Tube Change and Bias Questions

Discussion in 'The Workbench' started by Chris-in-LA, Dec 29, 2019.

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  1. MickeyJ

    MickeyJ Well-Known Member

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    now all you gotta do is work out why you cant get low on the current reading.
    youve asked what the bias tap should be, and you need to find out, may as well find out from the start of the chain.
     
  2. MickeyJ

    MickeyJ Well-Known Member

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    that's for cathode biased .
     
  3. anitoli

    anitoli Well-Known Member

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    Did he not state he has a 1 ohm resistor across the cathode?
     
  4. anitoli

    anitoli Well-Known Member

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    Right here post 1:

    Here’s my first question: I have a 1 ohm resistor soldered from ground to pins 1 and 8 on my power tubes. Would that have an effect on my pin 3 to pin 8 voltage reading?

    He is using a cathode resistor.
     
  5. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    You know what the dissipation should be yes,
    but you have not measured the plate actual milliamps.
    If you want to, you can set the meter for current amps. Then plug the probes into the "A" (10 amps) and common jacks.
    This "A" (amps) jack is a different jack from the "volt" jack on the Fluke meter.
    Then hook the meter as shown above to pin 3 and the OT to measure actual plate current.
     
  6. Nik Henville

    Nik Henville Well-Known Member

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    I
    use
    my ears
    (and my eyes)
    to set the bias, because life's too short.
    I have all the test gear, used to be a leccie-techie, but my ears are none the wiser.

    :hippie::pirate::uk:
     
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  7. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    If you measure across the 1 ohm cathode resistor you are reading screen and plate current combined.
    You are not reading the actual plate current.

    That is why it is nice to have a plate current probe...or you can use a current meter.
    Or you can measure the voltage across the OT winding, and divide the voltage by the resistance of the winding to find the current...etc.
    Or you can use a clamp-on probe to the plate wire.
     
  8. MickeyJ

    MickeyJ Well-Known Member

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    again the sarcasm. and again it goes over my head.
     
  9. MickeyJ

    MickeyJ Well-Known Member

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    nik henville how dare you do that.
    this, being your first rodeo, and your first amp, you will learn that doing it like that will explode the amplifier and possibly burn the dog.
     
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  10. MickeyJ

    MickeyJ Well-Known Member

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    hes using a resistor from the cathode, but its not a cathode resistor in the normal use of that term. its a clever use of voltage measuring, to determine bias current. very effective used by 98 percent of people who arent trying to convince people how clever they are.
     
  11. anitoli

    anitoli Well-Known Member

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    Whatever, it still will get you there.
     
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  12. MickeyJ

    MickeyJ Well-Known Member

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    Exactly.
     
  13. Nik Henville

    Nik Henville Well-Known Member

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    Ohh... you funny man and no messin'...
    My first JTM45 back in the late sixties, my innumerable other dogs along the way amongst which the most memorable were my Marshall plexi PA100, Selmer Zodiac Twin, Carlsbro CS100PA, Fender Champ 12, Fender Bassman 6G6A, Sound City Stidio 20, Traynor YGM3 MkIII, WEM Dominator, Matamp First Lady, Marshall JMP 2204, H&K TM36H and others... and not one exploded, or even red-plated, and no dogs were harmed, either...
    :hippie::pirate::uk:
     
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  14. RLW59

    RLW59 Well-Known Member

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    Hey, this is an improvement. At least he's stopped saying that anybody who doesn't use an oscilloscope is doing it wrong.
     
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  15. MickeyJ

    MickeyJ Well-Known Member

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    i think what helps to confuse people, myself included ( whenever i actually start to realize how these things work, I quickly try and forget, the more you know, the longer it takes to fix an amp) is that the 1 ohm resistor that is either usually there, or put there by people who don't want to use the shunt method, or own a bias probe , is at the the cathode itself, NOT the grid.
    To adjust bias in these amps with a pot, we control the GRID voltage in order to adjust the amount of electrons flowing from the cathode ( pin 8) to the plate ( pin 3).
    So what we are measuring at the pin 8 side of that 1 ohm resistor is the total current flow from inside that output tube, you're measuring all of the ' money shot' current.
    You're measuring all the ' work' that tube is doing in order to get notes/chords coming out of your speaker.( not heater current ' work' though)

    You're measuring/ adjusting one thing, which adjusts/changes another thing.

    All this really is, is a pile of shit when all you wanna do is play the damn thing, but if you DID want to know what people seem to get so excited about ( I most definitely don't) here is an excellent page that explains it.

    https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm
     
  16. Chris-in-LA

    Chris-in-LA Well-Known Member

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    Ok. So, 25 divided by the voltage at pin 3 (420), times .7=.041667. 39ma was too low, it’s now set at 42ma and everything is right with the world. I’ve been playing the amp all afternoon and it sounds great. Thanks to everyone for their input and suggestions.
     
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  17. South Park

    South Park Well-Known Member

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    Take out the 1 ohm resister you take the amp reading at the grid only. Grid current controls plate current . One reading does all . The only math is the wattage . Set bias to 70 percent. 35 miliamps is a good place
     
  18. FourT6and2

    FourT6and2 Active Member

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    Jesus Christ. AmpScientist dude just likes to be pedantic and confuse people. He does this in EVERY thread...

    When you measure your bias setting with 1ohm resistors, you do indeed have a reading that includes the screens. But it's ok. If you calculate your bias this way at 70%, it includes the screens and gives you a little bit of a safety margin so you're actually biased a little cooler. Totally ok. If you want to know how to calculate your screens do this:

    If your screen resistors are the typical 1K, you can just measure the voltage drop across them. i.e. put one probe on one side and another probe on the other and measure voltage. Since the resistors are 1K, you can use ohm's law. Let's say one side reads 460v and the other side reads 457v. That's a 3v drop. That means you can just add 3mA to your bias reading. So if you measure your bias via the 1ohm resistors to ground, and it reads 38mA, you're actually biased at 35mA since the screens eat up 3v. Understand?

    Easy.

    Now if you can't get your bias within range, that's another story. Probably just needs a bias-range resistor swap, which is also easy.
     
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