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Channel imbalance—suspected amp

Discussion in 'Tube Audio' started by bigjazzboy, Aug 10, 2018.

  1. bigjazzboy

    bigjazzboy New Member

    Messages:
    31
    Hey all!

    I apologize up front: this is kind of long. I hope it makes sense.

    I made a recent thread pertaining to tube biasing, and I got very helpful responses; this thread, however, is sort of an extension or explanation of that.

    I’m struggling with a channel imbalance on my amp (Antique Sound Lab MG SI 15DT). The right channel is noticeably—not grossly, but nonetheless noticeably—louder than the left. At various peaks, the difference—per the Decibel X app—is several decibels. I have tested numerous records of several different genres (jazz, soul, blues, electronica), and the difference seems relatively consistent.

    Now, I have contacted my turntable’s makers (U-Turn Audio) and followed their isolation strategy for the turntable (reverse RCA cables) and the imbalance did not switch channels; therefore, I was told, the issue is not the table. I trust their judgement, but am ultimately open to other ideas from y’all, of course.

    Some more info: this imbalance also occurs with CD’s played by my Onkyo player; I have switched speaker wires; I have, in fact, switched speakers.

    All said, this makes me the problematic component is the amp. Therein lies my problem.

    HOW to troubleshoot?

    As my earlier thread mentioned, the power tubes (not the preamps, just swapped them) are very old. Could they be the culprit? I hope so honestly; that’s an easy fix.

    As a workaround—if the tubes are the problem—could I bias the weaker channel tube a little hotter than the other? Would that correct it until I replace these old old tubes?
     

     

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  2. Tinkerbelle

    Tinkerbelle Everything in Moderation Staff Member Super Mod Moderator Subscriber

    Messages:
    6,265
    Location:
    Hudson Valley, NY
    From your introductory explanation it does seem to have removed the source and speakers from the list, so seems the issue is in the amp or pre-amp. It certainly could be a tube issue but could also be a component, voltage, contact or other such issue in one channel.

    Where is your other thread?
     
  3. peterh

    peterh AK Subscriber Subscriber

    Messages:
    1,869
    Location:
    Gothenburg,Sweden
    As this is a SE amp with cathode bias you could without adjustment swap the
    power tubes and see if the weaker channel follows the tube.
    And yes, old tubes might do this, tubes are "expendable" and will have a finite life
     
  4. bigjazzboy

    bigjazzboy New Member

    Messages:
    31
    Thanks for your response. How would somebody who—I’ll be honest here—doesn’t have much of a tech bone in his body go about discovering which of those possibilities (component, voltage, contact, and such) is the culprit? Or would a tech-blind person like myself do better by just taking the amp to a local tech for diagnostics?

    Earlier thread is from a day or two ago?
     
  5. bigjazzboy

    bigjazzboy New Member

    Messages:
    31
    That’s a helpful thought. That much I feel I can safely do haha.
     
  6. bigjazzboy

    bigjazzboy New Member

    Messages:
    31
    A hopefully helpful update: when I reversed the power tubes, there was no change in the imbalance, BUT I noticed this gunk when I took them out. Both photos are from the left channel power tube. Possible cause for the difficulty? Especially if the gunk pentrates deeper?
     

    Attached Files:

     

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  7. knockbill

    knockbill Addicted Member

    Messages:
    7,802
    Location:
    SE PA
    Cleaning and tightening the sockets may be the problem, they need to be cleaned regardless... If you have CDP/TT or other input source, a pre amp, and a power amp, take a good look at the RCA jacks and cables connecting them... They need to be clean a tight also...
     
  8. bigjazzboy

    bigjazzboy New Member

    Messages:
    31
    Gotcha. I purchased the amp second-hand and have not cleaned it before. Also--it's my first tube amp. So, how do I got about cleaning the sockets? My apologies if it's an easy process; I just am not familiar with it.
     
  9. knockbill

    knockbill Addicted Member

    Messages:
    7,802
    Location:
    SE PA
    I just use CRC contact cleaner, metal polish also works,,, on a Q tip for RCA jacks,,, pipe cleaners and CRC work well for sockets... Also use a small pick or screwdriver to tighten each socket contact... Do a search on RCA jack/tube socket cleaning, you'll find pictures,,, I try not to use abrasives, (sandpaper) as the contacts are plated, if it is removed the contacts will rust or corode...
     
  10. bigjazzboy

    bigjazzboy New Member

    Messages:
    31
    Got it. Thanks a bunch! I will get on with the cleaning soon.

    In related news—I reversed the preamp tubes and—to my pleasant surprise—the problem of imbalance seems to have gone away. I have no clue why. Maybe I didn’t have em in all the way. Consider me clueless.

    Thanks for all the help y’all
     
  11. knockbill

    knockbill Addicted Member

    Messages:
    7,802
    Location:
    SE PA
    Changing the tubes could have "cleaned" the sockets/tube pins just enough to make better contact,,, that's what scrubbing pins, sockets, jacks etc will get ya,,,
     

     

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  12. bigjazzboy

    bigjazzboy New Member

    Messages:
    31
    Interesting. I didn’t realize it could have so dramatic of an effect. I wonder what will happen with a good cleaning of the other sockets.
     
  13. triode17

    triode17 Super Member

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    1,320
    Don't spray the sockets with contact cleaner, this just makes a mess and can cause conduction between the pins. i bought a bottle of CAIG DeOxit D100 and simply brushed the pins with a brass brush, then I painted them with a bit of D100. Plug them in and out of the socket a few times and its done. Goes for power tubes and rectifiers also.
     
  14. bigjazzboy

    bigjazzboy New Member

    Messages:
    31
    Got it. I wasn’t sure what I should be spraying so that clarifies a bunch. Thanks!
     

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