4X12 8ohms and 4X12 16ohms question ?

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

  1. spacerocker

    spacerocker Well-Known Member

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    I joined this discussion with an open mind - not entirely sure what was meant by a "voltage doubler" in the context of a HT power supply of a valve amplifier.

    Having read with great interest the arguments being put forward - it is clear to me that some people like to argue for the sake of it, and have a very fixed view which is different to most other people on here who have amplifier and/or electronics experience.

    Santial has repeated (with great tact and diplomacy in fact...) - that it is more a matter of semantics than a proper technical discussion - in other words - calling something a name for that which is not (in this case calling a bridge rectifier power supply a voltage doubler). I can call a banana a potato if I like. Does that make it wrong? Perhaps not for me, if that is my view, but it does tend to cause confusion in the Greengrocers/ Supermarket!

    John has repeatedly failed to show what voltage is being doubled, or where in the rectifier circuit, yet he insists on calling it a "voltage doubler"

    Not really - I can't see any doubling anywhere, what voltage is being doubled? Where?


    I tried to explain, with the use of examples of a Simple bridge rectifier circuit with actual AV and DC voltages, but John has shown little interest in going down to this level of detail - yet accuses Santiago of being "light" with his responses:

    I don't see any clarity, just hundreds and hundreds of words.....


    This statement (quoted from another forum) is an attempt to explain why the bridge rectifier with bridge tap (which Santiago has already said is not fundamental to the operation of the circuit, but is a historic artifact, but also serving to keep the voltages on the series output caps balanced), is in some mysterious way, a voltage doubler:



    But this argument is like saying a loaf of bread is not a loaf of bread - it is a "half-loaf doubler" since if you cut it in half, and put the two haves together, you have a whole loaf! In-fact, I think from now on, when I buy bread, I am going to specifically ask for a "half-loaf doubler"! Imagine the fun and long protracted conversations I can have doing that!!!


    I have a little electronics experience, and have worked as a professional electronics engineer (although in the area of switched mode power supplies (where REAL voltage doubling is possible!) rather than audio amplification. My work these days is more Electrical (automotive) than electronics....

    But if I was a layman (with little or no electrical or electronics experience) reading this thread, I think I would be more inclined to believe Santiago than John Hammond, - that there is nothing special or different about a bridge rectifier circuit with a centre tap, compared to one without, and that the neither circuit is called, or acts like a voltage doubler....

    If anything I admire your confidence and self-belief, John. Santiago is well respected here for a reason - he designed some of the most revered modern Marshall amplifiers produced in recent times, including the JVM series, AFD and YJM amplifiers (plus some others...). I think he was head of his design department at Marshall while he was there? As I said before, if Santiago told me that I was mistaken about something, I would be inclined to believe him, and benefit from the learning being offered!

    So I don't think I am going to continue with this discussion (entertaining though it has been!) - I don't think anyone or anything is going to convince John that a simple bridge rectifier circuit with a centre tapped transformer is - well, just that and not some form of magical , fly-back boost regulator which doubles something, somewhere!
     
    Last edited: Sep 21, 2019
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  2. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    just a note: John Hammond has left the building.

    while, it's ok to further discuss his points (right or wrong), let's just not discuss him (personally), from this point on.

    thanks...
     
  3. santiall

    santiall Well-Known Member

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    Well, it's not about me being right or anything like that... I tried to explain that while technically it's not a voltage doubler, some people call them that and then I left it there.

    Now, should we discuss that a guitar tremolo bridge is actually a vibrato? :D
     
  4. Kelia

    Kelia Active Member

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    Good day Santiall , I have a question for you , I have a 2555SL
    and love the amp and was wondering why I get no ear fatigue with it even after playing it loud for a while compared with Plexi's I had and lots of others ?
    Is it because it has even harmonics or something ?

    Tx
     
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  5. SkyMonkey

    SkyMonkey Well-Known Member

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    Wrong. Wrong. Wrong.

    It's called a WHAMMY.

    And I'll fight anyone here who says different!!!
     
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  6. spacerocker

    spacerocker Well-Known Member

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    NO! It's called a "PMA" (Pitch Modulation Apparatus) - because I SAY SO! :lol::lol::lol:
     
  7. Nik Henville

    Nik Henville Well-Known Member

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    Hmm...

    Hope ol' hambone wasn't offed because of this debate.
    I was so disturbed by this 'discussion' that I went back to my ancient notes made at Hackney Technical college in the early seventies...

