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Vacuum Tube "grids"

Discussion in 'Electronic Design' started by Jon Slaughter, Oct 22, 2008.

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  1. Anyone know if essentially the design of the grids(gird, screen, suppressor,
    etc..) are virtually identical? i.e., could some application potentially use
    the suppression grid for the normal grid and the circuit still work if it is
    not too critical?

    I have some pentodes I'd like to play around with for audio amplification
    and want to try some ideas out such as tieing all the grids together. As far
    as I can tell the construction for the different grids are electrically
    equivalent? Maybe different voltage/current ratings and obviously different
    geometrically but seems beyond that one could play around with it?
  2. MooseFET

    MooseFET Guest

    The positions of the grids matters a lot. The suppressor grid is very
    near the anode. It can't do much to control the current flow. You
    need to make sure that it always stays more negative than the plate.
    It usually is connected to the cathode to do this. If it comes out on
    its own pin, you can run it at other voltages and capacitively couple
    it to the plate.

    The screen grid is sort of in the middle. Its voltage will control
    the current somewhat. It normally is allowed to pass about 10% of the
    plates current. You can run it at a lower voltage and a lower current
    at the cost of some gain. The screen grid is normally at the same AC
    voltage as the cathode so that it protects the control grid area from
    the AC on the plate. You can drive it with a signal to modulate the
    gain of the stage.
  3. Contrary to the popular belief, it is not the grid which controls the
    current from the cathode, it is the electric field in volts-per-metre
    which the grid is partly responsible for creating.

    This may seem like nit-picking, but you can deduce from the correct
    definition that the closer the grid is to the cathode, the steeper is
    the voltage gradient which it can create for a given signal voltage
    swing (thus the greater the amplification of the valve).

    The second and third grids of a pentode are a long way away from the
    cathode and give very little amplification, they also have widely spaced
    turns which give poor control of the current. They are there for a
    different purpose and are custom-designed for that purpose only.

    You could spend the rest of your life learning about valves and still
    only know a fraction of what has been discovered about them - but a good
    place to start for a proper understanding of the basics would be:

    "Radio Engineering" F.E. Terman (McGraw-Hill 1932 or later editions)
  4. Ok, thats all I needed to know. That tells me that the grids are
    functionally identical. Doesn't matter the distance or anything because that
    only modifies the output which I'm not worried about(well, in fact that's
    why I want to know so I can actually modify the output).

    If I can tie them all together that means I can play around with them and
    treat them the same. i.e., treat each one as a control grid using the same
    circuit topology and get different effects. If one grid, say, was
    functionally different than another I couldn't do that. Having a different
    geometry or physically being further away isn't a functional difference.
    Two random resistors are physically and geometrically different yet are
    functionally the same. (This is not to say that changing the resistance
    value won't have a signficant effect on the circuit)

    What you have told me is what I assumed but needed verification. (although
    it's similar to Jan's "No" answer but since I have done my own reading and
    concluded that you have only reinforced it. Jan's "No" contradicts what I
    have read and he doesn't support his claim at all)

    I know how the tube grids work in standard applications but I am using a
    pentode for audio freq amplification which normally one uses a triode. I
    could just disconnect the screen and suppressor but I was wondering what I
    could do to take advantage of them.

    The first idea would seem to be to tie them all together like you mentioned
    as it would act like a "super grid". I do not know how it sounds but thats
    the whole point. There are other possibilities too such as using the screen
    for an attenuator and the supressor as a mirrored control. Again, I do not
    know how well it would work or sound but the point is that I'd like to try
    it but I needed to know if each grid was functionally equivalent.
  5. Tim Shoppa

    Tim Shoppa Guest

    Suppressor grids are sometimes used for AGC or for keying/gating in
    pulse circuitry.

    It is not too uncommon to use tetrodes or pentodes in "triode mode" by
    tying the grid and screen together. That doesn't mean the screen grid
    is the same as the control grid, in fact in beam power tubes the
    screen grid is sometimes better identified as a "beam forming plate".
    A good chunk of tube design for many years was heavily influenced on
    whether you wanted to pay patents to company 1 or company 2, and part
    of the distinction was whether the screen was a screen or a beam-
    forming-plate, and sometimes the words have their definitions twisted
    to mean something you wouldn't think they mean in an attempt to use
    notation to sneak around patent issues.

