Eico ST70 - loud initial hum that subsides

Discussion in 'Eico Place' started by oldman55, Aug 26, 2017.

  1. oldman55

    oldman55 Well-Known Member

    Messages:
    758
    Rebuilt/modified ST70 which started out fairly quiet but has developed a loud hum at start up, even at no volume. The hum (both channels) decreases with time and at about 30-45 seconds it is at a manageable level. I was thinking that maybe the CL80 is involved in this problem? Or a tube warming up? Its running at 117v.
     

     

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  2. gadget73

    gadget73 junk junkie Subscriber

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    could be tube mismatch. If you've got the ability to check the tube current individually that might tell you the story. Push-pull amps rely on good tube balance to keep them from humming. Not sure if you have the ability to adjust the bias individually but if they aren't matched, tweaking them to match might quiet it down, or shuffling the tubes around until the two in each channel match up better could be a fix.
     
  3. oldman55

    oldman55 Well-Known Member

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    758
    Went back and checked photos, they have test points and 10 ohm resistors so I am sure I matched up what 7591s I had at the time. But things change. As much as I regret having to do so, I'll put it back on the bench and check. Backbreaking work with this unit. I think they named it a ST70 because it weighs 70 pounds.
    The strange thing is that after heating up, the hum goes away.

    Edit: went back to the old rebuild thread http://audiokarma.org/forums/index.php?threads/one-more-eico-st70-rebuild-thread.757584/page-22
    and can see that they were somewhat mismatched. This was one of the first units that I butchered. Been a long way since last February and I appreciate you guys that have helped me along this far...not that I've gotten that far down the road. Just hope I dont run out of age before I finally get there.
     
    Last edited: Aug 26, 2017
  4. gadget73

    gadget73 junk junkie Subscriber

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    35,829
    Location:
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    It may also be a difference in how fast the tubes warm up. If you've got enough meters to connect to each of the cathode resistors you'd see it. Honestly if it goes away when it warms up I wouldn't be overly concerned about it.
     

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