Impedance math question!

Discussion in 'Amps and Cabs' started by bscepter, Nov 22, 2005.


  1. bscepter

    bscepter Member

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    Okay, so I've borrowed a friend's Fender Custom Shop Vibrolux Reverb, which has an internal and external speaker jack. The internal speakers are wired in parallel at 4 ohms. The external jack does not indicate what load it neads, but says that the total should not be below 2 ohms.

    Could I hook up my 16-ohm series-wired 2x12 to the external speaker out? Could I rewire my 2 8-ohm speakers in parallel for a 4-ohm load and then run it as an external speaker cab? What happens if you mismatch impedances? Can you harm your speakers and/or output transformer, or would you just not get the right sound/power?

    Thanks for any and all help. (I went to film school. The only math I understand involves 24 frames per second...)
     
  2. bscepter

    bscepter Member

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    Bonus question:

    According to the manual, "... the amplifier is optimized for a 4 ohm speaker load, and the speakers in the cabinet are wired for 4 ohms. Should a total load of more or less than 4 ohms be used, the amplifier will not put out its maximum power output before distortion occurs."

    So what would happen if I plugged a 16-ohm cab into the 4-ohm (main) internal speaker jack? Would the amp no longer be putting out 40w? Would it endanger the OT or speakers?
     
  3. drbob1

    drbob1 Silver Supporting Member

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    Here're your options and the result:

    Plug in only the 16 ohm cab-you blow up the amp, bye....

    Plug in the internal speakers (4 ohms) and the 16 ohm cab, you get 3.2 ohms, perhaps a slight increase in percieved volume, but most of the power (80%) still goes thru the old speakers.

    Rewire the cab for 4 ohms-you now get 2 ohms (that's safe), perhaps a bit of an increase in percieved volume but a change in tone that's probably noticeable.

    Rewire the internal speakers to 16 ohms and run the external cab at 16 ohms, you've got 8 ohms and a different, different tone...

    I'd just keep the cab at 16 ohms and plug it in when necessary myself.
     
  4. bscepter

    bscepter Member

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    Even if plug it into the main speaker out rather than the external out (which you're not supposed to use by itself)?

    In other words, let's say it's a head, not a combo -- if I were to plug a 16-ohm cabinet into a 4-ohm jack (say on a Bassman)... what would happen? Buh-bye?

    I should mention that I'm really just trying to break in the speakers in the external cab, which is why I want to run them by themselves. I don't plan on using them with the Vibrolux for very long.
     
  5. jokerjkny

    jokerjkny Member

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    yea,

    with the impedences you want, plugging the whole setup would probably cause a small explosion. :eek:

    but seriously, do the fender's speakers have to be wired in parallel? that's the main problem in this senario. re-wire the fender speakers to series for 16 ohms parallel. plug in the other 16 ohm cab, and hit a nice and safe 8 ohms.

    each speaker should get enough power equally. course, the speaker with the highest "sensitivity" will seem louder and dominant.

    also, i doubt the amp is "optimized" for a specific impedence. more like 4 ohms is the lowest possible impedence, where the amp'll run safely. so its not gonna sound any better, and is just a precaution.

    and finally, most people will agree that impedence makes LITTLE difference to the tone. its perceptile to a degree, but on gig, i doubt you or your audience would notice.

    still, many say higher impedences like 16 ohms utilize more of the amp's transformers making for a more textured brighter sound, while lower impedences like 4 ohms make for a darker more smoother sound. but if you can tell the diff, you should work as a submarine sonor captain.

    BUT, IMHO, i can hear a difference in wiring. if you wire speakers in "series" wiring where they're daisy chained together, one after the other, you'll get a more textured sound. frequencies get filtered by the first speaker thus sending that affect signal to the next speaker causing the speaker to not reach its full tonal potential. rather a modified one.

    wiring speakers in parallel, where each speaker gets its own positive and negative tap from the speaker input makes for a smoother sound. also, each speaker gets an equal amount of pure signal from the amp, allowing each speaker to really shine.

    for cabs loaded w/ similar speakers like a pair of Celestion Blues, series wiring is actually great, cause you exaggerate all the frequencies that the speakers are known for. BUT, with cabs w/ dissimilar speakers like my Bob Burt 2x12 loaded w/ a Tone Tubby Hemp-E & Alessandro G12, using parallel wiring allows both speakers to receive an equal amout of pure signal, helping them sound their fullest potential.

    wow, that's alot longer than i wanted, but hopefully, you get the drift.
     
