Technical tube question...

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Ferg Deluxe, Aug 20, 2004.


  1. Ferg Deluxe

    Ferg Deluxe Gold Supporting Member

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    Over the last few weeks, I've been doing a lot of reading about tubes -- specifically how they work -- and trying to figure out exactly what's going on inside a tube guitar amp. This is just for my own edification and curiosity, not because I'm troubleshooting a specific problem. Anyhow, during my internet travels I have come across many references describing the origins of tubes and how they work. However, two of these in particular made a reference to a concept that seems "backwards" to me.

    Here are the direct quotes from both references (I'm going to try to give as much context as I can without making the quotes too long):

    1. Suppose now an electrode in the form of a plate is introduced in the bulb and a potential positive with respect to the filament is applied to it. The negative electrons will flow to the plate. Instead of thinking of a flow of electrons from the filament to the plate, we generally think of a flow of current from the plate to the filament; that is, from the positive to the negatively charged body.

    2. The electrons move from the cathode (K), the negative electrode, to the anode or plate (P), the positive electrode. Conventional current is in the opposite direction.


    Both of these references seem to be saying that, inside of a tube, electrons flow from the cathode (negatively charged area) to the anode (positively charged area), but this movement is different from conventional thought about current.

    :confused:

    If electricity is the movement of electrons (which it is), and electrons are negatively charged (which they are) then movement would always be from an area of high negative charge to an area of lower negative charge. How could it go the other way? What am I missing here? These don't seem like misprints, and it struck me when I saw two different pages essentially saying the same thing.

    Thanks!!

    Ferg

    :)

    PS: here are links to the pages where I got these quotes.

    1. http://earlyradiohistory.us/1920vacu.htm

    2. http://www.du.edu/~etuttle/electron/elect27.htm#Theory
     
  2. loverocker

    loverocker Member

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    It's because the early scienticians f*ck*d up. :) They knew diddly squat about electrons and so they assumed that whatever electricity was, it was flowing from the positive to negative/ground. They just couldn't envisage that it might be the other way round.

    50:50 guess and they got it wrong, and every electronics text book and reference since has had to explain why electron flow is in the opposite direction to what we think of as current flow.

    It's like in the power supply of a valve amp - it's conventional to think of the filter caps and resistors as 'supplying' the electricity to the plate, when - of course - they're providing a route for electrons going in the opposite direction.
     
  3. John Phillips

    John Phillips Member

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    Yep, major cause of trouble for every electronics student for the last hundred years! :)

    Those references are exactly right.

    The easiest way to get around it from a practical point of view is to forget all about electrons and just imagine 'positive current' that flows from the plate to the cathode, to start with.

    It doesn't take too much experience and familiarity before you can square the two concepts at the same time.

    Of course it's obvious when you see the way a tube works which way round it is, but the early electrical scientists were working before the invention of the tube (1904, I think), and in batteries, lamps, resistors and such it really isn't at all clear what's really happening.
     
  4. Ferg Deluxe

    Ferg Deluxe Gold Supporting Member

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    Thanks for your responses, guys!! :) Couple questions:



    I'm not sure I understand what you're saying here....to me "electron flow" and "current flow" are the same thing, moving in the same direction in a circuit. Are you implying that most folks have it backwards in their heads and don't know any better?

    You've lost me on this one. :eek: Why would I want to think of current this way? Are my assumptions above incorrect?

    Ferg
     
  5. John Phillips

    John Phillips Member

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    Back before the discovery of electrons, scientists knew that electrical current flowed around a circuit, but they didn't know what carried it. They developed a theoretical model based on positive charge... since until you get to thermionic tubes there really isn't any practical difference. They had a 50:50 chance of labelling this theoretical 'current flow' one way round or the other - they just guessed wrong, that's all.

    So 'conventional current' flows in the opposite direction from the real electrons in the circuit. It's not that most folks don't know any better, just that a system of describing current arose which appears backwards, once you know the true cause of current flow.

    Why would you want to think of current that way? Because from the point of view of most circuit design, with a common negative ground, it's easier.

    In a typical tube, it makes practical sense to have a negative ground, since you don't want the cathode and the heater circuitry (which are in physical contact) separated by a large potential difference.

    Once you get used to it, it makes sense, and you'll have no difficulty working with 'positive current' circuits, even though at the same time you know that electrons are in fact negative.
     
  6. spaceboy

    spaceboy Member

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    owww... my head!

    actually, hold on, i think i already knew this... sort of... i mean, in standard grade chemistry we learned that electrons, being negatively charged, flow towards that which is positively charged; but in tech studies, we learn that current goes from + to - .... wow, and I never even put the two together until now!

    so since we now know that scientists got it wrong, why don't we now think of it the other way around - current flowing - to + ? are people just too lazy to rewrite all the text books?
     
  7. John Phillips

    John Phillips Member

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    In a typical tube circuit it is still easier to think of 'positive current', because of the common negative ground.

    The best advice I can give is: until you can square the two ideas in your head without having to think about it, completely ignore the fact that electrons exist. Just imagine that current is positive and flows through a tube from plate to cathode. It will make understanding circuits and schematics a lot easier.

    If you try to think of it from the 'real' electron flow point of view, it becomes like trying to describe the heights of every object in a room relative to the ceiling, rather than the floor. Not impossible, but a lot harder.


