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Thread: Help. Sig:x squealing

  1. #1
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    Default Help. Sig:x squealing

    Hello

    I purchased a used vht Sig a few weeks back and it has running fine. today i switched it on and the amp is making a high pitched squeal whenever i stop playing. its not feedback, more like the whistle you get from a tea kettle. I figured that it must be a microfonic tube so i opened up the back and gave all the tubes a tap while it was on. Nothing.

    I have noticed that the whistling gets quieter when i keep my hands on the strings of the guitar. I have tested 4 different guitars and two cables and they all do the same thing.

    any other ideas as to what the problem could be ?

    Thanks

  2. #2
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    Turn it on ( low volume ) and pull out the pre ( 12ax7 ) tubes one at a time starting with v1 , remove 1 at a time only and see if noise stops once a certain faulty tube is pulled .
    Don't let it get too hot or electrocute yourself obviously

  3. #3
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    Hello cam. I actually did that last night. I had four unused tung - sol 12ax7s laying around. I tested all of them in every position with no luck.
    Unfortunately I do not have any extra kt88s laying around to test those.

    I also noticed that it is gain related mostly. On the clean channel there has always been a tinny bit of a rough sound. not really what i would call a driven sound. on the rhythm channel without the boost it dose not squeal until i add the boost or an od pedal. the lead channel squeals straight away unless i back of the gain.

    The previous owner said all the tubes where new... i have taken the amp out of its case to see if it has been biased correctly. maybe its a dying kt88.

    I have also noticed that on the boards that this amp is a revision 1.0. I have heard about some kind of update for these older 2009 models. any idea what that is ? I cant send my amp from Sweden to the US to get upgraded or repaired.

  4. #4
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    Ok. so i tried to set the bias with limited luck. first off both 40w and 100w where at about 85 - 89 vdc.
    So I adjusted the LP up to 140vdc, then i switched to 100w mode where it was now running at 139vdc.
    I turned the trimmer pot down but it did nothing, I turned it back to the middle and still nothing...
    The HP trim pot seems to do nothing at all.

    I tried all the different channels and all 40w - 100w switches with the same result.

    I tested the resistance of the HP trim pot and it is working fine. I searched for continuity from the output of the HP trim pot to the bias test point and there was nothing. Am i missing something here ?

  5. #5
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    could it be that the amp is stuck in 40w mode ? broken relay ? that would make sense to me. especially because I do not notice any difference in sound when switching between 40 and 100w.

  6. #6
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    Ah ok , weird - does it still squeal with no guitar plugged in at all to the input on a high gain setting ?
    I'm not that familiar with the sig x - if its not just a faulty tube I'm out of ideas , unless a bad power tube is making it stuck in 40w mode .
    Or maybe power source related ?
    Are you plugged in where it previously worked fine ?
    Not sure if swapping the phase inverter/ driver tube also would help .
    Could try emailing support ( Dave ) and see if he has any ideas if you have no luck ?

  7. #7
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    spent the afternoon talking back and forth with Dave. traced the 40w-100w issue back to an IC chip. ordered new ones and that should fix that issue.

    as for the squealing. its worst on the lead channel with the more switch and or boost enabled. so i suspect the issue ahs something to do with that tube. I will order some new preamp tubes since many of these look like the originals and its a bit of a mish mash of brands.

    Any good alternative to the 12ax7a ?

  8. #8
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    Ah good , sounds like its on the way to being sorted .
    As for tubes I'm just using a mish mash of brands also , I believe Fryette recommends the shuguang - I've used a few of the TAD rebranded ones and they have been great . I like the Gold lion also ...
    Or if you meant at - au etc I'm not sure with Sig x , they usually don't recommend it , Dave is probably best to answer that also .

  9. #9
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    So the 100w 40w mod is now working great. but the hum/squeal is still there. have not heard from support for over a week so i thought I would ask here again.

    It seems to be that this has something to do with the amps ground... with a cable and guitar plugged in the amp hums pretty loud. if I have the guitar in my hands and i touch any of the switches or knobs the hum and squealing stop completely ! also the knobs all make a scratching sound when turning them.
    This sounds like some kind of earthing issue to me

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