    Centre-Tap Full Wave Rectifier - two diodes and big iron/copper, centre tapped.
    As the spaces between each half-wave developed by each diode is now being filled in by the other diode the average DC output voltage across the load resistor is now double that of the single half-wave rectifier circuit and is about 0.637Vmax of the peak voltage, assuming no losses.


    I wonder if this kind of wording was the source of all the grief...
     
  8. santiall

    santiall Well-Known Member

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    Oh man, I should have stayed quiet...

    I'm not sure... There is a lot of personal taste and psychology in the way one perceives certain sounds. The 2555 have a certain type of distortion that makes it quite compressed whilst a plexi is much more dynamic and relies heavily on the power amp distortion.
    Probably we could analyse the harmonic content and come to some sort of conclusion but that won't be taking into account the 'feel' of the amp. We could have two amps that sound the same, with the same harmonic content but one of them being more dynamic or suiting our playing style more...
    It can get quite complicated so just go with whatever you like the most.
     
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  9. Kelia

    Kelia Active Member

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    Thank you Sir !!
    Really appreciate !

    Love it when you break silence !
    Haha!
     
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  10. spacerocker

    spacerocker Well-Known Member

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    Yes - maybe it was something like that? I had the feeling that there was something that he wasn't quite grasping, and he had this firm belief...... either way, none of the circuits being discussed used "half wave rectification", or even full wave rectification using two diodes...All the circuits being discussed used full wave, bridge rectification!

    I'm sure we have all been in discussions where we hold on to a belief despite being at odds with everyone else (I know I have!) - but there comes a point when we have to stand back and look again and question whether we have misunderstood something along we way? When an ex-Marshall senior amp designer gets involved, that should have been the point, really (instead of arguing the toss!) I wish Mr Hammond Well, anyway!
     
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  11. Dogs of Doom

    Dogs of Doom Moderator Staff Member

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    it's a whammy bar! :)...
     
  12. santiall

    santiall Well-Known Member

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    Definitely, that's usually the origin of those incorrectly named circuits or anything actually. Someone writes 'voltage doubler' and slowly becomes an misnomer.

    It's also not about me having been Technical Director or Marshall (something like CTO) I actually got a MSc in power electronics engineering and, as an engineer, we tend to be pedant and specific with the way we identify circuits. I do of course understand that people may use different terms and I'm sure I do the same for other things but, as said, again, that's technicality not a voltage doubler at all. You want to call it 'voltage doubler' then fine, what's the point arguing?

    In any case, bear in mind that engineers are never wrong! aren't we? :hmm:
     
  13. South Park

    South Park Active Member

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    The plexi cf in the preamp inverts the signal and makes third order distortion. It is the important part of the sound you get. A real work of art
     
  14. Kelia

    Kelia Active Member

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    Haha!..............I'm a house builder and deal with engineers and they are never wrong !!:cool:
     
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  15. Kelia

    Kelia Active Member

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    I have another question and been wondering !

    Why both of my 4X12 G12H Pre Rola cabs read under 15 ohms ?
    One reads 11.7 and the other 13.5 iirc !?

    Tx
     
  16. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    All of these products have tolerances which can sometimes be up to 20%.
    13.5 DCR to 11.7 DCR is about 13% out but do understand you are talking about products made and used some 40+ years ago.
     
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  17. Kelia

    Kelia Active Member

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    Thank you Mick !!
     
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  18. neikeel

    neikeel Well-Known Member

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    Thank heavens I was busy at work and missed this thread's developments. Funnily enough I had to make a voltage doubler this weekend, I had to put a Partridge 100w PT in an Orange ORST50. Had 360vac (no centre tap) and as typical with bridge rectifier got 500vdc. Problem was max negative bias voltage of 40v (needed -45 to get 35mA of cathode current at idle with EL34s). Drop in an extra 10uF cap in the feed, diode to ground - hey presto max negative volts of 80v = happy days!
    [​IMG]
     
  19. Gutch220

    Gutch220 Well-Known Member

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    From Marshall:

    upload_2019-9-23_13-30-16.png

    upload_2019-9-23_13-30-32.png

    upload_2019-9-23_13-31-10.png


    draw your own conclusions
     
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  20. SmokeyDopey

    SmokeyDopey Well-Known Member VIP Member

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    Thanks guys (and Joanna), I appreciate your time and patience.
     

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