Not getting a mA reading in one of the pwr tube sockets

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by BrentSP, Dec 21, 2004.


  1. BrentSP

    BrentSP Member

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    3 tubes show a mA reading but one of the sockets just shows 0.00...........I tried swapping out the tubes using the same socket and still 0.00. What does this mean? Am I running on 3 tubes?
    Also I get a plate voltage between 550 - 560 in all four of the sockets just not mA reading in one. Help!

    BTW: this is in a Bogner XTC
     
  2. John Phillips

    John Phillips Member

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    If you are gettting a plate voltage reading but no current (despite trying different tubes), then you almost certainly have a screen-grid resistor failure on that socket.

    Yes, you will be running on three tubes.
     
  3. BrentSP

    BrentSP Member

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    Now I'm not reading the pins as I dont' know how to do that. My mA and PV readings are coming from a Weber Bias Rite. I don't know if this has anything to do with it. By the way all four of the tube lights up and get just as hot as the other tubes.
     
  4. John Phillips

    John Phillips Member

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    The tube will heat up as normal if the filament is working - which is completely unrelated to the high-voltage stuff.

    If you're getting correct readings on the other three tubes, the Bias Rite is working OK.

    If you're getting a plate voltage reading on the suspect socket, both the plate connection and the cathode (ground) connection must be OK. If the tube isn't running away into overload (you'd see the plate glowing red, and measure a very large current), the grid connection - which supplies the bias - must be OK. That just leaves the screen-grid. Without this supplied with the right voltage, the tube will pass no current.

    Chances are that the resistor is cracked or burned out - sometimes they just fail internally for no very obvious reason too.

    The only other possibility is a bad connection between the tube socket and the tube pin, but actually a failed resistor is more likely.

    Check to see what happens when you flip from standby to on - if the tube is passing current at all, you'll see the glow inside the plate change, usually slightly brighter and a small amount of blue.
     
  5. Lonely Raven

    Lonely Raven Member

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    John, I hope you get thanked regularly for your technical
    help!

    You help bring things down to a level us beginners can
    understand rather then browbeating the reader like some
    seem to enjoy doing!

    Thanks!!
     
  6. BrentSP

    BrentSP Member

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    Thanks John. I talked to Mark Cameron and I may just send it to Bogner and let them tune it up for me.
     
  7. Greggy

    Greggy Member

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    Amen to that.:)
     

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