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Problem with microphonic tubes that are brand new.

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  • #16
    Came across a similar problem in an old Gibson that had a bad ground to a volume pot.

    It would sound like every tube was ringing, then be fine when the intermittent connection was making contact.

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    • #17
      Okay, I have isolated the problem though I'm not sure what to do. I have a lamp current limiter that is plugged into a variac. Whenever the amp is plugged into the current limiter it is fine. If I plug it into the variac or any other outlet, it starts to howl. Turning the variac slightly down does not help. Interestingly, the three-prong conversion was done on this amp but the death cap was still connected, so I just clipped it out. What is my current limiter doing to mask this problem?
      Last edited by jeffects; 12-23-2014, 09:30 PM.

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      • #18
        Did you try the NFB disconnect? I'm pretty doubtful about it but would like to eliminate the possibility of incorrect OT phasing.
        Originally posted by Enzo
        I have a sign in my shop that says, "Never think up reasons not to check something."


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        • #19
          g-one I haven't tried that yet. I got sidetracked when it started working properly. I definitely will tomorrow and let you know.

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          • #20
            I'm also going to go back and check voltages with it plugged straight into the variac.

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            • #21
              The current limiter limits the current,the variac lowers the voltage.Dont want to sound like a broken record,but the problem is current related and is most likely power supply related,filter caps.

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              • #22
                Yes, what stokes said should be the first thing to try:
                Originally posted by stokes View Post
                A bad decoupling cap would cause the problem you describe and should be the first step.If you have a spare 10,16 or 20uf cap,clip it across the appropriate stage and see.
                Originally posted by Enzo
                I have a sign in my shop that says, "Never think up reasons not to check something."


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                • #23
                  if I read correctly op said he replaced all the caps however, first thing I do when I get any amp 20+ years old or older is replace EVERY electrolytic cap in it regardless, they don't need to be puffy or leaking to be bad ( they could have just dried up and yes it happens I have seen it several times last was with a peavey VTM120 head).

                  I personally have found more instances of bad electrolytic caps causing howl, hum, hissing, and buzzing than anything else. the other causes I have found would be cold solder joints ( you might call them bad :P) which are easily identifiable by dull crystalized looking solder, bad grounds, and faulty jacks/bad cables.

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                  • #24
                    He said every cap EXCEPT electros.

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                    • #25
                      see post #3.
                      Originally posted by Enzo
                      I have a sign in my shop that says, "Never think up reasons not to check something."


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                      • #26
                        Okay guys, problem solved. It seems that drewl nailed it. Had nothing to do with tubes or microphonics. The volume pot was not properly grounded and the hum caused the entire chassis to vibrate. Literally every component in the preamp section became microphonic. Could have caught this a lot sooner had my current limiter not masked the problem while on the bench. (Still not sure why that was the case.) Anyway, I resoldered the ground to the volume pot and also resoldered the wires coming off the preamp tube to the input and volume because they looked suspicious as well. It's now one of the cleanest amps I ever played through. Thank you so much for your input guys. This business is a never ending learning experience.

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                        • #27
                          Glad you got it.
                          (post #13)

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