A question about tube biasing

Discussion in 'General Electronics Chat' started by boogie, Apr 19, 2011.

  1. boogie

    Thread Starter New Member

    Jan 10, 2010
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    Hello friends

    I'm having some difficulties trying to understand the theory behind tube biasing, specifically in the following example from the NEETS tutorial:
    http://www.tpub.com/content/neets/14178/css/14178_38.htm

    My problem is that in the above link they say that "the grid is more negative (-6 volts) than the cathode"
    Well how is this possible if in the diagram it refers to - http://www.tpub.com/content/neets/14178/css/14178_37.htm
    The cathode is connected to the (-300 volts) power supply. Obviously (-300) is more negative than (-6), so what am I missing?
     
    Last edited: Apr 19, 2011
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Well actually, the cathode is generally connected to ground, so it has a 0v potential.
    The plate supply voltage (usually called B+) is a high positive voltage.
    The grid bias is around -6v, and biased so that the output is around 1/2 to 2/3 the plate voltage.

    In the 2nd link you posted, have a look at the battery labeled Ecc; it's connected with its' positive terminal to the cathode, and the negative terminal to the grid via a signal transformer secondary winding. The winding keeps the average voltage on the grid at the cathode voltage, less Ecc, which is specified as -6v (it actually should have been specified as 6v, as the double negative confuses the issue).
     
  3. boogie

    Thread Starter New Member

    Jan 10, 2010
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    Thanks for your reply SgtWookie but how is this possible? In the diagram, you can clearly see that the cathode is connected to the negative side of Ebb (-300 volts) and from what I understand there must be some negative charge going into the cathode since it emits electrons...
     
  4. boogie

    Thread Starter New Member

    Jan 10, 2010
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    Wait a moment ... I just noticed that the negative terminal of Ecc is also in series with the negative terminal of Ebb so maybe that's it? it explains why the grid is more negative than the cathode (by 6 volts) although, the text presents the grid voltage as an absolute -6 volts (not -6 compared to the cathode which would make the grid -306 volts)
    So ... what am I missing?
     
  5. BillB3857

    Senior Member

    Feb 28, 2009
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    If you look closely at the diagram you referenced in your first post, there is no ground or zero volt reference. If you were to supply ground to the cathode of the tube, I think things will become very clear.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    The text does not give absolute voltages - just relative voltages - and the voltages of Ebb, Ecc.

    Ebb is given as 300 VDC, so the positive side of Ebb is 300 volts more positive than the negative side of Ebb. The negative side of Ebb is connected to the cathode, and also to the positive side of Ecc. Ecc is given as -6 VDC (it should have been specified as 6 VDC instead).

    The negative side of Ecc is connected to the grid via the secondary winding on the transformer, so the average voltage on the grid will be 6v less than the cathode voltage.

    As BillB3857 mentioned, it becomes easier if you consider the cathode as attached to ground, or zero volts.
     
  7. boogie

    Thread Starter New Member

    Jan 10, 2010
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    Thanks ... I think I get it now
     
  8. Adjuster

    Well-Known Member

    Dec 26, 2010
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    There are various ways to apply bias to a tube, not necessarily using a separate voltage source. Whatever method is used, the control grid potential is considered in relation to that of the cathode.

    The diagram you refer to shows battery biassing of the grid to -6V with respect to the cathode, and the anode returned to a load resistor 300V positive of the cathode. There is no ground connection shown, but this is not necessary to establish the relative potential of the electrodes.

    In a lot of tube circuits, the control grid bias is not provided by a dedicated supply. In this case, the grid is typically returned to the negative end of the main supply. Resistance is placed in series with the cathode return so that the cathode is raised to a small positive voltage, so that the grid is relatively negative. See link: http://www.tpub.com/content/neets/14178/css/14178_43.htm

    This wastes a little bit of power, and reduces the relative anode voltage, but it does tend to stabilise the anode current by means of a negative feedback effect.
     
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