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JTM45 clone High treble channel low volume and no gain when cranked

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  • JTM45 clone High treble channel low volume and no gain when cranked

    Hi to all,
    hope someone can help out.
    My brother bought a second hand JTM45 clone.
    It seems like the normal channel works fine and when you crank the volume pot it distorts nicely.
    The bright channel on the other hand seems a bit quieter and doesn't distort at all when cranked.
    I didn't get to look at it closely. I did quickly measure if everything is connected properly from inputs to about V2 Cathode follower out while parallel working with a schematic.
    The rest was all visual through photos.
    Anyway at first most of the preamp seems fine. Also the V1 cathodes share a 820ohma and 100uF bypass cap, so the channels should have same gain.
    I did notice from photos that the 3 of 4 resistors around V3 Phase inverter look as if they aren't connected.
    PI's pin 3 and 8 arw connected to 470 ohm and than the resistors are interconnected with each other as they should be. But it looks as if one 1Meg is not connected to Pin 2 same as the other to Pin 7. Also the 10kOhm seems as if isn't going to presence or anywhere else...
    Could this be a problem?
    Wouldn't this cause both channels not working or the amp completely?
    If not, what would be the symptoms of this?
    Here is the photo of this part:Click image for larger version

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  • #2
    The low gain difference between the inputs is most probably an assembly error.
    They should be identical.
    Then again maybe the builder 'wanted' the two inputs different.

    jtm45.pdf

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    • #3
      Hi Jazz,
      thanks for answering.
      Any thoughts about those Phase Inverter cathode resistors?
      Also some parts are different value than original.
      I noticed that both mixing resistors are 100kOhm instead of 270 or 470kOhm.
      Also i noticed that on normal channel one leg of the 22nF coupling cap that should be connected to the plate resistor is not connected anywhere as it seems. The other leg is good and going to the pot of normal channel. This is weird. It should mean that the normal channel wouldn't work at all, but it does....
      Last edited by torcamaniac; 05-20-2019, 07:45 AM.

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      • #4
        Check all the tube voltages,then you will understand what is working or not

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        • #5
          Yeah that is what i am planning on doing next.
          But I don't know when I will have time for checking it again...

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          • #6
            Regarding the pictured PI resistors, are you sure there's no solder joints made to those on the bottom side of the tagboard (that would also mean wires and/or components hidden behind as well)?

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            • #7
              Unfortunately I'm not sure. All I have for now are these pics....

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