Grounding Shield Of Coaxial Wire Shorts Input

Discussion in 'Building the Classics' started by mAx___, Mar 4, 2019.

  1. ampmadscientist

    ampmadscientist Well-Known Member

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    Shielded wire is a good thing.
    You should use shielded wire, for an extra gain stage has much more noise.
    Maybe the insulation is melting when you solder it?

    You should try teflon shielded wire which has a much higher melt point.
     
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  2. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    I know that but it is only 2" long with no supply voltage lines around it.
    As he indicated, the shield was disconnected and all seemed to be working fine.
     
  3. FourT6and2

    FourT6and2 Active Member

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    The reason it's only 2" is because you have the 68K grid resistor on the board. Move it to right on the tube grid pin so the resistor will function as a grid stopper to block RF/EMI. Then use a longer shielded cable. If you keep the resistor on the board and the wire is only 2" long, there is no real benefit to using shielded wire because the wire run AFTER the 68K resistor to the tube socket isn't shielded and the resistor can't function as a grid stopper.

    Grid stopping resistors need to physically be ON the grid of the tube. And they should be metal film to reduce thermal noise. As far as the coax shorting, if your soldering skills aren't good enough and you melt the insulation, try a coax cable made of teflon (it won't melt). I like Belden RG-188U. You can get it at Mouser or Digi-Key or Apex Jr.

    [​IMG]
     
    Last edited: Mar 14, 2019 at 11:24 AM
  4. mAx___

    mAx___ Active Member

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    That's a great tip right there. I will make sure to do that on my next build.
    Off topic but, what's your approach to diminish oscillation with an extra gain stage?
     
  5. FourT6and2

    FourT6and2 Active Member

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    Proper lead dress and component layout.
     
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  6. ampmadscientist

    ampmadscientist Well-Known Member

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    Except that the 68K resistor adds a huge amount of hiss.
     
  7. ampmadscientist

    ampmadscientist Well-Known Member

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    "what's your approach to diminish oscillation with an extra gain stage?"

    :lol::lol::lol:
    uh oh here we go...
     
  8. FourT6and2

    FourT6and2 Active Member

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    That 68K resistor is already in the circuit, so it's not "adding" anything by moving it to the tube socket. If you want to reduce hiss even more, drop it down in value to whatever you want. The lowest value possible while still blocking RF.
     
  9. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Would not a lower value grid stopper allow more upper frequency, noise and hiss?
     
  10. ampmadscientist

    ampmadscientist Well-Known Member

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    The existing resistor will not add any hiss when mounted at the socket but it can pick up AC hum from the filament voltage when mounted at the socket.
    I was taking about the hiss of the resistor itself.
    (resistor is in the magnetic field of the AC filament when mounted at the socket)
    DC filaments will be used...
    That's why they put 68K on the board or at the input jack...
     
  11. FourT6and2

    FourT6and2 Active Member

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    Yes. Which is why I said what I said about using the lowest value possible... while still blocking RF. A lower value has nothing to do with blocking hiss. Hiss is created by the higher value. Thermal noise increases as resistance of the component goes up. Which is why you use a lower value to reduce hiss.
     
  12. FourT6and2

    FourT6and2 Active Member

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    I'm going to repeat myself a third time here for posterity: A grid stopper isn't a grid stopper unless it's... on the grid. Using shielded coax from the input jack to the grid resistor (when not used as a grid stopper) is pointless because the path after the resistor, to the tube grid, isn't shielded. If you're going to use shielded coax to the input grid, the grid stopping resistor needs to physically be on the grid pin of the tube. That way the entire path from the input jack to the tube grid is shielded and will block RF and EMI.

    If you are concerned with hiss, then go read the article by Randall Aiken about resistor types and values, and how they each relate to different types of noise.
     
    Last edited: Mar 14, 2019 at 8:22 PM
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  13. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    You kinda said ....68k.....drop it down in value.....
    That made it seem as use a lower value than 68k.
     
  14. FourT6and2

    FourT6and2 Active Member

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    Yes.
     
  15. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Okayyyyy.
     
  16. FourT6and2

    FourT6and2 Active Member

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    What are you not understanding here?
    Did you read this?
    https://www.aikenamps.com/index.php/resistor-types-does-it-matter
    https://www.aikenamps.com/index.php/grid-resistors-why-are-they-used

    The more resistance you have on the input grid, the more thermal hiss the component will introduce into the circuit. Resistance = heat = hiss. If you use a higher wattage resistor, it will dissipate more heat = a little less hiss. If you use a lower value resistor, it will have less resistance = less heat = less hiss.

    Higher values will block more external RF. That has nothing to do with thermal hiss.

    So you can lower the value of the resistor to reduce thermal hiss, but if you go too low then you will start to let RF noise in. You can go down to like 34K before RF will really matter. You can also use no resistor at all and block RF by other techniques.
     
    Last edited: Mar 14, 2019 at 11:38 PM
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  17. Guitar-Rocker

    Guitar-Rocker Well-Known Member

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    A 7pF 1kV cap in addition to the grid stopper ( the cap is soldered between the plate and grid on the tube socket) works well to silence most anything left, after using the above mentioned methods. I've used that trick plenty.
     
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  18. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    I just said "ok" and I do understand. Do not get your panties in a bunch.
    Still the thermal noise is negligible with decent resistors. Also to mention is no one is frying grid stoppers with their high output pickups.
    I really don't care to read Aiken's stuff though it is generally helpful to a lot of people.

    Hiss/noise is generated by many things and runs the gamut on our hearing bandwidth.
     
    Last edited: Mar 14, 2019 at 11:54 PM
  19. ampmadscientist

    ampmadscientist Well-Known Member

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    "The more resistance you have on the input grid, the more thermal hiss the component will introduce into the circuit."

    He's right.
    But it's not "just" thermal noise, there is several other types of noise from resistors too.

    "Higher values will block more external RF. That has nothing to do with thermal hiss."

    He's right.

    "So you can lower the value of the resistor to reduce thermal hiss,"

    He's right.
    But you can also use better resistors (like bulk metal foil). Or eliminate the resistor.
     
  20. ampmadscientist

    ampmadscientist Well-Known Member

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    "Hiss/noise is generated by many things ..."

    The hiss is mainly from the resistors. The resistors account for at least 70% of the hiss. (verified by extensive testing)
     

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