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Another one about tube amps & impedance...

Discussion in 'Amps and Cabs [BG]' started by goran, Aug 5, 2003.


  1. goran

    goran

    Dec 17, 2002
    Croatia
    Endorsing Artist: Bartolini
    I was just wondering, I know that solid state amps don't tolerate impedance LOWER than their declared minimum impedance. Until recently, I thought that same thing goes with tube amps. Then I found this page:

    http://www.bassbacke.de/hints/bass/cabproperties.htm

    This guy claims that tube amps DO tolerate impedance lower than their declared impedance. Is this true?

    Doeas that mean that I can hook another cabinet (I'm currently using Peavey BVX115, 4 ohm) to my Fender Bassman 100 (4 ohms total impedance declared)
     
  2. wulf

    wulf

    Apr 11, 2002
    Oxford, UK
    He says:
    I still wouldn't try it, certainly not with two 4 Ohm cabs unless the amp is rated to work down to 2 Ohms (the combined resistance). Otherwise, you could find that rather than giving you more volume you end up with a short lived pyrotechnic display and then have to go out and replace your whole setup.

    What problem are you thinking of addressing by hooking up both 4 Ohm cabs?

    Wulf
     
  3. Ask yourself this - are you willing for your amp to be harmed a little? Sure, it might work for a while, but then your tubes could die, or your output transformer could die. In the case of running with too small a load, it really starts to put a whooping on your output tubes because they are conducting more current than they are meant to conduct and this means more power dissipation, which equates to hotter tube, which eventually leads to thermal runaway and a gassy tube. When running too high a load, like open circuit, the voltages across the tubes and output transformer get insanely high due to inductive flyback action (the same phenomenon that generates the 20kV in your TV) and this usually arcs over the output tranny, killing it.

    Your amp probably wouldn't fry immediately, but are you willing to take a $150 risk on it?

    Run tube amps at their required load. Avoid trouble. Also, you've got the optimum setup with that 100 Watts through a 1x15. Consider just getting another cab that is 2x15 @ 4 ohms, then you can just use either cab by itself.

    Chris
     
  4. goran

    goran

    Dec 17, 2002
    Croatia
    Endorsing Artist: Bartolini
    Well, Throbbinut explained whole thing with lower impedance.


    What happens with tubes, transformer, etc. if you load amp with higher impedance? For what I know, worst thing you can do to a tube amp is running it with no load at all. Why?
     
  5. Throbb's actually explained that in his post, too.
    Most tube amps can tolerate a load mismatch of about a factor of 2 without much trouble. Speaker impedances aren't static so you'll never have the exact load anyway.
    You' re less likely to damage a tube amp by running it at lower impedances than higher ones, which is the opposite of SS amps.

    I would reccommend matching impedances as closely as possible just to be safe, anyway.
     
  6. goran

    goran

    Dec 17, 2002
    Croatia
    Endorsing Artist: Bartolini
    Yes, you're right. I haven't red whole post. Thanks for reply