4-ohm parallel or 16-ohm series?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by bscepter, Sep 29, 2005.


  1. bscepter

    bscepter Member

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    As i mentioned in an earlier post, I want to put a pair of 8-ohm Celestion Blues (15w ea.) in a Bluesbreaker reissue.

    The BB is wired with two 16-ohm speakers in parallel, for an 8-ohm load. The impedance selector on the amp has 4,8 and 16-ohm settings.

    Technically, I should be able to simply swap out the speakers, set the selector for 4 ohms and be fine. However, I've heard that it's better to use the highest impedance possible. So, in order to do that, I'd have to wire the speakers in series, for a 16-ohm load.

    Now, as the BB has a 1/4" speaker jack, I'm not sure how to do this. Can anyone enlighten me? (I know how to wire two speakers in series -- I'm just not sure how to do it into a 1/4 speaker cable).

    Thanks!
     
  2. John Phillips

    John Phillips Member

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    I would replace the stock speaker cable anyway, it's a piece of crap.

    You need a 1/4" plug and some 2-core power cable. Wire the cable into the plug as normal, and strip back enough at the other end so that the positive wire will reach the positive terminal on one speaker and the negative wire will reach the negative terminal on the other speaker - it's easier if you orient the speakers so the tag boards are facing eachg other. Then connect the remaining positive and negative terminals on the two speakers with another piece of the same wire (you can just strip down the cable).

    That's for series.

    I probably would go for series, not because it's "better to use the highest impedance possible" - which is a myth, it depends what kind of tone you want - but because series-connecting the speakers and running them at 16 ohms will give the most complex, characterful tone, which is what most people would look for out of a Bluesbreaker.

    Running in parallel at 4 ohms gives a cleaner, more even tone - if that is what you'd prefer, connect the speakers in parallel as before... I'd still replace the cable though.


    Be careful with those speakers... the reason Marshall moved to the higher-powered ceramic Celestions when they became available was because they kept blowing the alnicos. The BB may be called a '30W' amp these days - which it is, in true clean RMS terms - but in fact it peaks a lot higher than that, closer to the distorted output of a 45W amp.
     
  3. bscepter

    bscepter Member

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    Thanks for the invaluable info.

    Oh, and I forgot to mention, I'm running a germanium treble boost pedal, too -- which, I imagine, would present an even greater danger to the speakers. Maybe I should hold off.

    Now, the Valvetech I'm getting is a '62 or '63 JMI AC30 copy, which puts out 33w RMS in a cathode-biased circuit. As the Blues are stock on even the new 30s, and the new 30s are certainly driven hard by many people, is there a danger there as well, or is there something different about a so-called class-A amp?

    Thanks again.
     
  4. John Phillips

    John Phillips Member

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    Partly, yes - precisely because it's a cathode-biased Class AB. When pushed hard (and especially once the power stage clips) the bias voltage rises quite sharply, which limits the power output a lot. So an AC30 doesn't produce sustained power all that much above the 33W, even distorted. Not only that, the preamp is relatively lower gain, and the power stage reaches distortion either first or at about the same time as the preamp, so the compression kicks in as it's distorting.

    In a Bluesbreaker (and other non-MV Marshalls), the preamp reaches distortion first, which limits the 'RMS' power to below what the power stage is really capable of when fed a distorted signal. There is a range just above this where the amp isn't clean any more because the preamp is distorting, but the power stage is reproducing the distorted signal with the full dynamics of a clean amp. When it is pushed into distortion, being fixed-bias it compresses much less too, so the sustained distorted power it can put out is a lot higher... and finally, having negative feedback on the output stage makes the distortion itself harder, which is also not good for the speakers.

    I know some people do run Bluesbreakers with two Blues, without apparently frying them, but I'd be cautious. A trick you can use if you want to is to deliberately mismatch the amp by one step - this will reduce the power by about 30% and make the power stage distort sooner relative to the preamp signal. If you want to do that, wire the speakers in parallel for 4 ohms but set the amp to 8. It's slightly harder on the tubes but shouldn't do any harm beyond that.
     
  5. bscepter

    bscepter Member

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    Cool. Thanks. I might try the mismatch trick.

    Boy howdy, you sure learn a lot from these here internets! :D
     
  6. drbob1

    drbob1 Silver Supporting Member

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    Or buy a pair of lightly doped Weber alnico Blue Dogs in the 30w size. I'm running my JTM45 into an open back cab with a pair of those and it sounds really good!
     
  7. bscepter

    bscepter Member

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    Actually, I've already bought the Blues. Dave at Avatar says he's only had like two or three come back blown over the past few years, and he's sold a ton of 'em.
     
  8. KCblues

    KCblues Member

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    First off, let me say thanks to John... I've been reading a lot of your posts here, and sure appreciate your knoweldge and willingness to share.

    I haven't had this happen to me... but one thing I've always considered is that if you are running the 2 speakers in series and burn a coil in one, the amp then sees an open circuit, and the OT could be damaged... whereas if they are in parallel all you get is twice the impedance, and probably no damage is done.

    Comments?
     

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