What is this JMP mod?

Discussion in 'The Workbench' started by jhayat, Dec 11, 2019.

  1. jhayat

    jhayat Member

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    Just found this on ebay. Not interested in this, but I am curious nonetheless...

    e-jmp.jpg

    Not looking for en explanation on everything (but if you are in the mood, by all means feel free! :) ), however, if we look towards the front of the PCB, I am curious as to what the grey wire going from that resistor (68k?) to v1 is for. And is that dependent on other stuff?
     
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  2. ampmadscientist

    ampmadscientist Well-Known Member

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    The grey wire is a "hot shield" which is actually a factory design that was used in some models.
    However in this amp the hot shield was added, and not installed at the factory.

    The middle wire of the screened cable is connected to the grid of the preamp tube as usual.
    The shield / screen is connected to the plate of the preamp tube. (the screen of the wire is energized with DC plate voltage)
    The shield of the wire is not grounded, but instead energized.
    This forms a capacitance between plate and grid of the preamp tube producing negative feedback in the first preamp stage.
    The end result being that:
    The frequency response of the first preamp stage is limited; the highs are attenuated.
    The hiss noise will be less. (because the highs are snubbed out of the audio path)
    The microphonic tendencies of the preamp (ringing whistling) are greatly reduced.
    This was probably necessary because the gain of the amp was high and tended to whistle / ring / squeal / hiss / oscillate too much.

    But these problems are really caused by the layout and spacing of the components in the chassis, the noise of the resistors being amplified...
    It's just that the "hot shield" was a cheaper solution to the noise and oscillations. Compared to a more complex layout / screening / better resistors / parts...
     
    Last edited: Dec 12, 2019
  3. South Park

    South Park Well-Known Member

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    It is called a grid stopper it stops feed back in the gain stage. You can jump the resister and get more signal. Some amps it works some amps you get some noice. It is a old school gain mod
     
  4. ampmadscientist

    ampmadscientist Well-Known Member

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    "It is called a grid stopper..."

    [​IMG] [​IMG]
     
  5. Gunner64

    Gunner64 Well-Known Member Premium Member

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    Noise. :)
     
  6. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    Where exactly is this modification you are describing?
     
  7. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    There is a lot of other stuff, more complicated stuff, than that screened wire.

    I would not attempt explaining any of it without a proper diagram or schematic.

    That is why I passed on this thread when it was posted.

    :2c:
    :)
     
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  8. jhayat

    jhayat Member

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    Thanks guys - esp. ams for the full-blown explanation.

    Ok, so not all heads from the same era are the same. Good to know. Why not, tho?

    Cheers.
     
  9. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    No they are pretty much the same but that one has too many modifications to know exactly what is going on from one picture.
     
  10. neikeel

    neikeel Well-Known Member

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    I think the first reply only looked at V1 which looks pretty like the original arrangement in this amp (as mentioned the hot shield).

    What as not commented on is the actual modded part is the extra block on the front of the chassis and the non stock gain/mv pots/switching arrangement linked to the line out on the back of the amp. Why are the input jacks fitted to a piece of paxolin?

    It also appears to have a dual bias set up.
     

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