MC-60 tube replacement?

Discussion in 'McIntosh Audio' started by Neo-Luddite, Feb 11, 2019 at 3:17 PM.

  1. Neo-Luddite

    Neo-Luddite Freeloader

    Messages:
    146
    Location:
    Minneapolis, MN
    I had a 6550 blow up yesterday and there's a '27' written on it by the guy who rebuilt them. Each power tube has a different bias number written on it.

    What do I need to do? I can't imagine bringing the amps back in every time they need a tube??

    Thanks much~
     

     

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  2. Pio1980

    Pio1980 AK Member Subscriber

    Messages:
    28,280
    Location:
    Eastern Bamastan
    Replace both with matched pairs.
     
  3. mcgjohn

    mcgjohn Active Member

    Messages:
    144
    Location:
    Chicago area
    yep, agree with Pio1980. Also don't forget to check the audio coupling cap to make sure its not leaking DC.

    am also assuming you have updated the bias rectifier in the amp. If not, would do that before inserting a new pair of tubes.

    good luck.
     
  4. Neo-Luddite

    Neo-Luddite Freeloader

    Messages:
    146
    Location:
    Minneapolis, MN
    Thanks guys...I'm not sure what was done. The other tubes have numbers like 30, 32 and 29 written on them....so I can just buy any matched set?
     
  5. mcgjohn

    mcgjohn Active Member

    Messages:
    144
    Location:
    Chicago area
    you can try to match up a single with your non blown tube, but can be tough since you really dont know how many hours are on the tubes.

    a pair of 6550 or KT88s are not too pricy so would still recommend that option.
    But before you do anything, would suggest you check your coupling caps for leakage and replace the bias supply. Otherwise you will stick in your new tubes and run the risk of smoking your new tubes......If you dont have a variac, make a dim bulb tester so you dont draw too much current and take some DC measurements on the coupling caps. One side should show high DC, and the other side none.
     
  6. nj pheonix

    nj pheonix AK Subscriber Subscriber

    Messages:
    10,273
    Location:
    New Jersey
    Matched tubes aren't necessary in the unity coupled circuit. There's no harm in using them though.
     

     

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