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  • #16
    stacked plate resistors with a tap in between.
    "Tapping" the plate resistor creates a voltage divider with HT being AC ground. So available output voltage will be reduced. The RL in the gain formula means total plate load. As long as this doesn't change, gain at the plate output stays the same.
    What is the idea behind this question?

    Bypassing the cathode resistor in the input stage reduces heater hum.
    - Own Opinions Only -

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    • #17
      Hi. Input impedance should be huge at circuit input, so guitar signal should not be affected by pickups impedance,no matter how much volume pot is rolling off...I suppose. How big the input capacitor should be , please ?
      I already changed grid resistor with 2M for test and is still quiet.
      "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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      • #18
        Originally posted by catalin gramada View Post
        Hi. Input impedance should be huge at circuit input, so guitar signal should not be affected by pickups impedance,no matter how much volume pot is rolling off...I suppose. How big the input capacitor should be , please ?
        1M input impedance is typically considered high enough. A coupling cap of 10nF with an input impedance of 1M results in a cut-off frequency of 16Hz. A coupling cap is only necessary if the input grid is not at ground potential.
        - Own Opinions Only -

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        • #19
          Thanks. The grid resistor is already bootstrapped not ground referenced so I think it can be reduced in value if noise is concerned.
          "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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          • #20
            One drawback of the circuit is very hard to over drive, too much local feedback, need 10v at input to clip and when it do , it clip very hard..But as first gain stage it is not a concern in my project.
            "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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            • #21
              Originally posted by catalin gramada View Post
              Hi. Input impedance should be huge at circuit input, so guitar signal should not be affected by pickups impedance,no matter how much volume pot is rolling off...I suppose
              I don't think input impedance higher than 1M is going to help much in reducing guitar vol pot (HF) roll off because there is still the capacitance of the cable.

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              • #22
                Originally posted by Helmholtz View Post
                "Tapping" the plate resistor creates a voltage divider with HT being AC ground. So available output voltage will be reduced. The RL in the gain formula means total plate load. As long as this doesn't change, gain at the plate output stays the same.
                What is the idea behind this question?
                if we call stage gain = apparent output voltage / input voltage, then both techniques reduce gain. However, gain of the tube for the two methods is different and the stage's clipping behavior will be different. I see that now, thanks for that.
                If it still won't get loud enough, it's probably broken. - Steve Conner
                If the thing works, stop fixing it. - Enzo
                We need more chaos in music, in art... I'm here to make it. - Justin Thomas
                MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

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                • #23
                  then both techniques reduce gain. However, gain of the tube for the two methods is different and the stage's clipping behavior will be different.
                  Not sure what you mean with "both" techniques/methods?
                  What is the idea behind tapping the plate resistor?
                  - Own Opinions Only -

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                  • #24
                    Input impedance should be huge at circuit input, so guitar signal should not be affected by pickups impedance,no matter how much volume pot is rolling off
                    As Dave H wrote, high input impedance has no benefit regarding HF roll-off caused by turning back the volume. This effect is due to the low pass filter effect caused by the effective series resistance of the vol pot and the cable capacitance.

                    Higher input impedance reduces the damping of the PUs resonance. But as the PUs are already loaded by the controls, an input impedance > 1M won't be an audible improvement.
                    I see no reason for bootstrapping the 1M input resistor, which would increase impedance to several Ms.
                    - Own Opinions Only -

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                    • #25
                      Originally posted by Helmholtz View Post
                      As Dave H wrote, high input impedance has no benefit regarding HF roll-off caused by turning back the volume. This effect is due to the low pass filter effect caused by the effective series resistance of the vol pot and the cable capacitance.

                      Higher input impedance reduces the damping of the PUs resonance. But as the PUs are already loaded by the controls, an input impedance > 1M won't be an audible improvement.
                      I see no reason for bootstrapping the 1M input resistor, which would increase impedance to several Ms.
                      Alles klar!
                      "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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