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Relative voltage question

  1. Feb 24, 2016 #1
    I have a question about relative voltages and operating points of components, in this case tubes.
    Are these two functionally the same?
    relative voltagess.png relative voltagess2.png
    For a while now I never could understand how the bottom triode of the WCF in an amp I bought was biased at -100v on the grid and 0 volt on the plate which would put the tube way into cutoff. It just dawned on me that since the cathode is -75v then 0v is effectively +75v on the load line for the triode and -100v is actually -20v.

    It then occurred to me that if 0v = +75v as far as the triode is concerned then can high positive voltages relative to other parts of the amplifier be considered low voltages in the WCF? For example the picture on the left, to each triode the relative voltages are the same as the voltages on the right.
    Does this mean that they would both function the same? In this case is 170v on the output of the left picture effectively the same as 0v to a speaker I hook up to it? (Assuming the other end of the speaker is also 170V)
    Or perhaps if I connected the picture on the left directly to a 150v input stage would it still be considered the same as the picture on the right as far as the tubes are concerned?
     
    Last edited: Feb 24, 2016
  2. jcsd
  3. Feb 24, 2016 #2
    Voltages are really just voltage potentials, and as such they are relative to a reference point. What the reference point is is arbitrary. Meaning, if you add/subtract a fixed value to all voltages involved, nothing changes.
     
  4. Feb 24, 2016 #3
    Well, lets say I have a valve amplification stage and it runs on 150v on the anode.
    So to the amplification stage 150v is 150v.
    Then I directly couple that to the picture on the left.
    Will the picture on the left function exactly as the picture on the right?
    Does 150v become -20v on the grid?

    Does 170v become 0v DC to any speaker I connect to the stage? (assuming I use a balanced set-up)

    Sorry just need to make sure I understand correctly, because LTSpice is not agreeing with me.
     
  5. Feb 24, 2016 #4
    From anything I can see, the second one just has 170V subtracted across the board. As such, it should function exactly the same.
    Unless, you have a fixed ground somewhere that is 0V in both cases. Because then you introduced a common reference point, and when you then subtract 170V in one, the potential differences towards ground are now different.
     
  6. Feb 24, 2016 #5

    jim hardy

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    Does ltspice allow DC across your AC source on upper grid? Were that source a transformer winding , perhaps it'd need a DC blocking capacitor ?

    remember voltage is potential difference
    start at the cathode
    an electron that's just been emitted into the space charge surrounding the cathode of any of those four tubes
    sees a control grid that's 20 volts negative wrt cathode
    and beyond that grid, a plate that's 100 volts positive wrt cathode
    if it can squeeze between those negative wires of the control grid it'll accelerate toward the plate.
    courtesy http://www.recredible.com/view/arduino/488
    upload_2016-2-24_14-21-56.png

    any help ?

    old jim
     
  7. Feb 24, 2016 #6
    Well, the anode of the input stage is 150v above wall ground. If I directly coupled the grid of the output stage to the anode of the input stage then the grid of the output stage would be 150v above ground relative to the ground of the input stage but 20v below ground as far as the output stage sees it.

    This is correct?
     
  8. Feb 24, 2016 #7

    jim hardy

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    Only 150 i see is on grid of upper tube in left figure. Input output and WCF are a mystery to me.

    Electrons inside the tube are oblivious to "ground", they respond only to the fields arising from potential differences between cathode, anode, and grid.

    Your left and right circuits are equivalent for AC provided your AC voltage source does not pass DC.
    An ideal voltage source, as might reside in a simulation program, ought to be a short circuit for DC.

    That's what i think.
     
  9. Feb 24, 2016 #8
    The input stage is not shown in the drawing however the plate of the input stage would be, in this example 150v above standard ground(from the wall) while the grid of the next stage (left picture) would also be 150v above the standard ground used in the input stage if I didn't use a coupling capacitor between them. Just trying to be clear that in this situation the left and the right picture are essentially equal. It seems like a "too easy" way to ditch the much hated coupling capacitor that is steeped in legend as an evil necessity, surely there must be more to it?
     
  10. Feb 24, 2016 #9

    jim hardy

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    seems there is a lot left unsaid.

    Direct coupling tubes can be made to work. Early op-amps were tubes.

    Why do you say coupling capacitors are 'hated' ? They allow for significant AC gain without a zero adjust .
     
  11. Feb 24, 2016 #10
    Most people that talk about audio amplifier design talk about the evils of capacitors, that they only serve to degrade the signal.
    I've experienced huge sound quality increases by changing capacitor quality as well, the difference is equal to or greater than tube rolling.
     
  12. Feb 24, 2016 #11

    jim hardy

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    they're kind of an antenna i suppose.. i make sure to install them with outside foil on lowest impedance or lowest voltage to ground side of circuit , where possible.

    Are polypropylene caps considered "good" in audio circles? I used them for an electrometer with good results.
     
  13. Feb 24, 2016 #12
    Polypropylene caps are considered best next to PIO caps.
    There's been a lot of research into why caps sound different but I don't think anyone has come up with anything conclusive yet.
     
  14. Feb 24, 2016 #13

    davenn

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    because it's all hearsay and totally unscientifically based
    I cant believe some of the garbage relating to capacitors posted on audio forums
    from people who obviously have absolutely no idea what they are talking about

    Capacitors are an essential component in audio gear for AC coupling / DC decoupling between stages etc
    you are not going to get anywhere without them


    Dave
     
  15. Feb 24, 2016 #14
    I was a skeptic too before I tried it myself, there's no denying it now. I find that typically the skeptics are people that haven't done much or any personal testing themselves.

    Are you implying the set-up in my question won't work?
     
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