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AC30 TBR noisy phase splitter

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  • #1

    AC30 TBR noisy phase splitter

    This amp is very noisy. