JTM45 Clone troubleshoot

Discussion in 'Amp Tech Center' started by iSoft, Jun 2, 2019.

  1. iSoft

    iSoft TDPRI Member

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    I did it a bit above, desoldered HV1 and HV2 and at this point it doesn't blow the fuse :)
     
  2. Wally

    Wally Telefied Ad Free Member

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    My apologies for losing track. Is the bias circuit still connected when the fuse blows with the B+ disconnected?
     
  3. iSoft

    iSoft TDPRI Member

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    If either of both is connected, it blows the fuse (either B+, either Bias or both)
     
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  4. Wally

    Wally Telefied Ad Free Member

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    At this point, I would suggest disconnecting the bias circuit output from the power tubes’ control grids and disconnecting the filter caps’ connections to the B1/2/3/4 power supply nodes. Then see what happens when you fire the amp up in the ‘either or’ scenarios above.....bias alone, B+ alone and both.
     
  5. D'tar

    D'tar Tele-Afflicted

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    I do not design circuits but this looks fishy to me.... There is not a limiting resistor in your bias circuit. No bueno.IMO...



    upload_2019-6-5_8-49-3.png

    like this...

    upload_2019-6-5_8-55-20.png
     
    Last edited: Jun 5, 2019
  6. peteb

    peteb Friend of Leo's

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    Not sure but I think it looks ok, and the schematic looks like it’s been built many times.


    Bias circuits always seem to need two important resistances, one in series, typically before the diode. I would call this one the limiting resistor because the bigger it is less bias voltsge gets thru. On his px 45 schematic this would be the 150k resistor to the left of the diode, d1. Then there is the resistance to ground, which I would not call a limiting resistor because making this resistor big makes for more bias voltage as less bias voltage goes to ground. Together they make a voltage divider.


    On his circuit, both R106 and trimpot 1 are on the path to ground.
     
  7. D'tar

    D'tar Tele-Afflicted

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    @peteb,

    You're right. Disregard my previous post. o_O
     
    Last edited: Jun 6, 2019
  8. robrob

    robrob Poster Extraordinaire Ad Free Member

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  9. iSoft

    iSoft TDPRI Member

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    Hi everyone !

    Si i found the problem for this, it was actually a faulty rectifier Diode... Replaced it and working now ! But have another problem :/

    The power coming out of the rectifiers is around 530V, but on B1, B2, etc... i only get between 70 and 100V (they all should be between 275 and 475V)
    Any idea of what could case this ? Didn't"t find what was wrong.

    Thanks again :)
     
  10. Wally

    Wally Telefied Ad Free Member

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    Bad filter cap? I just recapped a 1991 JTM 45 RI....the first section failed.
     
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