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KLH 18 FET Front End Question

Discussion in 'Tuners' started by jlovda, Apr 16, 2018.

  1. jlovda

    jlovda Things I loved from the 60's and 70's Subscriber

    Messages:
    3,475
    Location:
    NE Ohio
    I have two KLH 18 tuners and one will be sold. I have too many tuners. One is a very early, first year model with bipolar transistors in the front end, the other is the last version with two FETs; one at the antenna input and a second at the mixer. When FET tuners were introduced in the late 60's, the big selling point was the ability to handle strong signals without overload. If I don't live within several miles of a transmitter, would it make any real world difference in reception between the two?

    Thanks,
    John
     

     

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  2. rcs16

    rcs16 Super Member

    Messages:
    4,070
    Location:
    Baldwin, Ontario, Canada
    Cheaper tuners used jfets and more expensive ones used MOSFETS
    I have a Crown FM2 it uses J309 jfets, it overloads easier than the FM1 which uses 3N141 MOSFETS
    Define several miles? I get signals that peak my sig meter that are 40-50 miles away.
     
  3. jlovda

    jlovda Things I loved from the 60's and 70's Subscriber

    Messages:
    3,475
    Location:
    NE Ohio
    This question was intended to be generic although my personal situation is with regards to my KLH 18 tuners.

    I will use ~50uV since that is the level at which most tuners reach their ultimate quieting.

    In that range,
    1) how much better are junction FETs than bipolar transistors?
    2) how much better are dual gate MOSFETs than junction FETs?

    At what input levels do FETs start to have superior characteristics?
     
  4. rcs16

    rcs16 Super Member

    Messages:
    4,070
    Location:
    Baldwin, Ontario, Canada
    A lot of this depends on circuit design, exact components chosen.
    Many MOSFET designs use the agc capability of a dual gate part to deal with RF levels.
    I have a Yamaha T-85, the way it copes with a very high level signal/over load is to bypass the 1st RF gain stage and present the signal directly to a balanced mixer which has a very high IM spec.
     

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