Purpose of grid leak resistor when no attenuator is present

Discussion in 'The Workbench' started by StingRay85, Dec 5, 2020.

  1. StingRay85

    StingRay85 Well-Known Member

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    Can someone explain me the purpose of the grid leak resistor circled in red?

    In the original Soldano SLO-100, there is a OD/clean switch in between, where in the clean channel there is a 2.2M attenuator resistor present, so it works as a voltage divider and signal is reduced.

    But in this case, I fail to see the purpose of this resistor when there is no attenuator present. I'm (a newbie) working on my own idea to make a two channel amp based on the SLO, and try to understand all nuances of this schematic

    Simplified_SLO-100_Schematic.jpg
     
  2. thetragichero

    thetragichero Well-Known Member

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  3. South Park

    South Park Well-Known Member

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    The grid leak resister is to ground out the signal so it knows where to go. The next stage grid is not enough ground for the signal. If nothing goes to ground is thar any power
     
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  4. myersbw

    myersbw Well-Known Member

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    What that resistor does is complete the voltage loop for the cathode-to-grid circuit of that stage. We usually don't think of "ground" with no apparent voltage being a circuit, but there is.

    At the cathode of that same stage...you see 1.8K resistor parallel with the 1uF cap. The cap||resistor sets up your audio frequency bandpass, but now we're just concerned with the 1.8K. When you measure, you're going to have a voltage drop across it. That DC voltage circuit wraps around....through ground and up the grid leak and grid stop resistors. Since, for our purpose, the cathode resistor is positive (acting as our source voltage), we end up with a negative DC voltage on the grid. Sound familiar? Yep, it's sets up the operating bias for that stage just like we do to the power tubes.

    So, when it comes to design, those values are chosen to get just the right swing of voltage for amplifying the smaller input signal. An extreme bias example would be the cold clipper. As the name implies, the bias point is set purposely (and extremely) cold to force a cutoff of about half the signal or to "cold clip" it. But, we like the effect there.

    Yeah, I know...a little lengthy, but...that's why that resistor is there.
     
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  5. thetragichero

    thetragichero Well-Known Member

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    best way to "hear" how the grid resistors work, much like grid stoppers, is to forget to put em somewhere in a build. at least for me
    then you can scratch your head as to why it *kinda* works but sounds godawful... sorta like... a misbiased fuzz
     
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  6. South Park

    South Park Well-Known Member

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    Has any looked at that schematic it looks a lot like a 2204
     
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  7. StingRay85

    StingRay85 Well-Known Member

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    Thanks for the very detailed description. I want to learn. Indeed some of the ideas of the SLO seem inspired by the 2204. It just wants to have one additional gain stage in between to boost the signal before it hits the cold clipper. When I measure mu on ecc83, I find values between 90 and 110. This must have a big influence on performance of the stages.
     
  8. thetragichero

    thetragichero Well-Known Member

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    what's neat about the slo is the relatively low value (1uf) cathode bypass caps keeping the low frequencies trim. kind of a must when you stack a few of em back-to-back and want to have any semblance of clarity (or just go full stoner doom with 50uf cathode bypass and 68nf coupling caps everywhere and low end mush city like the old orange/matamps)
     
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  9. william vogel

    william vogel Well-Known Member

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    The grid leak resistor is a ground reference point for the grid. Just like on the input jack that has a usual 1Meg ground reference/ grid leak, every grid must be referenced to a potential/voltage. Usually there is very little current that flows in the grid circuit of tubes but some current flow exists. This resistor allows a pathway to ground for this current to pass. Without the path, grid voltage will shift and bias will shift. AC coupling with a capacitor between stages has a voltage shift and sources current as the voltage swings with the signal. The resistor references the grid to DC ground potential and allows the AC signal swing to reach the grid without the DC potential. In a long tailed pair phase inverter you’ll notice the grid leak resistors are joined at the junction of the bias resistor and tail resistor. This elevates the grid voltage to the potential of the tail resistor but below the cathode bias voltage. The use of voltage dividers accomplishes two tasks at the same time. The divider references the grid to a potential and reduces signal strength also. The value of the grid leak resistor is important because it effects output impedance of the preceding stage, input impedance of the following stage and forms a high pass filter in AC coupled stages. Low resistance values reduce impedance which lowers gain and raises the high pass frequency of the filter. Usually a high value resistor 1Meg is used to minimize its effect of the above mentioned parameters. Because a SLO has many preamp gain stages, attenuation and shaping MUST be used to lower gain, adjust frequencies and reduce signal strength. Mike’s combination is wonderful and produced a great sound.
     
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  10. StingRay85

    StingRay85 Well-Known Member

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    Thank you. I love this place
     
  11. myersbw

    myersbw Well-Known Member

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    Yeah, and commands a hefty price, too! ;)
     
  12. mickeydg5

    mickeydg5 Well-Known Member

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    It is a load resistor. People with that damn "grid leak" name and other nomenclature.
    That resistor loads not only the grid of V3b for its cathode/grid circuit but it also loads the plate of the previous tube V2b. It sets the gain for V2b and the sensitivity for V3b.

    A RESISTOR is an ATTENUATOR.
     
    Last edited: Dec 16, 2020
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  13. StingRay85

    StingRay85 Well-Known Member

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    I'm still learning. The principle of ground is still a bit of a mystery for me. I see it as something that doesn't interfere anymore, like a raindrop that falls on the ground, it's gone and doesn't influence other raindrops, but that seems not to be the case. One day I will do the brain exercise to reverse the schematic and read it as if ground was B+. In the end the electrons flow to the + part of the voltage difference, it could help with the understanding also of this particular case
     
  14. thetragichero

    thetragichero Well-Known Member

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    don't know if i care too much for that explanation of ground. there's a reason why it's a good idea to keep the grounds of, for instance, the preamp and transformer center tap separated (and why some schematics, like the ampeg svt 3 pro i had, list signal ground along with dirty/sewer ground)
     
  15. mickeydg5

    mickeydg5 Well-Known Member

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    Still, all grounds lead to ground. :)

    What is ground some may ask? Why it is merely a reference, usually a plane of 0V for electricity practicality sake. Earth is Mother Ground for us.
     
    Last edited: Dec 16, 2020

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