JTM45 Newb Help! GZ34 Voltage

Discussion in 'Building the Classics' started by Joey Stockton, Sep 29, 2019.

  1. neikeel

    neikeel Well-Known Member

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    That is all good info for hum chasing but the OP definitely referred to hiss on the bright channel only.
     
  2. ampmadscientist

    ampmadscientist Well-Known Member

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    These are the wires that need to be shielded:

    MT45.png
     
  3. ampmadscientist

    ampmadscientist Well-Known Member

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    Not a grounding problem.
     
  4. Joey Stockton

    Joey Stockton New Member

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    Sourced some 56K bulk metal foil resistors and replaced the 68K grid stoppers on the bright channel. So far, little to no change in the hiss. In fact, when I initially turned the amp on, it seemed worse, but I’m not sure it didn’t improve after 5 mins or so of run time. Getting late, kids are going to bed so I shall test it further tomorrow. I do think the hiss is acceptable, but I have a JCM 800, DSL100H, DSL40 and a former JTM45 that are all quieter than this amp as far as I can tell. I don’t have a scope, but I’m probably going to start shopping around for one!
     
  5. South Park

    South Park Well-Known Member

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    It being a new build check for cold solder joints. You most likely have a ground loop.chop stick the amp
     
  6. Joey Stockton

    Joey Stockton New Member

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    Just a quick update... the bulk metal foil grid stoppers I installed were the wrong value (57ohm instead of 57k). Mouser sent the wrong ones. Received new Vishay 50k ohm bulk metal foil resistors and installed those on the treble channel. Noticeable improvement in the noise department. I find that the hiss is at an acceptable level now. However, I think I am going to try moving the grid stoppers off the board, solder them directly to the tube sockets and run shielded wire to them and see if it can get any better.

    On a different note, I installed a PPIMV and a split/shared cathode switch. The amp sounds great! Thanks to all that chimed in on this!
     
  7. Metroman

    Metroman Well-Known Member

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    Heres how George at Meto wired up my 2013 LTD ED GMP45 head. Dont know if this helps any.


    [​IMG]
     
  8. ampmadscientist

    ampmadscientist Well-Known Member

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    Sourced some 56K bulk metal foil resistors and replaced the 68K grid stoppers on the bright channel.

    What do these resistors look like?
    Can you post pictures?
     
  9. Joey Stockton

    Joey Stockton New Member

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    EF136C2D-5D54-4490-AC24-7A511D149F6D.jpeg Here are the Vishay resistors... as I mentioned above, mouser sent me 50ohm resistors initially. Installed those and was disappointed in the results. Once I realized the error, I reordered and the ones in place now are 50K, I believe. Definitely reduced the hiss to an acceptable level. As you can see, they are still mounted on the board, but I plan on moving them directly to the V1 socket and running shielded cable to them.
     
  10. MickeyJ

    MickeyJ Well-Known Member

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    Shielded wire, I find, has an entirely undesirable effect in that it wipes some tone out, I use it only to cure oscillation, and then in pieces as short as possible.
     
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  11. neikeel

    neikeel Well-Known Member

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    I just noticed in that pic (maybe I'm slow) but that is not a JTM45 with split cathode and a 0.68uF on V1b.
    Maybe post an overall pic of what you have built?
     
  12. Joey Stockton

    Joey Stockton New Member

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    This amp started life as a JTM45 kit with plans on mods. I've had two previous JTM45s so I wanted to build one to play around with. I added a switch to change between shared & split cathodes on V1, switchable bright caps and changed some other component values to taste, mostly leaning towards a typical lead circuit. Ill see if I can get some better pics uploaded.
     
  13. Joey Stockton

    Joey Stockton New Member

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  14. Joey Stockton

    Joey Stockton New Member

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  15. neikeel

    neikeel Well-Known Member

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    That is helpful as your V1a cathode cap has reversed polarity, should be negative to earth and positive off to the preamp valve.
    You may be able to simply reverse it but it may be irretrievable.
    Do that first and see what happens:fingersx:

    Oh and BTW adding all those cathode bypass caps will add brightness and his c/w a stock JTM 45.

    Try a new electrolytic cap on V1a cathode first
     
  16. Joey Stockton

    Joey Stockton New Member

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    Thanks for pointing out my rookie mistake! I think I have another 25uF cap to swap that one out with.
    To your other point above about the bypass caps adding hiss, is there a better way to add some brightness without introducing more noise? Again, I’m not trying to build a perfect JTM45, been there done that. This amp is going to be my play thing to mod and learn from. So far, I like the mods that I have done just trying to reduce the hiss as much as possible, especially in the bright channel.

    thanks again!
     
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