Matching sections for phase inverter (12AU7) tubes in an MC240

Discussion in 'McIntosh Audio' started by Paul K, Nov 13, 2017.

  1. Paul K

    Paul K Active Member

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    Been banging my head about this. (mainly because I have a pair of Mullard long plates that don't have matched sections)
    I would like to put the best in the unit of course but I also know that the electrical needs outweigh the sound sometimes (son of an engineer ya know!)

    Is it necessary to match the two sections? In the manual Mcintosh describe that you don't need to worry about matching but I suspect they were actually talking about the power tube sections and how they work with the 12BH7s...

    Anybody have a clue as to what is correct for this electrically?
     
  2. Searing75

    Searing75 AK Subscriber Subscriber

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    Output tubes should be matched. No worries on the preamp tubes.
     
  3. 62caddy

    62caddy Trust but verify Subscriber

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    From MC240 Owner's Manual p. 8

    "The excellent performance that is inherent in all Mclntosh amplifiers does not depend on the critical adjustment of bias or balance controls in the output circuit. The patented Mclntosh circuit delivers its advertised specifications without any need for these controls and is not dependent on carefully balanced tubes for its performance."
     
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  4. thevinoman

    thevinoman Nothin' but the Blues... Subscriber

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    So am I reading two different things?....Searing says to match the outputs, and no worries on the preamp tubes, and 62caddy quotes Mac and says the outputs do not require critical adjustment, implying that the preamp tubes are important.....sorry if I'm dense today....
     
  5. Pio1980

    Pio1980 AK Member Subscriber

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    Not to be argumentative, I'd want matched sections in a long tail pair inverter stage, matched everything.
     
  6. 62caddy

    62caddy Trust but verify Subscriber

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    It's been some time since I heard the explanation but it has to do with the way the tubes are coupled in McIntosh's Unity Coupled circuit. In other words, the OT pairs are coupled in a way to make "1 [effective] tube" rather than two independent tubes in which critical matching would be necessary in other amplifiers to perform properly. (I hope I have not botched/oversimplified this explanation too badly due to my poor recollection. :confused: )

    In any case, I accept McIntosh's statement on this point without question. This is not the type of claim Mcintosh would make lightly.
     
  7. Paul K

    Paul K Active Member

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    Interesting...but isn't that referring to the power tubes?
     
  8. 62caddy

    62caddy Trust but verify Subscriber

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    How do you "match" the 12AU7 to the outputs? [​IMG] Is that what you're asking?
     
  9. Paul K

    Paul K Active Member

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    Ah. I may be less eloquent in these matters here!
    Let me see if I can explain a bit better...

    I am asking about both sections in each 12AU7 tube.
    In most amps with this position it's recommended that these sections be matched.
    I see and hear conflicting info regarding whether this is important in a MAC such as the 240 I am using.

    I understand that they are not too bothered by the power tubes not being matched to each other...but in the 12AU7 I understand that this may be performing a more critical function before it gets to the next section of tubes.
     
  10. c_dk

    c_dk Addicted Member

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    Back when these things were current the quality of the tubes was so much higher that in the case of Mac's designs tube matching beyond a good and proper bench test was all that was needed.

    Things changed over next few decades and by the late 70s the new tube quality for output tubes was beginning to be so dismal DOB stopped retubing units at clinics. He would have to reject 7 out of 10 new production tubes to match what the old tubes would do. It got worse. By the mid 80s he would have to cull out 9 of 10 tubes to get the latest production GE6L6GCs to meet spec in a MC240......I got his last 8.

    The small signal tubes were fairing much better and at the same clinic I hosted in the mid 80s the small signal tubes in my 3 MC240s were allowing the unit to meet spec. Only 4 of the mixed bag of 12 output tubes got even close.....like 80%.

    At this point I would and do suspect all vintage tubes unless vigorously tested in the actual application....the same suspicion should be applied to new production also......trust but verify.

    A thorough bench test is the only way to really know.
     
  11. nj pheonix

    nj pheonix AK Subscriber Subscriber

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    First its been explained quite well by Dave Gillespie. Outputs clearly don't need to be matched in the unity output.
    If both sections are in spec though not matched, they should be fine.
    Find someone with a distortion analyzer if you doubt it. I wouldn't sweat it.
     