    I have seen some applications use beam power tubes with nothing - nada
    - attached to the anode plate. They use the screen or beam forming
    plate (which itself can have a respectable dissipation) as the anode.

    There are some interesting heptode/mixer/computer tubes with multiple
    grids for specialized purposes. Look up for example the 6BE6 and its
    many variants for very common examples.

    Tim N3QE
  6. If they were "functionally identical", you could swap the signals on
    the pins without any bad results. Trust me, that doesn't work.
  7. kevin93

    kevin93 Guest


    A more common way is to tie screen to anode.

    The suppressor grid is often not accessible and is tied to cathode

  8. Rich Grise

    Rich Grise Guest

    There are hundreds, maybe thousands, of "specific references", some of
    which have been pointed out to you, but you don't like reading stuff from
    people who are smarter than you.

    And I think it's very rude to get all snitty with the people whom you're
    asking for free help.

    Good Luck!
  9. Rich Grise

    Rich Grise Guest

    Everybody's been trying to tell you that they are NOT "functionally
    identical". They have very specific purposes. The control grid is
    generally the signal input. The screen grid reduces the interelectrode
    capacitance between the control grid and plate, to prevent parasitic
    oscillations. The suppressor grid is there to repel secondary emission
    from the plate, so that it doesn't fall back into the screen, wasting
    power and possibly degrading your signal.

    You can do anything you want, but it's a good idea to learn some
    fundamentals so you have at least a shadow of an idea what you're trying
    to accomplish.

    Good Luck!
  10. Rich Grise

    Rich Grise Guest

    OOps! The screen grid is NEVER a "beam forming plate". The beam forming
    plates replace the suppressor grid.

    Hope This Helps!
  11. Eeyore

    Eeyore Guest


    Their spacing is very important.

  12. Eeyore

    Eeyore Guest

    Then you're reading the wrong books. Even the alignment of the various grid
    wires can be inportant.

  13. Eeyore

    Eeyore Guest

    Slaughter is what should happen to him. Aside from Jamie, he must be the
    dumbest here.

  14. Eeyore

    Eeyore Guest

    WTH have valves got to do with quality audio reproduction ?

    Surely you mean 'audiophool' ?

  15. Eeyore

    Eeyore Guest

    Utter BOLLOCKS.
  16. Eeyore

    Eeyore Guest

    Your problen is that you're STUPID.
  17. You guys don't get it. I don't care about the output. It is for an audio
    project and and I am looking for different effects I can get which want
    those different possibilities. I don't know how many times I have to say it.
    I'm only concerned with the circuit topology used to drive the grids as
    somehow having to be different or I'll end up fucking up the tube. But now
    that I think about it it's quite obvious that the circuit topology can be
    the same.

    E = -phi_n+1 - sum(grad(phi_k)) - phi_0

    where phi_k is the potential of the kth grid, phi_0 is the potential of the
    cathode, and phi_n+1 is the potential of the anode.

    phi_k might depend on material, temperature, geometry, etc... but since the
    ultimate result is the potential(which is basically what I was trying to
    ask) it doesn't matter. I just have to put a voltage across it and only make
    sure I don't go over the current rating. That means that I'll get some
    effect. THE TUBE MIGHT NOT OUTPUT ANYTHING but that is of no concern to me
    as I mentioned I will be playing with "ideas". You guys seem to think I'm
    going to randomly hook up the grids in some configuration for some
    application then run it into mass production. You don't get that I am
    simply going to play around with some idea(And it shouldn't matter) and see
    what kinda outputs I get.

    What is important too me is that I don't blow up the tube because one of hte
    grids, say, happen to be created in a way that resonates at some frequency
    and when I hook up my circuitry too it I end up hiting that resonance and it
    creates an atomic explosion. I've mentioned many times before I'm not
    interested in the signal output but the not ruining the components.

    so, for you what works it's totally different from me. If I hook up
    something to the grid and I don't ruin the tubes then it works... regardless
    of the output. I guess this is a very difficult concept for people in this
    group to comprehend but I guess then again they never "play around" with
    circuit ideas and just copy shit out the book.
  18. Eeyore

    Eeyore Guest

    Slaughter has an aversion to that.

  19. Eeyore

    Eeyore Guest

  20. Eeyore

    Eeyore Guest

    No we get it EXACTLY.

    You're STUPID and ineducable.

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