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Old Tube amp

Discussion in 'General Electronics Discussion' started by chris cash, Jun 13, 2015.

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  1. chris cash

    chris cash

    Nov 8, 2014
    Hello all,

    I'm in the process of tearing apart an old tube amp and looking for the source of some issues. I figured I would start by checking the capacitors. On some of them, I'm getting a reading on the multimeter of about double the rated capacitance.

    For example a 22 microfarad cap is reading 46 microfarads. It's also slowly falling as I let the multimeter sit.

    Does this indicate a bad cap? I wasn't expecting to see these values, just super high or super low. if they were bad.

    Thanks all
  2. Martaine2005


    May 12, 2015
    Hi Chris, measuring capacitance wont tell you if the capacitor is bad.
    An ESR (equivalent series resistance) meter is the only method to know for sure if the cap is bad or not. Other than bulged, leaking electrolytics of course.
    On an old unit it is probably good practice to replace all electrolytics with a good make and low esr as they do dry out over the years.
  3. davenn

    davenn Moderator

    Sep 5, 2009
    were you actually using a proper capacitance meter ?
    It would be very unusual for the capacitance value to rise, more commonly they drop in value

    Ohhh and were these the electrolytic ones or not ?
    Non-electro's would probably still be OK ... I would sometimes replace them in old valve radios I restored
    But I ALWAYS replaced the electrolytic ones .... and a neat trick to keep the unit looking original is to pull
    all the guts out of the large canned electro's and mount the smaller new electros inside.

  4. shrtrnd


    Jan 15, 2010
    Don't forget to find somebody who has a tube tester, to make sure those tubes are working properly.
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