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measure bias OT resistance method

Discussion in 'Shock Brother's DIY Amps' started by duffer426, Nov 4, 2018.

  1. duffer426

    duffer426 TDPRI Member

    Age:
    64
    38
    Jun 3, 2016
    In.
    Hello: when I measure th OT resistance between center tap and tube plates, my fluke 115 reads OL
    is this normal? thanks
     
  2. David Barnett

    David Barnett Doctor of Teleocity

    This should be a current measurement, not resistance.
     
  3. duffer426

    duffer426 TDPRI Member

    Age:
    64
    38
    Jun 3, 2016
    In.
    on robs sight it says to shut off amp discharge caps, and measure OT resistance at center tap and both tube plates. Unless I mis read the instructions. thanks for your reply
     
  4. duffer426

    duffer426 TDPRI Member

    Age:
    64
    38
    Jun 3, 2016
    In.
    PT is a classic tone 40-18022
    OT is a Classic tone 40-18002
    wall voltage is 122 v
    V3 plate voltage 391.7 voltage drop 7.2
    V4 plate voltage 391.7 voltage drop 6.8
    OT voltage at center tap is 398.5
     
  5. peteb

    peteb Friend of Leo's

    Apr 25, 2003
    Cascadia


    You are on the right track. Your meter is being finicky. Do you have another?


    You should be able to measure a hundred or a couple hundred ohms on that one.




    I’ve seen a meter act up taking that measurement
     
    JuneauMike likes this.
  6. duffer426

    duffer426 TDPRI Member

    Age:
    64
    38
    Jun 3, 2016
    In.
    used my fluke 73 got areading v3 was 4.3 and v4 was 5.6 meter was jumping all over the place, forgot to unplug power cord after cord was unplugged i could not get a reading except OL
     
  7. David Barnett

    David Barnett Doctor of Teleocity

    Sorry, I was thinking of bias shunt method never mind
     
  8. RLee77

    RLee77 Friend of Leo's

    May 15, 2016
    Silicon Valley
    You’ll likely be reading around 30 to 70 ohms, or close to that range. If there is any residual voltage charge in the amp, your meter won’t work correctly.
    I also like to take the meter’s leads into account, so I short the leads I’m using and subtract that reading (around 0.2 or .3) from the actual OT resistance.

    If you get OL, it can just mean you are not making contact with the leads (open circuit).
     
  9. clintj

    clintj Friend of Leo's

    Apr 4, 2015
    Idaho
    If you touch your probe tips together, does it show near zero ohms?

    There may be some residual flux or some oxidation on the points you're measuring. The other possibilities are a blown fuse in the meter, or a bad battery in the meter. Try measuring a known resistor, and for good measure check voltage on a 9V battery. If your meter has an issue, you may not be getting a valid reading on your caps when checking the amp is safe to work inside.
     
  10. jtcnj

    jtcnj Tele-Meister

    Age:
    54
    472
    Feb 2, 2015
    Brick, N.J. USA
    +1 on all the above. Gotta drain voltage off before, verify meter operation, and fish around on the test points to get good contact.

    My experience was similar to yours the first time through.
    I meant to try cleaning up mine with a pencil eraser this time around when I biased a tube swap, but forgot.

    Idk if the pencil eraser would help or not; I meant to find out.
     
  11. jtcnj

    jtcnj Tele-Meister

    Age:
    54
    472
    Feb 2, 2015
    Brick, N.J. USA
    +1 on all the above. Gotta drain voltage off before, verify meter operation, and fish around on the test points to get good contact.

    My experience was similar to yours the first time through.
    I meant to try cleaning up mine with a pencil eraser this time around when I biased a tube swap, but forgot.

    Idk if the pencil eraser would help or not; I meant to find out.
    Not that it matters, but it appears your transformer model numbers dont jive.
     
  12. JuneauMike

    JuneauMike Friend of Leo's

    May 5, 2015
    Alaska
    Typically I remember to drain off the filter caps but wouldn't all electrolytic have the ability to store a charge, however small?

    Wouldn't that mess up a meter when trying to read OT resistance?
     
  13. David Barnett

    David Barnett Doctor of Teleocity

    Yes, many meters will show "out of range" if you try to make a resistance measurement with any DC present.
     
    JuneauMike likes this.
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