4X12 8ohms and 4X12 16ohms question ?

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

  1. john hammond

    john hammond Well-Known Member

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    zz..zz zzz
     
  2. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Top
     
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  3. Gutch220

    Gutch220 Well-Known Member

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    I just sent Marshall an email, just because, to see what they say.
     
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  4. santiall

    santiall Well-Known Member

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    I guess you guys are arguing about 'Internet semantics', one of those circuits that people call incorrectly.

    Technically it is not a voltage doubler, just a pretty standard full wave rectifier, either bridge or centre tapped, but for whatever reason people call it voltage doubler when it is not.

    Voltage doublers are used for example in pretty much every switched mode power supply with the 120/230V selector to double the 120V to 240 internally. They, and also voltage multipliers, are used sometimes for increasing the bias range or powering tubes in pedals and the likes but I'm not aware of them being used for the HT in tube amps.
     
  5. spacerocker

    spacerocker Well-Known Member

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    Thanks Santiago for clearing this up (finally) - I was about to ask how this circuit could be considered to be a voltage doubler, because "boost regulation" is something I am only experienced with in Switched mode power suppliers or DC/DC convertors...

    It looked like a standard bridge rectifier circuit to me!


    A couple of things though - can you explain the centre tap on some amps? is it just to ensure the DC voltage is split evenly across the caps? I notice on the JVM that point is connected to the end of one of the windings - which I guess is a 0V DC point and has the same effect?

    Finally - how much load mis-match can be considered "safe" for Marshalls - and is it the same on all (or most) or as some amps more at risk than others?

    Once again thanks for joining in!
     
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  6. john hammond

    john hammond Well-Known Member

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    Technically, it IS a voltage doubler santiall, and frankly im surprised to see you type that out.

    The argument that the naysayers present, in terms just a little more expanded than you've outlined here ( and which consists of something more than a vague reference to stacked burgers..what did the other one type out..stacked burgers??) is that without that centre tap which balances the voltage out equally ( and with the addition of some bleeder resistors)..the outer taps would simply feed a full wave bridge to provide the high voltage dc.
    to blow your argument out of the water, and the argument of mickeydog5 - the full wave bridge doubler doubles the centre tap voltage

    So, because the centre tap IS there, and because the bleeder resistors ARENT and because the voltage doubler doubles the centre tap voltage ( hehe which = b+ )...you're wrong.

    Thankyou.
     
    Last edited: Sep 20, 2019
  7. john hammond

    john hammond Well-Known Member

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    oh..did he now?
     
  8. john hammond

    john hammond Well-Known Member

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  9. john hammond

    john hammond Well-Known Member

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    you are now.

    fender used it
    Musicman use it.
    Marshall use it.
    Vase use it.
    Holden wasp use it.
    Eminar use it.
    Valvetone uses it.
    Lenard uses it.
    Fisonic uses it.
    Baez uses it.

    - and theyre just some? of the guitar tube amps..the hi-fi guys love voltage doubling. FYI these other brands utilise voltage doubling in the more conventional way, without a centre tap.( with the exception of the fender UL bassman ten, same as marshall voltage doubling)
     
    Last edited: Sep 20, 2019
  10. john hammond

    john hammond Well-Known Member

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  11. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Not that any of you do not trust me or believe in me.........................................:squint:

    BUT he (Hamman) now just told Santiago that he is wrong as well.

    Do you not just love this guy?

    Oh yes, kudos for all you professional and technical electronics types out there for weighing in on the subject. :squint: :squint: :D

    And for all you who listen to every word of Ham with little question, say you won't get fooled again.........................
    [​IMG].
     
  12. john hammond

    john hammond Well-Known Member

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    if someone here can express why i am wrong, and actually use a reason, or some terminology..then pls do so.

    mickeydog5, just talk electronics dude, or wait for someone who can, this is technical stuff my friend, now, excuse me I have an exciting new marshall mod to unveil.
    Thankyou.
     
  13. spacerocker

    spacerocker Well-Known Member

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    You know who he is? right?
     
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  14. john hammond

    john hammond Well-Known Member

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    i know what he typed out, thats all i know.
    seems like a nice guy.
    spacerocker, just get ready for a result youre not expecting, thats my advice to you.

