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  • #11815

    Les Dickson
    CVRS Member

    I opened up a Philips PH103 and there is a sort of divider on the underside of the tube socket for 6SK7GT as shown in the attached photo. I haven’t come across this before and I’m wondering if anyone can enlighten me as to what this is.

    #11819

    Alistair Thomson
    CVRS Member

    It’s hard to tell the exact reason for that screen. The valve base orientation can’t be seen from the pic, but this grounded metal plate is a screen between pins on the base, almost certainly to reduce RF coupling to later stages. This valve is the IF amplifier, so maybe the earlier stage has high levels of original RF frequency, and they wanted to suppress their transfer to later stages.

    This really isn’t much of a problem at broadcast frequencies, but the receiver can receive signals up to 22 MHz, where inter-stage screening becomes important.

    #11821

    John Greenland
    CVRS Member

    Hi,
    If you blow up the pic, the 2 heavy red leads are connected to pin 7.
    That means the plate is between pins 1 and 2 on one side and 6 and 5 on the other.

    This would separate the plate and grid 2 (both higher voltage and probably higher signal level leads) from the other leads.

    Why ???— The previous member’s idea sounds good to me.

    #11822

    Alistair Thomson
    CVRS Member

    I think it might be there to block the pin 4 input, the control grid from the previous stage, from the tube’s output on pin 8.

    Just a guess… it might be trying to reduce parasitic oscillation in a high-gain IF stage.

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