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Dummy Load Danger?

Discussion in 'Amps and Cabs [BG]' started by SteveD, Jan 26, 2012.


  1. SteveD

    SteveD

    Feb 20, 2006
    Denver, Colorado
    In my neverending search for tone, I whined to my tech, "My Ampeg B-15N sounds better than any amp I've ever tried, but it's only 30 watts. My Carvin BX1200 has huge power, but it's transparent and dry sounding."

    Tech: I'll build you a magic box. It will have an input and and an output and a 'dummy load' inside to fool the Ampeg and keep it from blowing up. A line level signal will come from the output, which you will connect to the input of your Carvin. Then you will have the B-15N for a front end and all the power you want with the Ampeg tone and sag."

    Some other wags are saying this is dangerous and still liable to damage the Ampeg.

    I love the feel and tone of a tube amp, but don't want to carry a heavy tube amp around, that's how this all got started.

    Comments?
     
  2. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Get a tube pre-amp to use with the Carvin. Try a few until you find one you like. An emulator is another possibility. Some find them OK, some don't.
     
  3. lowfreq33

    lowfreq33

    Jan 27, 2010
    Nashville
    Endorsing Artist: Genz Benz Amplification
    As long as it's done properly this shouldn't hurt the amp at all.
     
  4. SteveD

    SteveD

    Feb 20, 2006
    Denver, Colorado
    Thanks Bill, that's an obvious and proper solution. I have a Tech 21 VT Deluxe, it clones the tone fairly well, but of course it can't do that natural compression saggy thing that only tubes seem to do. My tech calls the B-15 sound "spongy". It's a heavenly thing.
     
  5. SteveD

    SteveD

    Feb 20, 2006
    Denver, Colorado
    Thanks, lowfreq33, my tech has been very good over the years, but that makes me feel better. That Ampeg is my
    bay-bee, since 1975 (she's a 67).

    I've had a bad 38 year habit of paying a lot more attention to my playing than techie-type stuff. I know how to plug in a cord, change a fuse and that's about it.
     
  6. will33

    will33

    May 22, 2006
    austin,tx
    Assuming he remembers to carry his naught and calculates the resistance and power handling right, it's perfectly fine. Techs use dummy loads to work on amps all the time. All it is is a group of resistors that take the place of the speaker. As long as you make it with a proper resistance for the amp (4ohm/8ohm, etc.) and use enough to be sure they can handle the power (dissipate the heat) the amp is sending their way, it's perfectly fine. Just in the interest of reliability/safety, tell him to over-engineer it a little bit, meaning make one that'll handle a little more power than yoir B-15 has to give, just so your not making the components work their hardest. A small heatsink or cooling fan would be a double-guaranty for constant use, 4 hour gig, etc.
     
  7. sevdog

    sevdog

    Mar 2, 2008
    ATX
    Not more than a couple of weeks ago I was just asking here on TB and at a shop in Austin about doing a similar thing with an Ashdown Little Bastard and a power amp. I was told the answer was a power attenuator which is exactly what your tech told you he would build...to me it became cost prohibitive at that point but if I had the $$$ I'd probably do it. So, I say do it and post a video review on Youtube for us to all see.
     
  8. SteveD

    SteveD

    Feb 20, 2006
    Denver, Colorado
    Great idea. The magic box is $200. Just got word he's got it done. Picking it up tomorrow and will try it at tomorrow night's gig and give verbal report Saturday.

    This place is the greatest, thanks everyone.
     
  9. will33

    will33

    May 22, 2006
    austin,tx
    That sounds more like a Marshall Powerbrake or other tyoe device. They let the tubes run at full output and then soak up some of that power before it hits the speaker without changing the impedance, kind of like a giant L-pad. That's a different deal and yeah, they do kill tubes.

    The dummy load is simply a resistor(s) instead of a speaker. It's actually a more "friendly" load on the amp than the speaker is....of course, it doesn't make any sound.:p
     
  10. will33

    will33

    May 22, 2006
    austin,tx
    Hmm.....I have a typo on the word "type".....wierd.
     
  11. SteveD

    SteveD

    Feb 20, 2006
    Denver, Colorado
    Ah HA! That's what one of the guys was worried about. He said guys put these "power soaks" on Marshalls trying to get bloody tone at lower volumes and it hurts things.
     
  12. will33

    will33

    May 22, 2006
    austin,tx
    Yes.

    Your dummy load will allow you to run your B15 without a speaker at all and won't hurt it. Your tech will explain it to you. Make sure he's giving you a dummy load and not a power soak.
     
  13. P Town

    P Town

    Dec 7, 2011
    The sound of the tube output is created by the interaction of the output transformer, and the inductive, + resistive + speker mechanical + cabinet effect on the sound. A resistive load probably would not present the same voltage signal, (not to mention phase relationship with the current flow seen by the solid state amplifer). And I would bet it would not produce the desired effect. You could take the signal from the tube preamplifier, and switch the power to the output section of the tube amplifier off, and send that to a solid state power amplifier, but it really is not worth it.
     
  14. SteveD

    SteveD

    Feb 20, 2006
    Denver, Colorado
    Yep, it's a dummy load. I'm getting kind of excited now. I took the B-15 to a quiet jazz casual last week and it sounded so sweet. I'm interested to hear what this setup will sound like through my two Avatar BR210 cabs, stacked vertical of course. This will be at my classic rock gig.

    I'll take my old Fender Mystery Jazz and my MIM fretless Jazz. The Mystery Jazz has 61 pots, a 63 neck and a 65 serial number. Got it that way in 84, no idea what happened there, but she's a good 'un.
     
  15. JGR

    JGR The "G" is for Gustav Supporting Member Commercial User

    Jun 29, 2006
    Maryland
    President, CEO, CFO, CIO, Chief Engineer, Technician, Janitor - Reiner Amplification
    The dummy load, especially if it is purely resistive, will totally change the sound and feel/responsiveness of the amp, and not in a good way. The right way to do it is to tap a slave signal from the speaker jack and keep the internal amp speaker connected. Otherwise you loose all the dynamics of the amp's power section reacting with the speaker/cab, which has a HUGE impact. Send that signal to another power amp and cab. That will allow you to get the tone you love at higher volumes. I've spent a lot of time and money buying attenuators and building custom reactive and resistive dummy loads, and running all sorts of re-amp configurations, and that is by far the best way. You have the advantage of wanting more volume rather than trying to reduce it which makes things easier; use the small amp as a monitor.
     
  16. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    A resistive plus inductive load would, and a good techie can come up with that no problem. Well, I could, so make that a really good techie. :D
    But I still wouldn't lug around a 30 plus pound amp and a ten pound dummy load.
    A tube compressor will. I wouldn't spend a few k$ on a Fairchild, but I imagine there must be a decent one out there that isn't too pricey.
     
  17. P Town

    P Town

    Dec 7, 2011
    Or, get a PF500.
     
  18. SteveD

    SteveD

    Feb 20, 2006
    Denver, Colorado
    I thought about an SVT and a teenager to carry it, but couldn't find a teenager who would leave his x-box/facebook/texting/WarCraft etc. long enough to help me.
     
  19. will33

    will33

    May 22, 2006
    austin,tx
    Those guys points about your dummy load not sounding the same are spot on. It may not have all the B-15 character you expect. Although, for $200, I'd assume your tech is putting together something more involved like these guys are describing and not just a box with $10 worth of resistors in it. In that case, it should do what you're wanting to do.
     
  20. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    You can build an inductive dummy load out of a speaker motor which approximates a real speaker.
     

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