    - and check out my new mod why dont you? tell me what you think.
    I posted it in workbench a few minutes ago.
     
    Last edited: Sep 20, 2019
  15. spacerocker

    spacerocker Well-Known Member

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    I just know I am going to regret this - but;

    So your argument is that because the centre tap of the transformer secondary is connected to the mid point of the two series capacitors across the rectified (DC) output, that makes it a "Voltage Doubler" rather than a normal Bridge rectifier?

    So suppose we have a transformer secondary with a tap like this 180VAC-0-180VAC, and you connect it to this "voltage doubler" circuit with the centre tap, you get out 2 x the centre tap voltage? So the DC output is - what 360V DC???

    Apart from the fact that we are now mixing AC and DC voltages in our analysis, to my mind, the AC inputs to the rectifier will see 360VAC (i.e. 180VAC winding in series with with the other 180VAC transformer winding). So the output from the DC rectifier will be approx 1.4 x 360V = 504VDC* How is 504VDC twice 180VAC? The tranformer centre tap will be sitting at 252VDC and 0VAC. The mid point of the output caps will also be at this same voltage, 252VDC** and 0VAC.....

    Now let's forget about the centre tap and the "voltage doubler" circuit. Suppose we disconnect the centre tap of the transformer, so it is not connected to anything! Now let's connect the ends of the secondary winding to our bridge rectifier. the voltage across the two ends of the winding is 360VAC (as before) and this is the input to the rectifier. The output from the rectifier is 1.4 x the AC input voltage* = 504VDC

    (*ignoring ripple and diode drops)
    (** due to very slight leakage current through the caps)

    Now I can't see any difference at all between the end result of either circuit (with the centre tap connected or not) - except that you are calling one a "voltage doubler" (even though it is not even remotely doubling any transformer secondary or, indeed any other voltages), and the other is just a full wave bridge rectifier circuit. Am I a missing something?

    It sounds to me as though you are just calling a circuit something it is not, and refuse to budge from that belief! That is fine - you can call something by any name (Fender call the mechanism that produces Vibrato on their guitars a "tremelo" - even though it does not create tremelo at all!) - but it doesn't mean any one else has to agree with you!


    Personally - I have to say - if Santiago told me I had got the wrong idea about something related to how amps work, and that I was mistaken - I would be inclined to believe him!
     
    Last edited: Sep 20, 2019
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  16. SmokeyDopey

    SmokeyDopey Well-Known Member VIP Member

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    No way man, thanks.
     
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  17. santiall

    santiall Well-Known Member

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    As I said this seems to be a semantics problem but that there is an unused doesn't change that it is a full wave rectifier. A voltage doubler is a completely different circuit (just Google and you'll see, they don't even have a centre tap... ) but doesn't really matter, Im perfectly OK being wrong if you state so and I won't argue at all, it isn't worth. It's fine, sorry to disappoint you.
     
  18. santiall

    santiall Well-Known Member

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    The centre taps are there because in the age of tube rectifiers every tube had (and still has) two diodes. By using a centre tap what we have are two half waves rectifiers 'interleaved', if that makes any sense. Employing two tubes is unpractical and very expensive as each rectifier tube needs its own heater current. Some rectifiers have indirect heater but the higher power ones like GZ34 are still directly heated.

    Once silicon rectifiers became popular then the centre tap becomes redundant as we can use 4 diodes in a bridge and only half the amount of copper in the transformer.

    If available and if we use two capacitors in series to increase the total voltage, using the centre tap to balance the voltage is convenient. That circuit is easier to visualise if you think of it like a symmetrical power supply, for example the typical +/-15V or for powering solid state power amplifiers. Instead of grounding the centre we ground the negative side and there you go, a full wave rectifier with balanced cap using the centre tap.

    About mismatch, usually you can mismatch as much as you want but I'd recommend to reduce the mismatch vs output power. If we play very loud then the more matched the better, if we play at whisper levels then you won't break anything.
     
  19. Jon C

    Jon C Well-Known Member

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    I’m just happy as heck that you’re still willing to chime in @santiall ……
     
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  20. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Yes, thank you for chiming in @santiall. :agreed:
     
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