonable volume.

    Solid state amps can easily run an 8 ohm cab if they're rated at 2 or 4 ohms min load. I think tubes can also, but not sure.

    The main goal is to match the impedence so the amp can put out max power safely. Usually the danger is from too much current from too low ohm load. If there's a transformer output on the amp it should have taps for the common "ohm"ages, but I'd guess its more likely to have 4 and 8 than 2 and 4.

  5. Joe P

    Joe P

    Jul 15, 2004
    Milwaukee, WI
    Sure. Techs use'em to test amps.

    You can get special non-inductive ones, but the regular veriety aren't a problem.

    Here at work I have a whole box of 300W, 8-Ohm rib-wound power resistors... Hm - I s'pose I could take one over to the inductance analyser here; just a sec... 19uH with the steel mounting bolt going through it: That's neglegeable - at 20KHz the impedance is only up by 0.9%, with a 4.3° phase shift. Yeah, that'll do. Yikes - these are 5% tolerance, but I found one that's at +0.03%; I think I'll keep this one for myself.

    Is it expensive sending these all the way over there? I suppose. If anyone needs a load, I have four or five extra racks of three each with mounting feet laying around. Three of these eight-ohmers would let you do several impedance combinations.

  6. Unlike solid state amps ( ive seen, where different impedance = different wattage) it is 300watts on either load

    And i dont know what to do with a resister, would i just connect it to the 2 ends of a speaker cable ? (essentially)
  7. Passinwind

    Passinwind I Know Nothing Supporting Member

    You'd parallel it across the leads, and half the amp's rated power would make heat through the 8 ohm resistor, so the speaker would see 150 watts. No free lunch available, as usual. :cool:

    BTW, don't run the amp into an 8 ohm load using the 4 ohm tap. Some tube amps will tolerate this, but it's never a great idea, especially with a high powered amp, in my experience.
  8. Cheers, i didnt like the idea of running it at 8 when the max is 4 on the switch :S
  9. Quality


    May 7, 2003
    Long Beach, CA
    I could be wrong here but...
    Shouldn't it be 2 or 4 ohms Min on the amp.
    An 8 ohm cab will just decrease the overall output wattage.
    For example a amp that is rated at 500 watts at 4 ohms will only put out say 225 or so into an 8 ohm cabinet. There's no risk to hurting anything as far as I can tell.
    2, 8 ohm cabs run in parallel will drop the load down to 4 ohms.
    *EDIT* - This pertains mainly to solid state amplifiers. Running speaker combinations with a higher impedance with a Tube Amp Is EXTREMELY DANGEROUS.
  10. Transverz

    Transverz believer of the Low End Theory

    May 3, 2004
    Los Angeles, CA
    Yup, I and a zillion others have contemplated this whole change the impedance things also, but the realization I eventually came to is that yes, anyone could include an extra dummy-type load to switch impedances at will, but the trade off of lower power was the only real barrier.

    Actually, going UP in impedance (more ohms) would suck way too much power (for example half or more) to be desirable, but wouldn't a load that reduced the impedance be good for when you want to run a single cab instead of two AND have the same wattage being put into it?

    So like let's say you have an 8 ohm cab with a swtich to go to 16 ohms. The power cut would either be unusable or just plain not cool. But if you had a pair of 8 ohm cabs and you only wanted to use one but retain the same wattage it saw in your two cab setup, wouldn't it be an advantage to be able to switch it to 4 ohms?

    I even see some use in that 4 / 8 ohm switch for when you have that mono amp usually only want to see two 8 ohm cabs, and you also have a stereo rig you want to run those same cabs but now in 4 ohms each. Eh?

    I probably make no sense. In that case, nothing to see here, move along... :bag:

  11. Joe P

    Joe P

    Jul 15, 2004
    Milwaukee, WI
    I think a series resistor would mess with the damping factor too (lower it).

  12. johnvice


    Sep 7, 2004
    Palmer speaker simulators make a speaker simulator box that you plug the output of your amp into as the device's input. The speaker simulator's output is a line level output that adds a colouring to sound like a speaker.

    Palmer has a load and no load model.

    Geddy Lee of Rush uses such a devices as he relies on an in-ear monitor. He said in a recent interview for BassGuitar magazine that he has a speaker cabinet under the stage as a load for the amp "to keep my amp from blowing up". This implies he uses a no-load vesion of the Palmer. It would make more sense to uses load version for impediance so he (er his roadie) wouldn't have to lug a speaker cab around.
  13. Transverz

    Transverz believer of the Low End Theory

    May 3, 2004
    Los Angeles, CA
    Yeah, I saw these a while back and thought they were really interesting and had a lot of uses. But nobody really gave me a real world example or actual use of it so I totally forgot about it. Hmm...seems really useful for the tube amp owners AND want to record quietly or play a show direct-in wihtout cab with only in-ear / stage monitoring.
  14. Transverz

    Transverz believer of the Low End Theory

    May 3, 2004
    Los Angeles, CA
    Darn, I think the Palmer site is "under construction". Anyone got a working link to check out their products?

    *edit - Now I kind of remember some of the thoughts I had a long time ago when looking at the Palmer site, now that I see some other sites talking about certain product models. I am seeing that they are really made for handling wattage in the guitar range (like 50-200w depending on model, I guess). But then again, I'd think Geddy Lee would play at least 200 or more watts? I dunno.*
  15. its got the switches for 2 n 4, so im going by 2 being min and 4 being max :p

    I dont like the idea of doing it, these valve/tube amps are full of way too many suprises for my liking :p

    And i know 2 8 ohm cabs would be 4 ohms, if there were 2 8 ohm cabs there this thread wouldnt be :p

    Im afraid most of the other posts have lost me :crying:
  16. jonathanl


    Jan 12, 2003
    Orinda, CA
    Just for your information, Palmer products aren't currently available in the US.
  17. Passinwind

    Passinwind I Know Nothing Supporting Member

    That's a very dangerous statement when related to tube amps, there is a significant risk in trying this (mismatching the speaker load upwards of the tap rating). Arc-over of the output transformer, tube sockets, or tubes is a very real possibility. In the case of an Ampeg output transformer cooking, you could be looking at a few hundred dollars repair cost.

    There's a nice detailed explanation of this here, about halfway down the page: http://www.geofex.com/tubeampfaq/TUBEFAQ.htm#transformers
  18. Im not in the US :p

    and thanks Passinwind, i thought it was a bit suspect!
  19. Passinwind

    Passinwind I Know Nothing Supporting Member

    Lots of people do it, and get away with it, especially with old Fenders. You stated that you want to crank the amp though, which makes it especially sketchy. :cool:
  20. Quality


    May 7, 2003
    Long Beach, CA
    That's why I said I might be wrong. I will edit the above post so no one gets incorrect info.
    I did check with Ampeg's and some othe websites and Passinwind is 100% correct.
    Do not listen to me, I'm more of a solid state guy.