    Now... if you really want you head to hurt, try reading up on solid-state theory ;).
     
  8. spaceboy

    spaceboy Member

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    so... does the output from a tube come from the cathode or the anode...?
     
  9. loverocker

    loverocker Member

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    Ferg - methinks you're trying too hard :) It is only by convention (= the mistaken scientific consensus in the early days of electrickery) that we still think of current flowing from positive to negative. We've since learned that the only movement is of negative charge in the opposite direction, but we don't care. For scientists and electrical engineers, it's just a tiny hiccup in a field with waaay bigger questions.

    Spaceboy - "lazy"? Sort of, but IMHO, it's more a case that by the time scientists are established enough to make a fuss about the mistake, they've already grasped that it really doesn't matter. 'Current' is just a label that happens to mean 'electrons travelling in the opposite direction around a circuit'.
     
  10. spaceboy

    spaceboy Member

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    ok, that's what i thought really, but just one thing - when u say "'Current' is just a label that happens to mean 'electrons travelling in the opposite direction around a circuit'," the electrons still aren't ACTUALLY going round the oposite way, are they? that would still be impossible right? it's just a label for the concept of electrons travelling backwards, to make things easier...?
     
  11. loverocker

    loverocker Member

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    No, what I mean is much simpler: 'Current flowing from A to B in a circuit' means 'electrons are flowing from B to A'.

    FWIW, my background is psychology, and when I first started learning about valve amps, I went through the exact same thoughts ("why don't they just rewrite it all in the correct way?"). But at some point, I stopped worrying about it, and realised the contradiction is just two ways of saying the exact same thing.
     
  12. spaceboy

    spaceboy Member

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    ok, but yet another thing has come to mind... if current is the flow of electrons, and in schematics/diagrams etc. they have the current flow wrong, that is, the current isn't actually doing what they say it's doing, how come these schematics work??
     
  13. John Phillips

    John Phillips Member

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    You're still trying to make it too hard.

    The current is doing exactly what it's supposed to be doing, except that the description of what causes current is in fact backwards. Ignore it, it is not necessary for understanding how circuits work. Forget about electrons for the moment. Just think of 'current', or 'positive charge' flowing round a circuit.

    The only time you even need to know that electrons exist is in describing what actually happens inside a tube, which is not necessary to understand circuits. The tube is like a 'valve' (which is why it's called that in British terminology) which regulates current, that's all.

    To answer the question of whether the output from a tube comes from the anode or cathode - neither. The tube controls the flow of current in the output circuit, and it's the current in the components that are in the rest of the circuit that provide the output - usually a plate resistor or an output transformer connected to the plate, but you can have the output load between the cathode and ground as well if you want.
     
  14. loverocker

    loverocker Member

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    :confused: In the schematics you find on the Net now and in reference books, it's all A-OK: the flow of electrons is known to be from cathode to anode (like the laws of Physics require), and the current flow is correctly imagined to be from anode to cathode (as John mentioned above).

    One and the same thing. You can't have one without the other, and whatever direction one is flowing, the other (by definition) is going in the opposite direction.
     
  15. Ferg Deluxe

    Ferg Deluxe Gold Supporting Member

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    Methinks you are quite correct! (And John too!).

    Thanks again for trying to get us straightened out. Looks like spaceboy and I had the same HS science teacher. :D

    Okay, that's easy to rectify in my own head. Ahem, uh pardon the intentional pun. So basically a lot of my assumptions about electricity have been false. See, I just need to make more time for that electronics class I keep promising myself I would take!

    So...is the common ground actually negative, or do we just think of it that way?


    This is starting to make some sense. My next question was going to be this: if, for example, a rectifier tube is supposed to rectify AC to DC, why is it that the AC is attached to the plate? This makes more sense now...let me see if I can get this straight. The AC current is attached to the plate and alternates between roughly +117 volts and - 117 volts. When the plate is carrying the positively charged side of the AC, "current" flows from the plate to the cathode, but in reality a mess of electrons is actually flowing from the cathode to the plate.

    Fergie
     
  16. Wakarusa

    Wakarusa Member

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    Just when you thought it was safe...
    Whenever you are considering current (or electron) flow and the terms "positive" and "negative", it is a good idea to always include the phrase "with respect to".

    Normally the common ground is zero -- neither positive or negative. However, it is negative with respect to the positive power supply rail, the plates of tubes, etc.
     
  17. loverocker

    loverocker Member

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    Yup - that's it. Although I corrected a minor mistake: strictly speaking, you cannot connect or 'attach a current'. You can only connect a voltage and the laws of Physics dictate if/when current will flow.

    And in a typical valve amp design, you're rectifying the AC after the mains transformer, so it's much higher than 117V, of course.
     
  18. Ferg Deluxe

    Ferg Deluxe Gold Supporting Member

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    Okay, yes that makes sense. You can only introduce a voltage difference -- it's the up to the laws of physics whether or not current will flow. It just so happens that metals conduct electricity because of their pool of outer shell electrons. There I go again -- sorry John! I keep trying to forget that electrons exist, but it's a bit tough since I was a chemistry minor. :D

    This whole thread should be called the Fergie Uncertainty Principle.


    Ah yes, of course. Seem to have, ah, um, forgotten about the mains tranny in my little description. :eek: :D


    You guys rock! :dude
     
  19. spaceboy

    spaceboy Member

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    :D i'll second that one!
     

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