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  • DC coupled C.F. question

    Hello, how to avoid grid current into a 12ax7 C.F. DC coupled ,please ?
    The tube is supplied at 300V around and coupled at 75V through a 1M/1M voltage divider from the 150V plate of previous stage.Should be biased at -2V arround but it is not. I instaled 1K grid resistor. I don't have a precise milivoltmeter but on my fluke175 still recorded 0.1mV over it which is pretty evident as time I cannot proper bias my C.F. stage. The actual cathode resistor is 100k and draw almost same amount of current as recorded over 220k plate resistor of previous stage. With 47k seems the derive is better somehow...Please let me know which is the best way to limit the current to none through the CF grid. Thanks.
    Last edited by catalin gramada; 01-12-2019, 06:33 PM.
    "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

  • #2
    Originally posted by catalin gramada View Post
    The actual cathode resistor is 100k and draw almost same amount of current as recorded over 220k plate resistor of previous stage.
    That sounds about right to me. (300-150)/220k = 0.68mA. 75/100k = 0.75mA.

    The plate of the stage driving the CF is at 150V so the grid of the CF should be at 75V and its cathode at 77V. Is it not like that?
    Last edited by Dave H; 01-12-2019, 07:34 PM.

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    • #3
      Originally posted by Dave H View Post
      That sounds about right to me. (300-150)/220k = 0.68mA. 75/100k = 0.75mA.

      The plate of the stage driving the CF is at 150V so the grid of the CF should be at 75V and its cathode at 77V. Is it not like that?
      Thanks for answering. No, the difference is 6V around. I wonder why?
      "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

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      • #4
        You are measuring with one probe on grid and your other probe on cathode? (neither probe to ground)
        Originally posted by Enzo
        I have a sign in my shop that says, "Never think up reasons not to check something."


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        • #5
          No, I measured voltage of divider and voltage over cathode resistor and I did the difference.
          "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

          Comment


          • #6
            Can you post a schematic of the stage with voltages?
            Did you try different tubes?
            At high cathode current grid bias of the DCCF may vary considerably with individual tubes.
            It is not uncommon to find positive grid bias in the DCCF of e.g. an AC30.
            - Own Opinions Only -

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            • #7
              Is a straight design, changed tube, measured resistors and here we go...
              Click image for larger version

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              "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

              Comment


              • #8
                Originally posted by catalin gramada View Post
                Is a straight design, changed tube, measured resistors and here we go...
                [ATTACH=CONFIG]51913[/ATTACH]
                So problem solved?
                - Own Opinions Only -

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                • #9
                  ...
                  Last edited by catalin gramada; 01-12-2019, 09:47 PM.
                  "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

                  Comment


                  • #10
                    Originally posted by catalin gramada View Post
                    ...
                    First of all your voltages indicate that the DCCF has negative grid bias.
                    Most DMMs have an input impedance of 10M, so voltages across an 1M resistor may measure up to 9% low depending on rest of circuit.
                    Do you still measure a positive grid-cathode voltage at the CF, attaching the red meter lead to grid and the black one to cathode with the changed tube?
                    Last edited by Helmholtz; 01-12-2019, 11:41 PM.
                    - Own Opinions Only -

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                    • #11
                      Originally posted by catalin gramada View Post
                      No, I measured voltage of divider and voltage over cathode resistor and I did the difference.
                      Your meter is loading down the voltage divider.
                      Measure the plate voltage of the driver stage and cathode voltage of the CF.
                      Do they measure as you would expect knowing the divider ratio and -2V CF bias voltage?

                      Edit: I see Helmholtz beat me to it

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                      • #12
                        Originally posted by Helmholtz View Post
                        Most DMMs have an input impedance of 10M, so voltages across an 1M resistor may measure up to 10% low depending on rest of circuit.
                        If I insert that parallel 10M into that voltage divider I calculate 71V at the grid.
                        Originally posted by Enzo
                        I have a sign in my shop that says, "Never think up reasons not to check something."


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                        • #13
                          Yes, should be like that. The divider resistors are perfectly matched so the voltage should be half (except if grid C.F. do not start to conduct) In this circumstance the difference was -2.8V around with 100k in cathode. Changed to 47k and adjusted the supply as same as before (300v) get -2.25V
                          "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

                          Comment


                          • #14
                            It have no sense to me. If I want to do the grid less negative in respect with cathode I suppose to raise the cathode value for the same fixed bias in grid.....but is happen exactly opposite....
                            "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

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                            • #15
                              Originally posted by catalin gramada View Post
                              It have no sense to me. If I want to do less negative I suppose to raise the cathode value for the same fixed bias in grid.....but is happen exactly opposite....
                              Why? A lower value cathode resistor requires more cathode current (as cathode voltage follows grid voltage within a couple of volts), thus more positive/less negative grid bias.

                              What is the actual problem?
                              Last edited by Helmholtz; 01-12-2019, 11:44 PM.
                              - Own Opinions Only -

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