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Fender Showman output?

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  • Fender Showman output?

    I have a 70's Fender Showman on the bench. When hooked up to a 8ohm dummy load I have a nice clean waveform with the appropriate amplitude on the output with the volume at 100%. When I connect a 8ohm speaker, my waveform begins to breakdown with the volume at 50% and above. I hear it as distortion. Sounds like a bad OT actually. I replaced the OT and its still doing it. I figure its a resistive load vs a reactive load thing but what is causing this and how should I troubleshoot this to a solution?

  • #2
    Start by connecting a different speaker cab.
    Education is what you're left with after you have forgotten what you have learned.

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    • #3
      The speaker I am using is a 8ohm 1x12 rated at 100w. I checked it with another amplifier and everything shows up fine on the scope as well as the audio. Is there any other reason to change the cab besides suspect of a bad speaker?

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      • #4
        What RMS wattage are you getting with the speaker hooked up?

        With the volume full up, what signal voltage are you applying at the input? As a yardstick, the output test for the twin states 12mv applied to normal channel with controls at full rotation, vibrato channel controls at 50%, reverb & vibrato off, vibrato controls at 50%.

        If you have a malfunction, this will normally show up on the dc voltages, list these please for all tubes, plus power tube plate current.

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        • #5
          1x12" "rated" 100W is no match for a macho amp like a Dual Showman.
          I bet it's on an open backed cab or a soft speaker.
          What frequency are you using?
          What speaker is it?
          If it put out high power into a resistor, your transformer is fine.
          Anyway Dual Showmans ask for 4 ohm loads.
          All this points to my suspicion that the speaker does not take it, it might even be bottoming.
          I bet that head run into, say, an EV , will show no problem at all.
          PS: when you test with a speaker, do you use the same sinewave as before or you play a guitar? Just wondering.
          Juan Manuel Fahey

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          • #6
            Also bear in mind that an "8 ohm" speaker is only 8 ohms nominally. It can be much higher near its resonant frequency, and this will affect the loading on the amp.

            When the waveform breaks down with the speaker connected, is it a higher amplitude than what you got for 100% into the dummy load?
            "Enzo, I see that you replied parasitic oscillations. Is that a hypothesis? Or is that your amazing metal band I should check out?"

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            • #7
              SO, it seems that my speaker was fine. My dummy load was the prob. It had broken a ground wire so that is why I was getting two different readings. After I repaired that I saw the noise on the scope. It ended up being a filter cap that was leaking causing "distortion" due to it not doing its job. Thanks for everyones suggestions and I am sorry for my poor test equipment.

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              • #8
                Thanks for posting the end of the novel.
                Often somebody posts an "impossible" problem, (by that I mean illogical), gets a couple answers and dissapears forever, leaving us clueless as to what really happened.
                Thanks.
                Good you could solve it.
                Juan Manuel Fahey

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