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Fender Twin motorboating at high gain

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  • Fender Twin motorboating at high gain

    Hello folks,
    I decided to share this faux pas of mine in the case anyone else runs into something like this.
    I was trying to ferret out what appeared to be a shielding issue with this amp and had just put 2 power tubes in so I didn't have all that heat being generated. After resolving that shielding issue, I noticed if I raised the volume on the vibrato channel between 9-10, the amp went into Thumping oscillation.
    I determined it wasn't the power supply. The only tube I removed that killed the oscillation was the driver tube. I eliminated the negative feedback as a possible path.
    Well, here's the faux pas:
    Well, I usually set the chassis vertically with the transformer on the bottom end. I apparently installed the power tubes without looking and put them in on phase of the power sockets (IOW, both tubes on one end of the 4 sockets. Oh no Mr. Bill!!
    I'm thinking technically that with only one phase of the Push-Pull circuit working, the common mode rejection ratio of the P-P circuit was not in affect and by some weird anomaly of the amp, it allowed the low frequency oscillation.
    Anyway, there ya have it. Even after all these years of doing this, I still do dumb shit .

  • #2
    Originally posted by Mars Amp Repair View Post
    Anyway, there ya have it. Even after all these years of doing this, I still do dumb shit .
    I think the noise you were hearing was the buzz of power supply charging. The more the legs of the output transformer primary are out of current balance, the more buzz shows up in the audio. With all the current on one side of the push pull pair and none on the other, buzz will be inevitably too loud and may make you think something vital is wrong. Which it is. And you sorted it out so next time you encounter that sound you'll know what to look for. Thanks for posting as I'm sure others will benefit from your tale.

    And dumb stuff, I've likely spend months of my life reversing my own blunders. Or trying. I'm sure lots of others vex themselves this way. And the rest blithely carry on as if everything they do is magnificent.
    This isn't the future I signed up for.

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    • #3
      Originally posted by Mars Amp Repair View Post
      ...Even after all these years of doing this...
      That type of experience really makes one remember much better than if we had a book of "Don't do this" stuff. You could read story after story in the book but it would not all lodge in your brain like it does after having the personal experience.
      Cheers,
      Tom

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      • #4
        Yep, we've all done dumb stuff. Sometimes, you do the same dumb stuff even though you know better. Just last week I soldered up a MIDI cable end- thin wires, close connector pins that were slightly corroded and wouldn't take solder well, etc. A real pain in the ass. And, ...... you guessed it. I forgot to put the sleeve over the cable first and had to start all over.
        "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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        • #5
          Originally posted by The Dude View Post
          Yep, we've all done dumb stuff. Sometimes, you do the same dumb stuff even though you know better. Just last week I soldered up a MIDI cable end- thin wires, close connector pins that were slightly corroded and wouldn't take solder well, etc. A real pain in the ass. And, ...... you guessed it. I forgot to put the sleeve over the cable first and had to start all over.
          I was going to post the same thing. Though it was just an instrument cable. Damn if I don't forget to put the sleeve on first half the fu&!n@ time !?! I always think "What the hell is wrong with me!?"
          "Take two placebos, works twice as well." Enzo

          "Now get off my lawn with your silicooties and boom-chucka speakers and computers masquerading as amplifiers" Justin Thomas

          "If you're not interested in opinions and the experience of others, why even start a thread?
          You can't just expect consent." Helmholtz

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          • #6
            Every time you forget a connector sleeve, you tell yourself, "I'm never doing that again". And then, you do it again.
            "I took a photo of my ohm meter... It didn't help." Enzo 8/20/22

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            • #7
              OH MAN!
              If we all had a few bucks for the times we soldered on even a 1/4" plug to a guitar cable & forgot the screw-on cover first!!
              Thanx for all the responses in support of all of us fallible human beings!
              Glen

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              • #8
                Originally posted by Leo_Gnardo View Post

                I think the noise you were hearing was the buzz of power supply charging. The more the legs of the output transformer primary are out of current balance, the more buzz shows up in the audio. With all the current on one side of the push pull pair and none on the other, buzz will be inevitably too loud and may make you think something vital is wrong. Which it is. And you sorted it out so next time you encounter that sound you'll know what to look for. Thanks for posting as I'm sure others will benefit from your tale.

                And dumb stuff, I've likely spend months of my life reversing my own blunders. Or trying. I'm sure lots of others vex themselves this way. And the rest blithely carry on as if everything they do is magnificent.
                I think the buzz issue, for which I changed out the straight wire for a shielded one, was when all the tubes were installed. This amp had some mods in it which most likely changed the gain of things. The lead I replaced had a long run from I think V4 to the front of the amp down to the trem controls, so a very long run to pickup all kinds of noise, esp with the increased mods.
                The cab the cust brought in with the cab was from a newer combo that did not have the shielding in the top panel, so that added to it, too.
                Thanx, Leonardo ('Leonardo, I'm not comfortable in here') Does that sound familiar?

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                • #9
                  Originally posted by Mars Amp Repair View Post

                  Leonardo ('Leonardo, I'm not comfortable in here') Does that sound familiar?
                  I think Mona Lisa said that while posing for the painting. It was a bit drafty in the room what with the big window behind her wide open. But she remained her semi cheerful self nonetheless.

                  Click image for larger version

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                  This isn't the future I signed up for.

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                  • #10
                    Originally posted by Leo_Gnardo View Post

                    I think Mona Lisa said that while posing for the painting. It was a bit drafty in the room what with the big window behind her wide open. But she remained her semi cheerful self nonetheless.

                    Click image for larger version

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                    Good one!

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