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Carvin 3200 conversion to JTM45 problem

Discussion in 'Amp Tech Center' started by gtmcgar1, Jan 17, 2021.

  1. gtmcgar1

    gtmcgar1 TDPRI Member

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    I'm doing a conversion for a friend and have hit a snag with the bright channel I hope someone can point me in the right direction. The normal channel seems to be functioning properly, the bright channel farts out with increasing vol (>2 or 3) and is worse with higher treble or middle, the bias reading jumps at the same time.
    I followed the triode electronics layout and component values. Voltages appear to be within 10% of specs. Tried various tubes and re dressed wiring. I'm using the original PT and OT (8ohm out with 2x12 8ohm speakers from the Carvin) Also I re configured the pot order to fit the Carvin graphics and have longer wire runs than I would like, maybe I shouldn't have done that. I'm ok with changing anything to improve the situation it doesn't need to be authentic JTM. Was thinking of redoing the pots first.
    Changes - addition of 5v fil tranny for rectifier, blocking diodes, pot order, Carvin iron, no standby, bias measuring jacks, artificial center tap. Any tips appreciated.

    thanks Gordon
     
  2. Peegoo

    Peegoo Poster Extraordinaire

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    Since one channel is working fine and one channel isn't, and you already swapped tubes around to rule them out as the cause, I'd look closely at the preamp tube sockets for cooked socket contacts and failed solder joints.
     
    boredguy6060 likes this.
  3. Paul G.

    Paul G. Friend of Leo's

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    The channels mix directly after the volume controls on that amp, so there are very few components that are not common to both. Since the bias seems to move with the bright volume pot, I'd zero in on the coupling cap between the plate of V1b and the volume control. If it's leaking AC...
     
  4. goldguitarguy

    goldguitarguy Tele-Meister

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    Sounds like your losing gain maybe if I’m interpreting “farts out” correctly. Could be an issue with the cathode bypass capacitor of 1b. Ensure it is polarized correctly against the schematic or try swapping it.
     
  5. gtmcgar1

    gtmcgar1 TDPRI Member

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    To clarify the bias jumps up and vol cuts out with both middle and treble increase, Vol not so much. I just changed the couple cap 0.022uF checked the old one, ok. Changed cathode bypass cap 330uF at 50v, removed one also checks ok.
    All sockets are new and I see no sign of a problem there. I also move the lead dress to the pot and it seems to have improved things, cuts out now at vol of 5 instead of 2! Thanks for helping me out.

    g
     
  6. dan40

    dan40 Friend of Leo's

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    Since you rearranged the pot order and therefor the wiring to the tonestack, you may have some oscillation occurring that is affecting the bright channel. The wire to the presence pot can be particularly sensitive in these circuits but the presence pot is common to both channels. Could you post some of pics of the pot and socket wiring?
     
  7. gtmcgar1

    gtmcgar1 TDPRI Member

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    I will re-order the pots and try it again I thought about this during the build but I guess I made the wrong call. I'll post a photo after re wire since Im going to swap them about. The Carvin chassis isn't ideal for board alignment to pots and sockets since I reused the pot and socket holes.

    thx for the help g
     
  8. goldguitarguy

    goldguitarguy Tele-Meister

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    You could just chop stick the wires when you’re getting the unwanted sound to see which wires affect it and then just reroute them...
     
  9. gtmcgar1

    gtmcgar1 TDPRI Member

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    Rebuilt the tone stack and re-ordered the pots to match the jtm layout as best I could, still have the problem but its not as bad. I think my next step is to recheck the voltages. Not much room to make the final connections but I checked
    them all with a magnifying glass and they seem fine.
    Pot and socket layout is governed by the re-purposed chassis layout this is about the best I could do to have the most direct runs.
    Any observations appreciated Im about over this build! where's the whisky.

    thx g


    IMG_2188.jpg IMG_2189 2.jpg
     
  10. gtmcgar1

    gtmcgar1 TDPRI Member

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    she's sorted, its was the OT wiring, long runs because of the original transformer position. I re directed the primaries away from the board and everything is stable again. Sounds brilliant on all inputs and quiet. Thanks for the help.

    g
     
    Paul G. likes this.
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