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do bigger caps = quieter amp?

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by brad347, Feb 21, 2006.

  1. brad347

    brad347 Member

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    Hi. I was wondering if (within reason) the uf values of the power filter caps could be increased in an amp (say an old Fender) to reduce noise (hum).

    Is this safe for the amp, and will it work?

    Discuss!
     
  2. Fifthstone

    Fifthstone Member

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    I've read conflicting opinions on this. Some say there is such a thing as "over filtering". It's also possible that the hum, if you have it, is a result of something completely unrelated to the power supply filter caps, like routing of leads, or a bad ground.
     
  3. WailinGuy

    WailinGuy Member

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    If there's a hum problem, replacing the filter caps with ones of higher value will not help, assuming the caps being replaced are in good working order.

    I don't recommend using filter caps that are significantly higher in value, compared with the original stock caps. But I don't have any problem with replacing the original 70uF/350V caps with 100uF/350V or original 16uF/450V caps with 20uF/500V. That's not a significant increase in filtering. (BTW, I always use Atoms when replacing filter caps in Fenders.)

    You do have to be careful with amps that have rectifier tubes. Tube rectifiers don't like to see more than a certain amount of initial filtering, since their life span can be shortened. For example, a GZ34/5AR4 doesn't like to see more than 60uF of main filtering. So you're OK if the 1st filter stage consists of two 100uF caps in series; the total capacitance is 50uF, which is under the 60uF limit. But you definitely wouldn't want to use two 220uF caps in series.
     
  4. donnyjaguar

    donnyjaguar Member

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    If you do the math you'd find you need surprisingly little capacitance in your power supply to filter out the 120Hz hum (try 10uF). By adding more capacitance you certainly can reduce the hum further but its the law of diminishing returns. By putting in too much you run the risk of fatiguing other components, as discussed. I'm running 150uF on my home-built amp. I don't need that much, but it was convenient as I had the parts handy and the other parts of the circuit can cope with the inrush current.
     
  5. Gordon

    Gordon Silver Supporting Member

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    The first question that you have to ask yourself is weather the hum that you're hearing is fillament hum or power supply hum. The filament supply should have a ground reference to help keep it quiet. Either a gounded center tap or two 100 ohm resistors to ground. Once you know the fillaments are not causing excessive hum then you should determine if the filter caps are in good workiing order since it's a vintage piece. Old caps should be replaced if you want to play the amp. Increasing filteration affects the tone and feel of an amp more than anything else. I prefer the way vintage amps feel with vintage levels of filteration. Increasing values tend to make the amp stiffer and less musical in my opinion.

    Gordon
     
  6. brad347

    brad347 Member

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    what it was was my fender reverb unit sitting on top of my deluxe non-reverb.

    Both have new filter caps... the reverb unit would hum a little, not bad... but when you cranked up the levels on the reverb to surf-like reverb levels, it would hum a little bit (still not too terrible).

    I figured out, however, that moving the reverb from sittting on the amp to sitting on the floor beside it made a big difference.

    I have concluded that the hum I was hearing was the power supply of the amp being too close to the signal path in the reverb unit... or vice versa.

    Thanks for an interesting discussion.
     
  7. VaughnC

    VaughnC Supporting Member

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    Yup, reverb tanks are great hum antennas! Get them too close to an amps power transformer and they will hum away.
     

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