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Using a big power resistor to change impedance of speaker...

Discussion in 'Amp Tech Center' started by MrCoolGuy, Sep 14, 2020.

  1. telemnemonics

    telemnemonics Telefied Ad Free Member

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    WRT the resistor idea that took some pretty mortal wounds early on in this thread, I'm thinking now that since we learned that the friend the OP was asking for wants to use an old grandpa speaker that can't handle the amps full power, maybe a resistor isn't such a bad idea after all.

    As far as hurting the amp, and managing the heat, I don't know the answers.

    As far as the speaker maybe/ sorta no longer being a reactive load when the resistor gets added?
    IDK that either but with no clear testimony stating that a resistor will harm the amp maybe it's a cheap east try it and see situation.
    Especially since it would help protect the $500 speaker from getting blown.
     
  2. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

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    No, the speaker can handle the load, it's just not the proper impedance for the OT. Though the back of his vibroverb clearly says "4ohm min"... I'm sure 8 is fine... and he likes it. But I didn't like the loss of power. Sounds great, but it's quieter than before.
     
  3. telemnemonics

    telemnemonics Telefied Ad Free Member

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    I don't know if the D130 was reconed recently but in general they were 100w speakers when new and have been called 50w speakers in vintage condition by educated guessers for a decade or more.
    Figure vintage speakers lose 10w of power handling per decade as a sort of general rule if you want to keep those vintage speakers working.
    A Vibroverb is rated at 40 clean watts then puts out a few more watts above clipping.
    So this could easily be a 45w amp into a 35w speaker.
    We also know that 80w amps used to blow new 100w D130 speakers, even in closed back cabs.
    For that matter, a pair of new 100w D120F would blow with only 40-45 watts on each in a Twin Reverb.
    A D130 handles more power in a sealed cab than with an open back.

    The general rule for speaker power handling is you want 50-100% more power handling than the amp wattage.
    So you want a brand new 100w D130, not an old 35-40w D130.
    Could be the old speaker has a new VC & cone?
     
  4. jimbo735

    jimbo735 Tele-Afflicted

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    Would the amount of power be the deciding factor?The reason I ask is my Pignose Piggy in a box will run any speaker ohm you throw at,it is there a magical transformer that compensates enough to do this.Seems there is gear that will take a speaker load or line load with magical transformers that allow you to run straight into PA or other things.Just asking.:)
     
  5. radiocaster

    radiocaster Poster Extraordinaire

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  6. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

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    You said it yourself. Which I think means we may be running a 40 watt amp into a 50 watt speaker
    (I understand your adjustment but don't agree)... Yes, twins would routinely blow these speakers, even when new. I've never heard of a vibroverb blowing one... and lots of people use them.
    Question... this is an 8ohm speaker. This Vibroverb has a 4 ohm OT...
    There is obviously a loss in volume using this speaker as opposed to using a 4ohm speaker. Where does that energy go?
     
  7. JRapp

    JRapp Tele-Meister

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    Who pays $500 for a D130F? 150-200 all day long...
     
  8. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

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    I think it was around 380
    I see them occasionally for 250 or so.
    But I also see them for 500 occasionally.
     
  9. telemnemonics

    telemnemonics Telefied Ad Free Member

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    You mean "obviously a loss of volume", as in that you hear when comparing the two speakers?
    Or based on the theory?
    I've hot switched 8 and 16 ohm speakers and could not hear any difference at all, not tone or volume.
    Both with the amp set for 8 and and for 16.
    4 and 8 would be the same as 8 and 16 in terms of one being a correct impedance match and the other being the mismatch.

    I generally find vintage speakers are less efficient than newer speakers, even if they used to be highly efficient when new.
    So any audible volume difference may be different speakers and not a result of the impedance mismatch.

    Solid State amps are different, and commonly deliver say 100w into 8 ohms and only 50 watts into 4 ohms.
    You can hear that halving of amp wattage.
    Tube amps don't work that way.
     
  10. NTC

    NTC Tele-Meister

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    Folks, Do you know how they deteemine how much power output an audio amp has? You connect a resistor as a load (no speaker) and run the measurements. The tube amps can drive reistive loads of the correct value.

    The sound of "American Woman" is from what was essentially a tweed Champ with a reisistor as a load and a couple more resistors to knock the signal down to drive the input of another amp. A Garnet Herzog? IIRC.
     
    twangdude and Digital Larry like this.
  11. NTC

    NTC Tele-Meister

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    Just keep the impedance correct for the amp.
     
  12. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

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    Yes, by "obvious loss of volume" I mean when you compare the vintage JBL to a 4 ohm 10F150. But, yes, you're right.
    Maybe it's just not as efficient as the Weber. 10F150's are pretty loud.
    Really? American Woman? Interesting.
     
  13. Bendyha

    Bendyha Friend of Leo's Silver Supporter

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    I just ran across this, and thought of this thread, sort of seems relevant.
    upload_2020-9-28_22-16-36.png
     
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