4X12 8ohms and 4X12 16ohms question ?

Discussion in 'Cabinets & Speakers' started by Kelia, Sep 15, 2019.

  1. SkyMonkey

    SkyMonkey Well-Known Member

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    I have almost no understanding of the electronics involved in tube amps, so do not take my word for anything. But.....

    For mismatching with 5 jack amps like the JVMs and new DSL40CR & 100HR, I have seen it explained like this:

    The OT needs to 'see' 1 correct load of 16 (1x16), 8 (2x16), or 4 (2x8) ohms.
    Plugging a 16 Ohm speaker into one of the 8 Ohm jacks shows the OT 1/2 of a correct load.
    Plugging an 8 Ohm speaker into one of the 4 Ohm jacks shows the OT 1/2 of a correct load.
    2 x 1/2 of a correct load = 1 correct load.

    Can someone explain why this might be bullsh#t, if it is bullsh#t.

    BTW I have NOT tried this out.
    But have heard of people posting in this forum using it with success.
    No one has posted saying this blew up their amp (that I have seen anyway).
     
  2. JohnH

    JohnH Well-Known Member

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    Im not an amp expert either, but Ive thought quite a lot about impedances etc, and have my head reasonably well wrapped around general electronics, having been into it for over 40 years, although my day job as an engineer is not electronics.

    So, all I want to say is that the idea above of using the jacks to combine different cabs, as told to us by Santiago and confirmed by Steve Dawson (two of the very best amp designers ever), makes total sense to me. It seems perfect with no compromise that I can see.
     
  3. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    I don't think the OP has a 4 ohm tap on his amp...
     
  4. Im247frogs

    Im247frogs Well-Known Member

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    I blasted the shit out of the 67 Bassman I just completely overhauled into the 8 ohm tap on the multi impedance jack on a 1936-style 2x12 for several hours over the weekend and I had a stiffy the whole time.
    No funny smells, no smoke, nothing too hot other than the original GE 6L6's that it left the factory with. I feel totally comfortable continuing to run it in similar fashion moving forward. I also have a sneaking suspicion that it's been impedance mis-matched for most of its life.
    I might build a dedicated 4ohm cab to go with it, esp in the interest of selling the whole rig at some point, but I'd rather put the cash into another guitar build.
     
  5. SkyMonkey

    SkyMonkey Well-Known Member

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    Apologies. I was aware of the DSL50 switching configuration.
    But I was actually trying to reply to @Gene Ballzz comment on the 5 jack amp mismatch solution but forgot to add the quote (below).

    As an amp tech, can you comment on the 5 jack mismatch solution @ampmadscientist?
     
  6. junk notes

    junk notes Active Member

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    Licking the frosting; You can safely add an 8Ω dummy and have your cake.
     
  7. spacerocker

    spacerocker Well-Known Member

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    OK - I'm going to buck the trend here!


    I say that if the 4 Ohm output is used it will be perfectly OK

    The 8 Ohm and the 16 Ohm cabs will be in parallel. So the total impedance will be around 5.3 Ohms. That is not even a 1-step mismatch (and it is an upward mis-match) so the O/P Transformer will be perfectly OK

    With the 20V winding selected on the Transformer (i.e the 4 Ohm setting), a 4 Ohm load would give 100W. Therefore a 5.3 Ohm equivalent load would dissipate 75W, split 50W/25W between the 8 Ohm cab and the 16 Ohm Cab.

    So - apart from the fact that the 8Ohm cab would potenially be 3dB louder than the 16 Ohm Cab (i.e only a slight difference), everything is fine! The transformer is nearly matched, and nothing is trying to take more power from the amp than it was designed to make!


    Hopefully someone like Santiago will be along shortly to agree with me!
     
    Last edited: Sep 16, 2019
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  8. spacerocker

    spacerocker Well-Known Member

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    Firstly - you have answered your own question! Does the explanation make sense to you? If it does, then there is your answer!

    It is most definitely NOT Bullshit - and here's 3 very good reasons why:

    1) It makes perfect sense!
    2) This was originally posted by Santiago Alvarez who designed many of the Marshall amps, including the JVM, YJM and AFB (but not the DSL). Santiago is very unlikely to be wrong about something as simple as this!
    3) Santiago posted the maths behind this. The simple form is exactly what you posted. The more complicated form was from his own hand-written notes. Not surprisingly, his analysis made perfect sense!


    HOWEVER - none of this answers the OP's question because he only has two speaker connections (which are actually in parallel, and connect to the transformer selector switch) and the output selector switch itself, which has 3 settings, 4, 8 and 16 Ohm.

    See my analysis (above) for why I think the OP can actually safely do what he is suggesting....
     
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  9. Kelia

    Kelia Well-Known Member

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    Yes it does have a 4ohm !!............Very happy I posted this thread,
    lots of great info and very interesting at the same time !!
    If I'd be able to start everything again ,.....I would
    have studied electronics !.....and understand more of what some of you are
    debating !

    But I'm going up high on a roof tomorrow to fix it !.........Arrrgghhh!!:erk:
     
  10. Kelia

    Kelia Well-Known Member

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    What do you mean by adding a load ?
     
