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Define "leaky" caps...

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  • Define "leaky" caps...

    I just picked up a nearly-original 1965 Deluxe. The main filter caps have been changed and a 3-prong cord added. That's it. The internals are completely untouched. I biases up fine - aside from the 450 plate voltage - but admittedly, the pots are *slightly* scratchy. The amp is very quiet, even with the volume all the way up, but if you touch the tone pots in particular they sound like a Marshall presence pot (otherwise quiet when left alone). Most of the blue caps are showing around .4 or .5 mA dc (200mA setting on my meter) on the negative side, but the channel one cap before the mix resistor is showing 39 mA dc and the channel 2 cap before the mixer is showing 8 mA. I don;t know if this could be bleeding back into the tone pots? Or maybe the pots are just old and scratchy? I would prefer to just leave it alone and play it, so my question is, am I hurting anything by having these amounts of leakage? Oddly enough the dual 25uf electrolytic cathode caps are all original, perfect and not leaking at all, nor do the old ceramics seem to be leaking! Any thoughts much appreciated - how far do you let it go before it is really necessary to change out caps?

  • #2
    Leaking caps are...well... leaky.
    Capacitors have many functions.
    The plate coupling caps are the ones that take a beating.
    Plate voltage (DC) on one side & ideally (depending on the circuit) zero volts DC on the other side.
    If the coupling cap in question is leaking, it will leak volts DC.
    This leaking voltage will affect whatever it is coupling to.
    Is this bad or good?
    It all depends on the circuit.
    ps: your post is referencing ma's.
    Should this not be mv's.
    Last edited by Jazz P Bass; 06-19-2010, 08:00 PM.


    • #3
      It is usualy acceptable to leak just under 1/4 volt dc.To me if it leaks anything I would change it,it will likely get worse sooner than later.


      • #4
        Electrolytics get leaky too....more often than film types. Ever had an amp that would blow fuses with the output tubes out? Chances are the pwr supply lytics were off the scale leaky. They draw current on thier own when in this condition.
        The farmer takes a wife, the barber takes a pole....


        • #5
          Oops, yes I meant mV. So 1/4 volt would be ... 250 mV? Wow, nothing leaking that much. Every time I look at that untouched board, I think I can live with a tiny bit of scratchiness in the pots. For now...


          • #6
            Those blue molded caps that came in that amp are pretty good sounding caps....I wouldn't replace those unlss absolutely necessary. Clean the pots with some DeOxit5 and see if they are still scratchy. Any electrolytics in the amp that were not replaced before should be, as they will fail sooner or later, and will affect the tone right now in a poor way since they are so old. Film and ceramic caps don't go bad nearly as often as electrolytics, and ceramics are usually used for oscillation suppression and/or in a trem circuit. You may need to change the plate resistors in the preamp and phase inverter as those could also be causing scratchy or crackling issues. If you do, use the same carbon comp that came in the amp and you'll be golden. Cool amp!