HometechnicalGrid Leak Bias or Contact Potential Bias.


Grid Leak Bias or Contact Potential Bias. — 6 Comments

  1. First thank you very much for a very comprehensive reseach about this fenoma.
    Do you think this contact potential current & voltage differs conciderebly in hi mu frame grid tubes, like Tesla E83CC, TFK ECC803S & E283CC. (They look from outside more like ECC88’s)
    The TFK ECC803S (frame grid) reputation & price has gone through the roof! To confuse things even more, both AEG & Tesla/JJ ECC803S are long plates, but Tesla E83CC & JJ ECC83 are frame grid types.
    There must be a real technical reason these (specially TFK) are prefered by so many listeners. I’ve considered grid current reason involved, even still biased in the “no grid current area”
    The grid must be more out of the space charge cloud in this high mu type of design. The data sheets for Tesla & TFK frame grid versions seems just to be copies of their earlier long plate designs, but the Philips E283CC shows a little different figures and max 25MOhm for grid current bias.

    How does the contact potential current behave when you bias conciderably above this voltage with f.ex. a cathode resistor? Is the same CPC still flowing through the grid leak, or is it decreasing next to zero?

    Is there any influence of the anode voltage to the grid leak current at the same current if biased above the CPV?

    Many questions, but very intresing field to investigate.

    • Hi,
      Yes, it is an interesting field to explore.
      The contact potential voltage depends on the material used for the cathode and the grid, to be more specific, the potential generated by two metals in contact or substantially close to each other in air or vacuum. the potential depends on the work function of the materials used (or the surfaces of the materials and the heating of these), and I don’t think that a frame grid valve would be any different regarding this. The contact potential voltage will add to the grid voltage set by the bias, if the resistance in the grid circuit is large enough so that the cp voltage can build up over the grid resistor.

      The current due to the contact potential voltage is an extremely small current, and you need a very high resistance in the grid circuit for the contact potential voltage to remain at the grid, and don’t “leak” to ground.

      TFK ECC803S is among the finest valves ever produced, it has low microphonics, very low noise and the two sections are very well matched, but it was produced in a very limited quantity and are practically impossible to obtain today, and if you find it the price is extremely high.

      The measurements I have done sow far, show no influence of the anode voltage on the grid leak current, it also seems to be independent of how the grid is biased. But remember, as said before, it is an extremely small current.

      What is very different when you look at a frame grid valve is, of course the frame grid itself, which is very tightly and precisely wound, and with a very thin wire, but also that the distance between the cathode and the grid is very small compared to a “normal” valve. Both these things together should give us a valve with a relatively high gm. ECC88 has a gm of 12,5 ma/volt.

      The tight and precise winding of the very thin grid wire in a frame grid valve will in most cases give us a valve with very smooth and linear curves and thereby, theoretically, less distortion than a valve with an “normal” grid, and this may be one of the reasons why these valves are preferred by many.

      But as said before, it is a very interesting subject an I will try to test some frame grid valves and compare these to “normal” valves to see if there is any measurable difference when regarding the contact potential voltage, and other parameters measured and publish the results here.

  2. Amazing body of work. I have not seen such in depth discussion of hollow state since college in the early 1960s. Thank you for the engineering level presentation.
    I linked your page on “contact potential bias” in a discussion of vintage amateur radio gear. I hope the brief quote is OK.
    “For more information, consult this web site:
    Which states:
    “The manufacturers did try to control contact potential in the valve types commonly used in this mode, but there may anyway be considerable variations between similar valves, and during valve life span, and this must be taken into account when designing such circuits. Now comes the question; how do we calculate the bias voltage when using contact potential bias; the answer is, it is practically impossible. If the valve is biased, by other means, outside the contact potential bias region, but is driven into it by positive peaks that causes grid current to flow, the signal source is also loaded, and the result could be distortion in audio amplifiers.” This is particularly important, considering the proliferation of Chinese tubes these days.”

    • Thank you for the nice words. It is perfectly fine for you to use this as long as you keep the reference to my page

  3. What are the effects of different value grid leak resistors? I see 5-20M as a common range. The Bogen CH-18 I have uses a 1M grid leak for a 6SF5

    • Hi, different values of grid leak resistors are not going to make much of a difference, as long as it is large enough, in the order of 5 – 20 Mohms. 1 M grid leak resistor is a very small value to be used in a grid leak biased system, but it has been used in several PA amplifiers for mic. input. The 1 M resistor will make a very low bias voltage, almost “zero” bias. But it is used as a mic input, and the microphone signals usually has a very low value, in the order of 0.001 to 0.01 volt so it should not be of any problem. If you are going to use this input for something other than a mic, I would suggest that you modify the input circuit to a ordinary biased stage with a cathode resistor.

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