Home Forums GuitarPCB Build Support Sunn Model-T question (answered)

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  • #4452
    Anonymous
    Inactive

    I build the Sunn model-T (and Ratt) and I have some questions. (It all seems to work, but maybe I can tweak it some more)

    Both the gain pots do not go to zero, even complete cc there is still gain left (both channels). The volume is zero completely cc. Because of this if I select only one channel a fair amount of hiss can be heard from the channel that is not used.

    It sounds like the volume increases if i turn the treble cc, bass and mid do not seem to do this. I have the pictures on the show your build tread, I can post them again but I thought I save a little space on the servers.

    I did the biassing (I think) I measured the voltage between the right and left leg of the transistor. 1 and 2 is easy, but 3 and 4 very difficult if I get one right the other is way to high or low. Best I could get is one at about 3.8 and 4.15. Is this the right way?

    Again thank you for all your assistance and if you need more info and measurements please let me know.

     

    #4453
    Barry
    Keymaster

    You need to refocus on biasing. If you are running one too high (6v & up) you will induce noise. Half the supply voltage is what you are looking for at optimal settings.

    It would be much more helpful if we knew the exact voltages on all transistors by number.

    Also it is always helpful to place a photo in the working thread if there is something we need to see so we are not trying to jump back and forth between threads.

    Additionally since the project is designed to emulate an overdriven Sunn type amplifier it will not completely clean up unless you lower the volume on your guitar. (Your guitar pickups are always part of every circuit). We are not emulating the power section of the amp and only the preamp section which is desired for its clipping characteristics.

    Hopefully you socketed your transistors so you can carefully try them in different locations in an effort to get the bias better because no transistor is identical even from the same plant on the same day.

    #4457
    Anonymous
    Inactive

    Thank you. If necessary I will post the pictures if needed. I will post the numbers also. And by your suggestion I socketed the transistors. Is the way I measure the voltage ok (so left and right leg of the transistor?) The voltages of the 2 other transistors are around 4.5 V but I will test some more. Even without the guitar plugged in there is some hiss. I will do some more investigation and post everything including a sound/video clip but this will take some time.

     

    #4459
    Barry
    Keymaster

    If you have noise there is likely another issue but I cannot be sure without testing all transistors first.

    They need to all read 4.5v to 5v.

    If you get that accomplished and still have undesirable noise I would start check  and re-flowing all common grounds both on and off board as well as using an audio probe if that does not work out to more easily find the offending area.

    My circuit does not make undesirable noise and in the year we have been selling them it has never been mentioned as an issue so start with transistors then re-flow common grounds and finally check the entire audio path with an audio probe.

    #4460
    Barry
    Keymaster

    Also have you done the battery test while only using the Sunn-T by itself with no other circuits hooked up?

    Batteries cannot produce noise and by doing the battery test while only using 1 pedal and 2 good cables you will rule out about 15 possible problems that would have to do with power supply.

    #4462
    wilkie1
    Moderator

    You measured the transistor bias incorrectly.  You need to place one probe on the Drain of the transistor and the other probe on a ground point.  The desired voltage is 4.5V- 5.0V.

    #4464
    Anonymous
    Inactive

    Thank you Mr. Wilkie1, that was the solution, all my measurements where to high. Did them the right way and now I was able to get the right voltages on 3 and 4. Also the sound cleaned up nice. When gain is set to zero nice almost complete clean tones (gain at zero is still gain, but no problem) and it crunches nicely when I turn it up. Only thing is you have to set the other channel gain low when you only use one channel for almost clean tones.

    For me that is not a thing, I am going to put a muff and a Ram (Ram is already there) in front of it and distort it like crazy! Just wanted to have a clean tone to be sure the pedal was working fine. I have a very nice test setup so I can reach everything without risk of shortings. It’s an old computer sidepanel.

     

     

    #4470
    wilkie1
    Moderator

    Fabulous!  We look forward to seeing your finished work!

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