Another vintage Fender amp impedence question.

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by bno, Jul 21, 2006.

  1. bno

    bno Member

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    Okay, I think I figured it out, but I just want to be sure. The two speaker jacks are both tapped off the transformer at 4 ohms and putting a 4 ohm cabinet on each of the speaker jacks does not mean the tranny is seeing one 2 ohm load (the outputs are not actually parallel to the same tap). Using one or two 4 ohm cabs does not create an under impedence mismatch. The original Fender cabs were two 8 ohm speakers in parallel (4 ohms). The amp was designed to safely drive two of these cabinets.

    In other words it is perfectly safe to use one or two 4 ohm cabinets on a vintage Fender amp.
     
  2. Dave C

    Dave C Gold Supporting Member

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    The output jacks are in parallel , 2-4 ohm cabs would be a 2 ohm load and Fender OT's will generally take a 100% mismatch...4-2 or 4-8 ohms quite well but no more than that.
    Dave C
     
  3. bno

    bno Member

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    Dang. I'd swear I read that the speakers jacks were on separate 4 ohm taps. So, would using one 4x10 8 ohm cab along with the 2x12 4 ohm cab be okay? Wouldn't that be a 6 Ohm load and less stress on the OT? Or does it matter?

    I have a 65 Bassman head that has be modded and I love the way it sounds. I currently play it through a 2x12 4 ohm. I want to make a second 2X12 (4 ohm) or a 4x10 (8 ohm) and plug it into the ext speaker jack. Or would it be best to have two 8 ohm cabs when I use the bigger rig.
     
  4. Dave C

    Dave C Gold Supporting Member

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    8 ohm cab and 4 ohm cab would be 2.66 ohms , which would work . Max power throughput will be at 4 ohms but the difference is not that much.
    Dave C
     
  5. bno

    bno Member

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    Great. Thanks for the help.
     
  6. stratzrus

    stratzrus Philadelphia Jazz, Funk, and R&B Supporting Member

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    I've always wondered about this: If the BF Super Reverb is wired for a 2 ohm load, what is the effect of adding another speaker cabinet? Specifically an 8 ohm 2x12, or does it have to be another 2 ohm cabinet, or does the Super really not want an extension?

    stratzrus
     
  7. John Phillips

    John Phillips Member

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    The Super has a 2-ohm output transformer which means that it will drive another 2-ohm load as well, if you want. 2 ohms is the correct match so it will handle down to half that safely (ie 1 ohm total). All the BF/SF Fender amps were designed this way.

    EXCEPT: late-70s 'Ultra-Linear' - 70W and 135W, and the "75" model - amps, which do have two separate taps, and the output jacks are actually wired in series. Plugging into the Ext. jack resets the amp to 8 ohms. (That's why they say "4 ohms" under each speaker jack, rather than under both.)

    ... and early-80s 'Rivera series' models which are the same except that the jacks are wired in parallel and the Ext. jack switches the amp to half the usual impedance (4 or 8 ohms depending on model).
     

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