  6. bscepter

    bscepter Member

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    So... if I had a 4-ohm Fender Bassman head and I wanted to plug in a 16-ohm Marshall cabinet, my Bassman would blow up?

    Sorry about the confusion.
     
  7. Boogs

    Boogs Member

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    Perhaps this is a silly question, but what are going to use them with, and could you break them in with that?


    That aside, if I had to break them in with that amp, I'd rewire the cab parallel in 4ohms and run it from the "main speaker" output by themselves, which sound like it's designed to run at that impedance.
     
  8. bscepter

    bscepter Member

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    I'm waiting... waiting... waiting... on a Hayseed 30 head! My Avatar cab arrived today, and I wanted to break in the Blues before my head arrived.

    BTW, rewiring the cab for 4-ohms seems to make the most sense. I was just curious if I could unplug the Fender's internal speakers and plug in the 16-ohm cab in their place. In other words, the Fender would be expecting a 4-ohm load, but would actually be seeing 16-ohms.

    I still don't believe that nobody has ever run a Marshall 4x12 with a Bassman successfully (a 16-ohm load on a 4-ohm head).
     
  9. jokerjkny

    jokerjkny Member

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    lol,

    not really. was being hyperbolic. BUT, IMHO, the amp wont push out the same amount of power to each cab. you might hear more of the fender than you will of the 16 ohm cab.

    go back up and check out the other stuff i added to my post. hope it helps!
     
  10. wsaraceni

    wsaraceni Member

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    most people seem to say that a 2:1 mismatch in either direction "should" be ok. I personally wouldnt chance it with a 16ohm cab.
     
  11. bscepter

    bscepter Member

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    Thanks -- that helps a lot. I was just being lazy, I guess -- hoping I didn't have to rewire my cab in parallel. It's not that difficult, though, so I'll probably wire it for 4 ohms and try it out. I've got a pair of Yellow Jackets in there right now, so I won't be pumping 40 watts into a 30 watt cab.
     
  12. bscepter

    bscepter Member

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    Ah, thankfully, it's all moot -- Rob just e-mailed me to say my Hayseed's shipping next week! :dude
     
  13. Shea

    Shea Member

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    It'd be simpler and, IMO, better to re-wire the extension cab to 4 ohms and run a total load of 2 ohms. A downward mismatch is a little safer than an upward mismatch. And if the transformer is anything like the ones Fender used in my silverfaces, it'll sound a little better than way too. The biggest advantage is that any time he wants to play it the normal way, without the extension cab, he won't have to re-wire the combo speakers.

    Of course it is. There's no way it couldn't be.

    With any tube power amp, there's a certain plate-to-plate impedance which is optimal for max power and widest bandwidth. If you deviate higher or lower from that impedance, you'll lose either power or bandwidth, or possibly both.

    The purpose of the output transformer is to mutiply the impedance of the speaker load up to the optimal plate-to-plate impedance that the tubes want to see. Most Fenders have only one output on the secondary of the output transformer, so that means this amp is optimized for one speaker impedance - in this case, 4 ohms. The transformer in this amp converts a 4 ohm speaker impedance to something in the area of maybe 4000 ohms plate-to-plate impedance.

    Well, you might be right about the effect, but not the cause. When people say that running 16 ohms uses more of the amp's output transformer, they're talking about amps in which the output transformer has multiple secondary taps. When you set the speaker impedance on such an amp to 16 ohms, then the output current will be flowing through the entire secondary winding, so some people think that will make the amp sound better because "you're using the whole transformer" or something like that.

    But when the transformer secondary has only one output lead (and one ground lead), as in a Fender, then it's ALWAYS using the entire secondary winding, no matter what you plug into it.

    However, when you mismatch impedances, i.e., use a speaker impedance that is different than the one your amp is set up for, then a higher impedance will generally result in a brighter, spikier, sometimes buzzier tone, and a lower impedance will result in a darker, but somewhat smoother tone. So in that sense you're right, but I wouldn't run a 16 amp speaker load with this amp.

    When a Fender combo has an extension speaker jack on the back, that's a pretty reliable indicator that the output transformer can drive a load of half the usual impedance without any problems. And they sound pretty good that way, though the high end may be a tad less edgy.

    Shea
     

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