HometechnicalGrid Leak Bias or Contact Potential Bias.

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Grid Leak Bias or Contact Potential Bias. — 4 Comments

  1. 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.
    https://wireless-girl.com/Projects/AMTransmitters/HeathkitDX60.html
    “For more information, consult this web site:
    https://tubes.njunis.net/?p=256&lang=en
    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

  2. 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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