Ampeg PF-50T hum fix

Discussion in 'Amps and Cabs [BG]' started by KF2B, Oct 4, 2017.

  1. KF2B


    Jan 28, 2013

    Bumped onto this article and found it interesting.

    Ampeg PF-50T Humbuster

    Any thoughts / experience?
  2. coreyfyfe

    coreyfyfe Supporting Member

    Nov 19, 2007
    boston, ma
  3. bassicstu


    Jun 27, 2007
    I'm having the same experience with my PF 50 T. I'm an electrical engineer. I'm planning on doing the modification. In my area of experience as an electrical engineer, whenever we have a noise issue we add a 0.01 micro farad (uf) capacitor between the positive and or negative voltage supplies and ground at every component. I don't know why it works but it does. I plan on putting a 0.01 uf cap across all the large caps when I do the mod. I don't have access to an oscilloscope or spectrum analyzer. If someone does and has done the mod, and for grins and giggles wants to do a before and after spectrum analyzer screen shot with and without the 0.01 caps I'd love to see if it makes a difference.
    chupacerveza likes this.
  4. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    If you don't know why it works, then perhaps it's not a good idea. In fact, it can be a bad idea as well as a good idea depending on the exact cause of the noise. A big part of engineering (speaking as an EE) is understanding what's happening in a circuit and how each component influences performance. If you don't understand the problem, how can you understand the solution?

    In the case of the hum in the PF-50, adding .01uF will do absolutely nothing because that has nothing to do with the cause of the hum.
  5. JimmyM


    Apr 11, 2005
    Apopka, FL
    Endorsing: Yamaha, Ampeg, Line 6, EMG
    Just follow Bertr's directions and don't question it. I had it done to mine and it's perfect.
    chupacerveza and agedhorse like this.
  6. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    A small sized capacitor in parallel with a much larger capacitor is used to filter high frequency hash that the large capacitor doesn’t catch. You wouldn’t expect to see high frequency noise in this bias circuit. Some rectifiers do have switching noise which can be cured with a cap in parallel with the diode. Not just any cap will do though.
    chupacerveza likes this.
  7. agedhorse

    agedhorse Supporting Member Commercial User

    Feb 12, 2006
    Davis, CA (USA)
    Development Engineer-Mesa, Product Support-Genz Benz
    This was a common trick with (some) old caps that had an effective series inductance which caused a rising impedance as frequency increased. This is generally not the case with modern caps unless there is a large amount of high frequency noise (think SMPS, class D, boost/buck converters, etc).

    Most of the noise issues are better addressed in the design, layout and parts choices. Caps will not (reliably) "fix" a poor design or layout.
    Bassdirty and beans-on-toast like this.
  8. beans-on-toast

    beans-on-toast Supporting Member

    Aug 7, 2008
    Capacitor technology has made great strides. More so than other components I think. It is true that design and layout can circumvent their need. Yet, I still see a lot of small caps in the power supply chain. An example is tube heater circuits. The design world is far from perfect. Some might be forced to use a less expensive lower quality capacitors and fix it by adding a two cent patch.

    An interesting summary of sources of hum in tube amps: article.pdf
    Last edited: May 6, 2020
  9. micguy


    May 17, 2011
    I am myself, and have worked with a lot of EE's - I've had a rather large number of them on my teams, and/or reporting to me over several decades. If someone said those two statements to me in their first year out of college, I'd be gracious and suggest they might want to go and figure that out. An Engineer can and should learn new things - that's part of the job. A few years, in, I'd be rather surprised to hear someone say those two things in a paragraph, unless they were seeking my expertise in learning what was going on, or were reporting something they just found, and said they were working on understanding what was up.

    Power amplifiers are serious business - there are safety issues, which, if you don't know what you're doing, you can shock people or burn their house down. I've designed some, made a few mistakes (which thankfully never hurt anyone), and learned from those mistakes. Never once did I put parts into a design where I didn't know why I was putting them there, or have a good grasp of how they did what I wanted them to do.
    Hawkbone, Richard IV and agedhorse like this.
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