  11. spacerocker

    spacerocker Well-Known Member

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    What Skymonkey meant was " does it have a SEPARATE 4 Ohm socket" (like the JVM series, which have FIVE separate output jacks in total) On the JVM, the ideal solution would be to connect the 16 Ohm Cab to the 8 Ohm output, and connect the 8 Ohm cab to the 4 Ohm output. Your amp DOES NOT have this option - it only has ONE output which is connected to two jacks (i.e you have two sockets but they are both connected in together in parallel internally). You select the impedance using the rotary selector switch!



    This is a suggestion to add an 8 Ohm resistor in series with your 8 Ohm Cab to make the total resistance the same as the 16 Ohm cab. This is to achieve a perfect match with the amp output impedance. This would work, but has two big problems:

    1) You need a resistor that can handle about 40W. These are available, but you will need to mount it in a massive head sink otherwise it will melt!

    2) It is completely pointless, as the speakers will still be dissipating different amounts of power, and thus one cab will still be 3dB louder than the other. Therefore there is no real advantage over the method I described, which I am repeating below:




    So - you connect both 8 Ohm and 16 Ohm Cabs to your output sockets, and set the impedance on the amp to 4 Ohm.


    This method is fine, safe, (especially if you set your impedance selector to 4 Ohm as I have described), and you have already heard both cabs connected at once and were happy with the sound. You really don't need to ask any more questions!.....
     
    Last edited: Sep 17, 2019
  12. john hammond

    john hammond Well-Known Member

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    The problem with people really knowing what to do here is based in their utter lack of understanding as to why it matters, if it matters, and when it matters in the first place.
    Im referring to the actual electro- mechanical aspect of the (alleged) danger.
    Until people start discussing that, its just one group of people ( who actually use mis-matches all the time with no problem) trying to convince the other group of people that in fact, their amps don't blow.
    And the other group aren't giving up, they are repeating everything they've ever read online, which STILL doesn't relate to the actual electro-mechanical risks.

    There are actually two alleged areas of ' risk' when mis-matching

    1) stressing the tubes
    2) stressing the windings on the output transformer

    - One is a true risk , but only if you use dud output tubes. ( solid tubes couldn't care less about plate to plate impedance at a 100 percent mismatch, maybe 200 percent, my mesa mk1 doesnt mind a 200 percent mismatch one bit, and mesa dont warn anyone either, in fact they dont even fuse the o/t.)

    - One is a myth . ( the second one, unless your amp has voltage doubling )

    pls note, this post doesnt go into it too deep either, its just a brief description.

    Could people care less for a proper look into it?..not one bit, they like to argue.
     
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  13. Nik Henville

    Nik Henville Well-Known Member

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    This...
    ...is the way it is, and always has been.

    Belief is not something you can argue with sensibly, or even discuss.

    :hippie::pirate::uk:
     
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  14. spacerocker

    spacerocker Well-Known Member

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    Very helpful, I'm sure - but what are you actually advising the OP to do? Do you agree with my suggestion or not? If not what are you going to tell him?
     
  15. john hammond

    john hammond Well-Known Member

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    ive already told him.
    my advice is rock on with what he likes.
     
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  16. Kelia

    Kelia Well-Known Member

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    Thanks for the explanation !
     
  17. Kelia

    Kelia Well-Known Member

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    What is voltage doubling !?...............Plexi vs my amp for example?
     
  18. john hammond

    john hammond Well-Known Member

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    its the way the amp gets its H.T ( high voltage)
    voltage doubling is cheaper, because you can have smaller iron doing the same job, but it does have a drawback in that it induces flyback spikes, big transient notes cause current to go backwards past rectifier diodes ( i'd say because filter caps get drained), then, like an ocean tide, the energy changes direction again to the ' normal' direction. problem is in by doing so, it goes through rectification diodes again, this means 182vac becomes 475vdc ( i know thats not doubled..in fact that 182 gets mulitplied by 1.3, then THAT total is doubled) , then, during transient spikes, that 182 actually becomes 364 which is then mulitplied by 1.3, then doubled = 946vdc...if you mismatch by 100 percent, guess what..during those transient spikes your output windings see 2000v.
    Modern windings , good quality ones are rated at 2kv, but vintage iron isnt.

    The way other amps get their H.t is simply a couple of windings fed through some diodes, that multiplies the voltage x 1.3 and turns it into dc once it gets in contact with a filter cap...so a normal power supply, non voltage doubling may have 350vac H.T windings, which is multiplied x 1.3 = your plate voltage.

    Voltage doubling is a good way to get your H.T because it has a stronger push, more bass. its a ' fast' sound, good for plexi delivery type.
    Hi-fi guys like voltage doubling, more bass.
    but, you have to consider the flyback spikes...you can run three in4007 diodes to ground from your two inner output tubes to mitigate any damage...ken fischers idea.
    i do this on my voltage doubled amps.
     
    Last edited: Sep 18, 2019
  19. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    It has also been confirmed and explained by me on more occassions then I remember.
    :D :scratch: :squint:
    :thumbs:
     
  20. Kelia

    Kelia Well-Known Member

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    Thank you John for this great explanation ,...appreciate!
     

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