Replacing Electrolytics - Schematic or Actual Values?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by JCMNelson, Feb 11, 2012.

  1. JCMNelson

    JCMNelson Member

    Sep 30, 2008
    London, UK
    Hi everyone,

    I've just bought a 1974 Bassman 50, which sounds ok, but is lacking volume. While this may not be the issue, I of course need to replace the electrolytic caps. Trouble is, the values in the amp don't seem to relate to any of the possible schematics (AA371, AA270, Bassman 50).

    Here they are:

    2x 70mfd 350V
    3x 20mfd 500V
    1x 16mfd 475V
    1x 1mfd 50V
    1x 5mfd 50V
    3x 25mfd 25V
    2x 80mfd 75V

    They all seem to be original (photo's available) as they are Mallorys and look old!

    So, should just replace the caps with as similar as possible, or work off a schematic?

    Thanks for your help!
  2. VacuumVoodoo

    VacuumVoodoo Member

    Jan 9, 2005
    Gothenburg, Sweden
    These are not absolute values. Electrolytic caps have pretty wide tolerance specs. They can spread +50%/-20% or +20%/-20% or anything in between. Most when new tend to fall within +15%/-15%.
    The thing you need to pay attention to is voltage rating and not replace them with caps rated lower.
    Also, bear in mind that purely decimal series of capacitance values are obsolete, we now have something called E-series. For example E12 series will carry values 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82 and their multiples.
    2x 70mfd 350V ---> buy 68uF or 82uF
    3x 20mfd 500V ---> buy 22uF
    1x 16mfd 475V ---> buy 15uF or 22uF (18uF is rare)
    1x 1mfd 50V ---> buy 1uF
    1x 5mfd 50V ---> buy 4.7uF
    3x 25mfd 25V ---> buy 22uF or 33uF (27uF is rare)
    2x 80mfd 75V ---> buy 82uF

    "m" nowadays denotes "milli" not "micro"
  3. guitarcapo

    guitarcapo Senior Member

    Mar 16, 2010
    I'd pay close attention to those 25uF/25v caps that sit on the cathode of the preamp tubes. If those dry up you get that loss of volume. Of course it could also be weak tubes (preamp or power) Also check voltages everywhere.
  4. Ronsonic

    Ronsonic Member

    May 14, 2006
    Sunny Tampa, Florida
    I say go 100uF @ 350 as the time tested and readily available sub. An' I don' wanna hear about no "overcapping" stuff.

    This one will end up being 100uF @ 100. Nice standard value that doesn't require another catalog and another shipping fee.

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