Upgrading a VOX AD50VT Valvetronix

Discussion in 'Amps & Cabs' started by Dorian, May 8, 2005.

  1. CharlieB

    CharlieB Active Member

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    First, it wasn't an assertion... more like a "I wonder if...."

    On to the wave form thing......

    Every Class A/B audio amp must be push pull. It cannot be any other way. All push pull amps have some sort of phase inverter to which provides near identical, yet mirror image waveforms to the output. This is the same on solid state, tube, or hybrid amps. They all have phase inverters. The adjustment (circuit design) of the phase inverter determines the output symmetry, regardless of the hardware that does the output.

    Tubes like the 12AX7 run class A. There is no reason at all to run a 12AX7 itself as a push pull design. Class A amplifiers are biased such that the output is set between shutoff and saturation... at the midpoint, and both top and bottom halves of the signal waveform are conducted. Why two triodes then? Just to have another one to use for whatever.

    So, the 12AX7 is best used as a class A amp, in the signal path (if its used as an amplifier), and the phase inversion and symmetry (or not) is taken care of immediately before the output section.
     
  2. Dorian

    Dorian Active Member

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    CharlieB: I am not able to comment on the relative importance of the phase inverter versus the output tube itself, especially in a circuit with feedback. Thank for pointing this out.

    As for your doubts about the possibility of AB operation with the 12AX7, I copied a few relevant paragraphs from the operating manual of the AD50VT (and AD30VT, AD15VT). There is no lack of hype in these paragraphs, but my own reading is that the push-pull class AB mode is really emulated, meaning that both triodes in the 12AX7 must play complementary roles for some of the amp models. This is also shown in a block diagram in the manual, although this diagram is pretty blocky. I suppose this may all be just hype, but I am inclined to believe it because I like the tones.

     
  3. CharlieB

    CharlieB Active Member

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    Feedback has nothing to do with it really. Feedback in the output will provide a good deal of damping, plus control the high end oscillation - which may be ultrasonic but very powerful. In this discussion its a non-issue.

    I see what you've read.

    Let me explain -

    What they're saying there is.... our modeling SOUNDS like ClassA/B, ClassA etc etc.. They are not, repeat, NOT putting the amp iself into those classes. The amp is a Class A/B amplifier, probably closer to the B side than A, as is the case with solid state amps.

    Which brings us to the discussion - what exactly does Class A sound like? If done correctly, it sounds the same as Class A/B, or in many cases it has higher fidelity than Class A/B. If done correctly. In guitar amps... hehehe... its not. In fact, there are only a very FEW handful of Class A designs out there. We must not confuse (but we do) Class A with "cathode bias". Most so called, Class A amps are actually cathode biased Class A/B amps, exhibiting the tendancy to compress in an unnatural (and overblown) way. A true Class A design will not compress. The reason for this is that the current per duty cycle remains the same regarless of the input signal - from no signal to maximum signal. Only when the current varies, do we get compression. Yes, as voltages drop we get compression but they drop because of greater current use.....

    So, its interesting to see that Vox uses the vulgar terminology rather than the precise, when describing its operation. I guess this to be expected.
     
  4. Dorian

    Dorian Active Member

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    CharlieB: Clearly we need to see an actual circuit diagram to settle these questions. Vox literature in one place states that their circuit is "patented" and in another place "patent applied for". I could not find a U.S patent number in the manual or printed on my amp. (Maybe someone else can see this on their AD30VT or AD50VT?) My resources to search the U.S. Patent database are limited without more information than I have now, and I was not successful in a cursory search.

    In the meantime, please look at the web page:
    http://www.voxamps.co.uk/products/valvetronix/vtoverview_inside_story.htm
    Their figure is pretty clear that the 12AX7 dual triode is run in push-pull mode. Note claimed location of phase inverter and feedback circuit. Whether this is actually true is another issue, of course.
     
  5. E.Buzz

    E.Buzz Member

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    "Ditto!"
    "Ditto?...Ditto, you provincial putz?"
    [​IMG]
     
  6. CharlieB

    CharlieB Active Member

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    Sure looks like PP to me according to their diagram! Then again, they're a little free with the descriptions of things. I retract what I said, because it looks like they're doing it in PP. Just how... thats another issue.

    I'd still like to see what happens when that tube is pulled!
     
  7. ess

    ess Guest

    somebody pull a frikken toob will ya?!!! :lol:
     
  8. CharlieB

    CharlieB Active Member

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    Pull the toob, but never ever pull the string..... EVER!
     
  9. skidshark

    skidshark Active Member

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    isn't vic spose to dive in there soon?? :shock:

    8) i think he's holdin out for an etsg electric screwdriver!!! 8)
     
  10. vic108

    vic108 Well-Known Member

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    and a throw pillow......
     
  11. ess

    ess Guest

    cant find no string? :x
     
  12. CharlieB

    CharlieB Active Member

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    Maybe its a chain, like on the tinklebox?
     
  13. ess

    ess Guest

    im feeling flush :oops:
     
  14. skidshark

    skidshark Active Member

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    ess about to earn his red wings cb??
     
  15. TNT

    TNT Active Member

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    :shock: TMI TMI TMI TMI
    Is this a fambly forum? :D
     
  16. Gplayer

    Gplayer Member

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    Skids...are you aware of the "military" definition of earning your Red Wings? :lol:
     
  17. Voxman

    Voxman Moderator Staff Member

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    Thought it was the Bibical sense here? :lol:
     
  18. skidshark

    skidshark Active Member

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    i just know the old biker way earning em G !! ....(NO)
     
  19. ess

    ess Guest


    if its anything like the cookie race i aint in. :x
     
  20. skidshark

    skidshark Active Member

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    hmmmm....would someone tell me what a cookie race is?? ( do i dare ask???
     

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