  12. nj pheonix

    nj pheonix AK Subscriber Subscriber

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  13. Paul K

    Paul K Active Member

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    Thank you. So I assume that this means that my Mullard tubes, although within the range of MU measurement and are not equal in both sections of the tube, are good to go.
    What then would be the advantage of installing my RCA long greys which do have matched sections?
     
  14. 62caddy

    62caddy Trust but verify Subscriber

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    McIntosh had always been designed so that tubes are given a great margin of operation. They are never "run" at 100% capacity nor do they need test 100% in order for McIntosh amplifiers to make spec.

    Even if output tubes are under par, it simply means the maximum output may be somewhat lessened in order for the amplifier to remain under 0.5% THD which was McIntosh standard. The net result is you have perhaps a 15 or 20 wpc amplifier (@ 0.5% THD) instead of 40 or 50 wpc in the case of an MC240. If running a highly sensitive pair of speakers such as K horns for example, the extra power may never be missed so the question is relative.

    It is simply not the case that performance suddenly falls off a cliff simply because one or several tubes are do not test 100%. Not in a McIntosh product.
     
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  15. nj pheonix

    nj pheonix AK Subscriber Subscriber

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    Without the correct gear to test I won't make a blanket statement.
    What I'll say. Tube testers are at best fair to determine tubes' health .
    They generally don't test at actual operating voltages and loads.
    That said,
    Try this.
    Get your wife or your friend (or whoever).
    Sit in your listening position.
    Try a dozen cycles with each unknown. (DON'T SHORT CYCLE AMP) Give it a few (5 maybe?) Off before you turn back on.
    I'd be surprised if you can tell the difference period.
    Or just spend a week listening to either.
    I've found some gear does change sound presentation with rolling.
    I have found Mac tube gear remarkably consistent regardless of brand , type of tube. (Better design, operating points?)
    You be the judge
     
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  16. Paul K

    Paul K Active Member

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    Wonderful advice from everyone. I am amazed at how wonderful this amp sounds.

    It was working and brought up to spec by Michael Samra's great recommendations using Jim McShane's work on his Citations.

    It is marvellous sounding and in great health with strong vintage tubes in it. GE5751 in the first slot, RCA Black Plate 12BH7s, a pair of long gray RCA 12AX7's in the final slots and GE 6L6GC NOS all bought from Brent Jesse.

    I love this amp very much (although have just heard in my house a set of MC30s and really do love them..possibly more? Too different to be honest!)

    Powered by an MX110Z and into a pair of Tannoy 15" Monitor Golds.

    I am a very lucky guy indeed.

    Thank you for all the suggestions and advice everybody!
     
  17. c_dk

    c_dk Addicted Member

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    I may step in it here but just so you know the original designers of these amps, while they would never say anything to you as a Mac owner, behind the scenes if I would bring up as a dealer or salesperson the things you do about judging your tube selection......well they would tear me up!

    Having worked the test bench with DOB a dozen times if the tubes did not allow the unit to make spec into the trash they would go.....black, long, clear, Mullard, GE, RCA, blessed by monks, didn't matter, met spec or did not spec out....DOB wasn't a bad shot into the circular file for a short guy who did not play basketball.
     
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  18. Paul K

    Paul K Active Member

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    Question for you then....

    What would make an input tube not pass muster?
    Could this be measured by a tube tester or audibly?
     
  19. c_dk

    c_dk Addicted Member

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    I would expect to see notch distortion on a oscilloscope, obviously distortion levels would be higher....I can hear Gordon's or DOBs voice in my ear....."some people find some distortion to be pleasing.....you have to tread carefully to not insult them or hurt their feelings".......HOWEVER........
     
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  20. 62caddy

    62caddy Trust but verify Subscriber

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    The instant a brand new tube has power applied, some of its life has been lost. To what extent a tube (or tubes) deterioration must occur before performance is measurably and/or audibly affected is a whole different discussion.

    Any amplifier design that does not allow for the slightest deviation of tube operational spec isn't going to be of much use to anybody.
     
    Last edited: Nov 13, 2017

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