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    ST120 Blown Bias Resistor

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    Kramer

    Posts : 33
    Join date : 2018-02-11
    Age : 30
    Location : Chicago

    ST120 Blown Bias Resistor

    Post by Kramer on Fri Mar 02, 2018 10:13 pm

    Hi

    I recenty built a st120 kit and everything was going great for a few weeks but recently I had a fuse blow. Unfortunately I was in the other room at the time so I did not see exactly what caused the issue but I had checked the bias of the tubes a few days earlier. I replaced the fuse and installed a different rectifier tube. After startup I tested the bias and one of the bias points tested way high like 23V. I opened the amp and tested the bias resistors, sure enough one has popped.

    Is the blown bias resistor always a sign of a bad output tube?

    I purchased these output tubes just recently and unfortunaly don't have a tube tester. I have some replacement resistors here somewhere and will dig them up tomorrow but would it be worth it to contact the tube seller to find out about replacement or should I wait and test the tube in the amp with a new resistor? Visual inspection of the output tube doesn't show any marks or burns but I know that doesn't mean much.

    Thanks in advance for the advise.

    Ben
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    Bob Latino
    Admin

    Posts : 2647
    Join date : 2008-11-26
    Location : Massachusetts

    Re: ST120 Blown Bias Resistor

    Post by Bob Latino on Fri Mar 02, 2018 10:24 pm

    Kramer wrote:Hi

    I recenty built a st120 kit and everything was going great for a few weeks but recently I had a fuse blow. Unfortunately I was in the other room at the time so I did not see exactly what caused the issue but I had checked the bias of the tubes a few days earlier. I replaced the fuse and installed a different rectifier tube. After startup I tested the bias and one of the bias points tested way high like 23V. I opened the amp and tested the bias resistors, sure enough one has popped.

    Is the blown bias resistor always a sign of a bad output tube?

    I purchased these output tubes just recently and unfortunaly don't have a tube tester. I have some replacement resistors here somewhere and will dig them up tomorrow but would it be worth it to contact the tube seller to find out about replacement or should I wait and test the tube in the amp with a new resistor? Visual inspection of the output tube doesn't show any marks or burns but I know that doesn't mean much.

    Thanks in advance for the advise.

    Ben

    Ben,

    Most of the time a blown 10 ohm bias resistor does mean that tube has a problem. It could also be a contact issue with pin #5 on that tube socket. Pin #5 is where the bias is "injected" into that tube socket. If the tube in the socket loses contact with pin #5, then the bias control for that tube is gone and that tube will draw as much current as it can and blow the 10 ohm resistor which goes from pins 1/8 to chassis ground on that tube socket.

    Bob
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    Kramer

    Posts : 33
    Join date : 2018-02-11
    Age : 30
    Location : Chicago

    Re: ST120 Blown Bias Resistor

    Post by Kramer on Fri Mar 02, 2018 10:45 pm

    Bob Latino wrote:Ben,

    Most of the time a blown 10 ohm bias resistor does mean that tube has a problem. It could also be a contact issue with pin #5 on that tube socket. Pin #5 is where the bias is "injected" into that tube socket. If the tube in the socket loses contact with pin #5, then the bias control for that tube is gone and that tube will draw as much current as it can and blow the 10 ohm resistor which goes from pins 1/8 to chassis ground on that tube socket.

    Bob

    Thanks Bob, I will check the connections on pin #5 and replace the resistor. If the resistor blows again I will replace the tube.

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