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Mcintosh MC30s Restoration (Looking for correct schematic)

Discussion in 'McIntosh Audio' started by Tysen, Jan 9, 2017.

  1. Tysen

    Tysen AK Subscriber Subscriber

    Messages:
    245
    Location:
    Palm Springs, CA
    Well, I'm still in love the macs. They're better than my knight kb-85, and they're better than my scott 296.


    I have a 8 RCA 6L6GCs...they seem fine, but i have no way of matching/testing them. Is okay to run them in the amps? Obviously, if they glow funny or make noise that's no good, but if things sound good and the amp tests okay, does matching the outputs matter all that much?
     
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  2. x3workshop

    x3workshop AK Subscriber Subscriber

    Messages:
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    As long as they are good tubes (not worn out, shorted, etc.) they don't really need to be matched in a McIntosh amp. The unity coupled circuit performs extremely well in spite of potential mismatches.
     
    Bill Ferris and Tysen like this.
  3. Tysen

    Tysen AK Subscriber Subscriber

    Messages:
    245
    Location:
    Palm Springs, CA
    great. I may need some help sorting the tubes out however.

    I have 5 that appear to match....a sixth that looks to have the same innards as the other 5 but is taller...the other two appear to be grey plates. I'll post photos tonight to get some opinions.
     
  4. Tysen

    Tysen AK Subscriber Subscriber

    Messages:
    245
    Location:
    Palm Springs, CA
    For those that are interested, here are photos of the amps, totally restored!

    IMG_1639.JPG IMG_1640.JPG IMG_1642.JPG


    Using Telefunken 12AX7s; Amperex 12U7s; RCA 12BH7s; RCA 5U4Gs; and Sylvania STR 415 6L6GCs.

    The amps are nearly dead quiet, even with my semi-sensitive Altec 604s. They sound so freakin good.


    Just step is to get a MX110z.
     

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  5. x3workshop

    x3workshop AK Subscriber Subscriber

    Messages:
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    Location:
    Harlem
    Looks good! Bet it sounds great!
     
  6. Tysen

    Tysen AK Subscriber Subscriber

    Messages:
    245
    Location:
    Palm Springs, CA
    Hi All,

    Today I did the yearly tube socket cleaning...my amps have been working great since last March.

    Well, when I turned the amps back on after cleaning them, one of the amps made a pop and one of the output tubes started red plating immediately...I such the amp down quickly.

    I have a replacement tube...is it safe to try turning the amp again? Or should I bring it up slowly with my variac? Transformers are tapped for the 125v line, so I'm running the amps at 125v measured out of the variac...previously the amp biased perfectly at 125v.
     

     

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  7. x3workshop

    x3workshop AK Subscriber Subscriber

    Messages:
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    Location:
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    When you were cleaning the sockets did you also retension them? That is something that I do if I clean the sockets as you may have bent them in the cleaning process.

    I'm not sure what the pop would have been from but it would be wise to take a look underneath for any obvious signs of a problem such as a burned resistor or cap that has purged its guts.
     
  8. Tysen

    Tysen AK Subscriber Subscriber

    Messages:
    245
    Location:
    Palm Springs, CA

    Sorry, X3, missed the notification that you responded.

    I didn't re-tension—still need to learn how to do that!



    Update—I think I may have a slightly bigger problem than I thought. One of the amps has been humming and the volume is now mismatched with the other working MC30. So I got out the old multimeter today and...whenever I try to take voltages from pins 1&2 on V1 & V2, the amp pops and makes a bad noise. I changed out both the 12ax7 in V1 and the 12au7 in V2, but the problem remains. Could this problem be a leaky cap? It would explain my humming, why one of the output tubes blew, and why another one started red plating.

    Please advise!




    UPDATE ***. I turned on the amp without having the 12AX7 in V1 in place—no hum. Amp is dead silent without it.
     
    Last edited: Apr 13, 2018

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