Tube Issue on MA2275

Discussion in 'McIntosh Audio' started by Matt The Cat, Feb 8, 2019.

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  1. Matt The Cat

    Matt The Cat Prowlin' At 78 RPMs

    Messages:
    370
    Location:
    Salem, MA USA
    Hi all,

    I noticed the other night as I was going to turn my MA2275 integrated tube amp off that the tube cage was WAAAAY too hot. I determined that the front, left KT88 was extremely hotter than the other 3 KT88s and also smelled bad. I immediately shut the amp off. Next day, I turned it on and sure enough, the front, left KT88 got very hot, very fast, while the other KT88s remained cool to the touch.

    Hoping this was just a tube issue, I removed the 2 KT88s from the left channel and replaced them with the McIntosh issued tubes that came with the amp. I am currently using the reissue Gold Lions. The original Mac tubes worked fine, EXCEPT the front, left KT88 is still way too hot. Again, hoping it was just a tube issue, I replaced the 12AT7 for the left channel, but that didn't sold the issue.

    So, did I blow a resistor or something?

    I'm gutted as I hate to take this 77lb beast out for repairs. What a hassle, not to mention the financial setback. I bought the amp used in 2008, but it was produced in 2005.

    Any help/advice would be most appreciated.

    Thank you.

    Matt
    Salem, MA
     

     

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  2. Brice

    Brice AK Subscriber Subscriber

    Messages:
    1,555
    Location:
    Hoboken NJ
    I never had a MA2275 but if you changed the tubes and sure enough, that tube gets super hot, I am afraid that it means there is something wrong with the amp.
    When you mean super hot, do you see it red plating quickly?
    It could also be the tube losing it's bias from a bad grid contact at the socket level. That happened to me once.

    My 2 cents.
    Brice.
     
  3. TR6man98

    TR6man98 Well-Known Member

    Messages:
    748
    Location:
    Seattle, WA
    Did the offending tube get red?
    I had similar issue on one of my restored MC60 a month or do ago and definitely will need to open the unit, check the voltages, then resistors etc.
    I think you showed issue is not the tube and it's a time for tech to look under the hood.
    I hope you have one close by but i feel your pain; MAC amps are haavy and bear to move.
    Good luck
     
  4. Matt The Cat

    Matt The Cat Prowlin' At 78 RPMs

    Messages:
    370
    Location:
    Salem, MA USA
    Thanks guys,

    I appreciate the responses. No, the tube wasn't red and it didn't look different from the cooler ones, but it was WAY too hot and there was a slight rumble in the left channel (same channel as the offending tube). It was just a little bit of distortion, which was not in the right channel AND did not get louder when the volume was turned up. After turning the amp on, I would say the tube got very hot after about ten minutes or so, while the other 3 KT88s remained cool and could be touched. There were no pops or anything in the left channel either. There was that sort of electric burning smell, which made me nervious that it was probably the amp itself.

    Moving this unit from the attic is going to suck. It's times like this when I regret some of my audio choices...like whenever I have to move my record collection...bad choices, man.

    Matt
     

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