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Blues junior v5 running extremely hot after mods

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  • Blues junior v5 running extremely hot after mods

    I have a cream board 2002 blues junior and the sound has become a little thinner and the el84 in v5 gets very hot and the tip section glows really red hot. I added a allen amps ot20 output transformer,changed r52 from a 22k to a 27k resistor. I also added a choke inplace of the resistor at r47. Any idea what i can check to see why v5 is so red and hot and v4 seems normal. Any help much appreciated. Thanks,bluestom

  • #2
    Swap the output tubes from one socket to the other.
    If the red hot tube stays red hot, then it is bad.

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    • #3
      Originally posted by bluestom50 View Post
      I have a cream board 2002 blues junior and the sound has become a little thinner and the el84 in v5 gets very hot and the tip section glows really red hot. I added a allen amps ot20 output transformer,changed r52 from a 22k to a 27k resistor. I also added a choke inplace of the resistor at r47. Any idea what i can check to see why v5 is so red and hot and v4 seems normal. Any help much appreciated. Thanks,bluestom
      [edit] do what Jazz P says, if the problem stays with the socket, then try... [/edit]

      Links to schematics, gut shots of the work you did, etc. will help everyone to be on the same page with you.

      Did you lose the bias to V5? Check the voltage at pin 2 of V4 and V5. If R32 came loose while soldering R52, there might be a connection (or rather - not a connection!). Did you lose one of the diodes D11 or D12 when soldering in the OT? That's another nearby component.

      Check the bias, check the supply and heater voltages while you're at it. Maybe V4 is not running at all, and V5 is doing all the work. See how close to the diagram all your voltages are now. It's not a bad idea to do that even when the amp mods are good: that way you have a handle on what changed systemically.

      If all the voltages are reasonable and you've touched up all the connections that you made, I'd suspect the OT first. Something in the way it's wired up?
      Last edited by eschertron; 10-21-2013, 11:29 PM. Reason: Jazz P beat my reply
      If it still won't get loud enough, it's probably broken. - Steve Conner
      If the thing works, stop fixing it. - Enzo
      We need more chaos in music, in art... I'm here to make it. - Justin Thomas
      MANY things in human experience can be easily differentiated, yet *impossible* to express as a measurement. - Juan Fahey

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      • #4
        Have you checked the static dissipation of the EL84 plates, eg with a bias probe, after the mods?
        The lower resistance of a choke (~100 ohms compared to 2k2 of the resistor) will cause the screen grid voltage to rise.
        This will make the tube conduct more than before and may result in over dissipation.

        Also the value of screen grid resistors R24 and R25 may now be too low for reasonable tube life, see JJ_EH_tube_tests

        Oscillation is also another cause of overheating, see Billm Audio » Cream Board Phase Inverter Oscillation
        Pete
        My band:- http://www.youtube.com/user/RedwingBand

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