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Mesa Dual Rectifier 2 channel blowing fuse with new 5U4GB tubes

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  • Mesa Dual Rectifier 2 channel blowing fuse with new 5U4GB tubes

    I got this dual rectifier a few months ago. I noticed some strange sounds and checked the rectifier tubes - I had tried EL34 in place of the stock 6l6s and then when back to the latter - and I noticed one of the stock Mesa brand 5U4G tubes had the keyway broken off.

    I wanted to experiment with new preamp tubes so I also ordered some JJ 5U4GB tubes. Upon putting them in and turning the amp on, the fuse blew. I tried the amp again, this time powering it up with the Silicon diode mode and the amp ran fine till switching to the rectifiers. Rects lit up and the fuse blew.

    Putting the old Mesa tubes back, the amp works fine in the rectifier mode.

    1.) I figured the stock rectifier with the broken keyway meant the tube was no good. Is that not the case?

    2.) From my understanding, there is no difference between the 5U4G and 5U4GB so is it just that the new tubes are duds?

    Click image for larger version

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  • #2
    Tube QC isn't what it used to be. I wouldn't be surprised if you got a new defective tube. Try the new rectifiers one at a time to see if you can determine that one of them is bad. There's no problem using a tube with a broken keyway, other than the possibility of inserting it into the socket wrong. The tube itself is probably fine.
    "Yeah, well, you know, that's just, like, your opinion, man."

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    • #3
      Nothing is fu**ed!

      Thanks, Duder.

      I have a few extra fuses at the hardware store - ordered more online - so I don't want to blow any more but I did see some arcing so I imagine that was it.

      Good to know about the broken Keyway.

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      • #4
        A light bulb limiter would help here- instead of using a box of fuses.
        "Yeah, well, you know, that's just, like, your opinion, man."

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        • #5
          You can use it without the keyway if you want. I usually put a scratch in the base so it's easier to see where to line up.
          If you have the old key you can also glue it back in place.
          Or you can buy replacement slide on type: https://www.tubesandmore.com/product...ce-octal-tubes
          "Everything is better with a tube. I have a customer with an all-tube pacemaker. His heartbeat is steady, reassuring and dependable, not like a modern heartbeat. And if it goes wrong he can fix it himself. You can't do that with SMD." - Mick Bailey

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          • #6
            Just make sure you put it on right! I had a guy bring in an amp that was blowing fuses once. He had put on one of those slide on ones oriented the wrong way. I didn't notice it right away and drove in circles chasing my tail for a while.
            Last edited by The Dude; 05-30-2020, 01:54 AM.
            "Yeah, well, you know, that's just, like, your opinion, man."

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            • #7
              Originally posted by The Dude View Post
              A light bulb limiter would help here- instead of using a box of fuses.
              Ahh. I'll grab my limiter from my shop and just double check the JJ 5U4GBs tomorrow to rule them out.

              And regarding the pin orientation, the other Mesa 5U4G still has it's keyway so I just visually watch that one go in and line up the other accordingly.

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