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Predator 400 amplifier

Discussion in 'Amplifiers' started by DR456, Jun 14, 2018.

  1. DR456

    DR456 Member

    Jan 23, 2017
    Likes Received:
    Can anyone explain why the rf output is wrapped around the input. Is this some kind of cooling choke?


    Attached Files:

  2. Dmans

    Dmans Well-Known Member

    Jan 22, 2017
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    Very odd.
    I've never seen anything like that (said the Blind Man!)
  3. Shadetree Mechanic

    Shadetree Mechanic 808 On The North Side of Dover

    Oct 23, 2017
    Likes Received:
    Seems like it would make the amp run away with feedback like that.
  4. nomadradio

    nomadradio Analog Retentive

    Apr 3, 2005
    Likes Received:
    Billy's ghost appears again.

    As in Billy D. Ward. Pretty sure this was his idea in the old Gray 300 amplifiers.

    Textbook name for this is a "gimmick" capacitor. Wrapping an insulated wire around another one gets you a small-value capacitor.

    The intention is to neutralize the amplifier, and prevent it from oscillating.

    Barely works.

    Better solution would have been to put a negative feedback network across each transistor. A 100 ohm 2 Watt resistor in series with a 0.1uf ceramic disc capacitor is placed with one end of the series pair on the collector of each transistor, the other end on the base lead.

    Works for Texas Star.

    Never did find out why Billy didn't apply such a simple solution.

    Good chance the transistors are MRF455. They have a lot of gain, and this makes them likely to oscillate.

    Once the NFB network is in place, one on each transistor, you can unwrap the blue wire and put a short piece of wire directly from the center pin of the coax socket to the circuit board.

    You should place a SWR meter and coax jumper between the radio and the radio socket on the amplifier after doing this. Key the radio with the amplifier operating, and read the SWR between the radio and the amplifier.

    If it's much over 2 to 1, this could give the radio a headache. Won't try to cover how you would remedy this problem for now. And if it reads below 2 to 1, it's not worth improving.

    #4 nomadradio, Jun 15, 2018
    Last edited: Jun 15, 2018
    Dmans and Shadetree Mechanic like this.

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