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How to select a ferrite bead to suppress high frequency

  1. Sep 6, 2016 #1
    Hi guys,

    I see ferrite beads used on the input of V1 on some high end tube guitar amp designs without any indication of type and value.
    Can you give me some idea how to select this component and at what other places in a tube amp it could prove useful?


  2. jcsd
  3. Sep 6, 2016 #2


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  4. Sep 6, 2016 #3


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  5. Sep 6, 2016 #4


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    I am guessing this has to do with the RF interference in your homemade amp? While I won't try to talk you out of installing ferrite beads I would suggest you isolate the interference to a particular stage if you have not already done so.
  6. Sep 6, 2016 #5

    Yes, thinking about ferrite beads did get kicked off by the RF interference issue. I was not however thinking about using ferrite as a solution to that specific issue. The amp chassis setting on the bench had none of the typical shielding that is included in the design and is not overly subject to RFI as best I can tell. The question was more generic in nature.

    I had seen ferrite beads used on other amps and assumed it was there to suppress high frequency. What frequencies they were trying to suppress was/is unknown to me. The schematics I have seen gave no indication of the resistance or inductance values. As they were always placed on the input of the first pre amp tube, I assumed they were there to stop high frequencies that could come in on the guitar and even more so on a long guitar chord. In truth I never paid much attention as they seem never to fail or cause obvious issues.
    As I got to thinking about all this I also wondered if they could be used in some way to prevent high frequency oscillations in tubes in general. The use of ferrite beads in guitar amps is a pretty modern idea or at least I have not seen them in older amps.

    Thanks to everyone for posting the links. The Analog Dialog Article was very useful. Thanks berkeman.

    The problem with electronics is the more I know and come to understand the better I understand how little I know...lol

    Thanks for putting up with me!

  7. Sep 6, 2016 #6


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    My old ham radio mentor always said; "To eliminate noise, go to the source." It's always worked for me. :oldbiggrin:
  8. Sep 6, 2016 #7
    LOL...I agree Don...when I turned off the source, like magic, the noise stopped!!

    For all the consternation the RFI caused it has proven to be a very useful learning tool. The road to success is paved with bricks of failure or so I have been told...

  9. Sep 6, 2016 #8


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  10. Sep 7, 2016 #9


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    The ferrite/inductor works with the input impedance of the amplifier to form a low pass filter (eg that allows audio and blocks RF frequencies).


    Scroll down this page to "RL low pass filter" for the maths.


    The value of the inductor might not be super critical as there is likely to be a reasonable difference between the frequencies you want to pass and those you want to block.

    In some situations ferrites are used to stop noise coming out of an input. For example if you have a fancy computer controlled mixer there might be clock frequencies in the GHz range floating about inside the chassis. These have a habit of finding their way out in unexpected ways. Putting ferrites on inputs might sometimes be done to ensure the input leads don't stop the equipment meeting FCC/CE emissions rules.
  11. Sep 7, 2016 #10
    Thanks for the links. I had not thought of the FCC possibility.

  12. Sep 8, 2016 #11


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    Valve circuits once used series resistors having a low terminal capacitance, with the choke coil(s), lattice-pi-wound on them in electrical parallel with the resistance. They were dipped in paraffin wax to hold the winding in place and to keep out moisture. Those early RFCs dropped little voltage for the DC bias and audio, but presented the resistance to RF signals.

    TwoPies,Lattice-wound-RFC .jpg

    Using a modern low-loss ferrite core alone on a valve electrode connection can make a resonant circuit with electrode capacitance at some frequency. Without resistance, an RF choke makes an energy reflector that keeps the RF energy bouncing around in the circuit. There needs to be somewhere for the unwanted RF energy to go, to be converted to heat and so reduce the gain below unity and kill any possibility of ultrasonic or RF oscillation.

    See; http://www.rfcafe.com/references/electronics-world/select-rf-chokes-may-1966-electronics-world.htm
  13. Sep 8, 2016 #12


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    A 'low spec' ferrite with a lot of HF loss is often a good way to eliminate RFI. No resonance problems,
  14. Sep 9, 2016 #13
    After a good bit of digging into this subject it looks like ferrite beads are not used much in guitar amps. Several of the guys from the DIY community have explored some of the ideas I had been thinking about without much conclusive results.

    In some ways tube guitar amp are pretty low tech devices to begin with. The more I explore the schematic from all the major builders the more I see very similar circuits at least in non hybrid amps. I do see ferrite in hybrid amps with lots of transistors.

    I have done a good bit of experimentation lately and have been amased at how much I can change a circuit and how little difference those changes made in the sound.

    I have no doubt that the ferrite education will prove useful at some point.


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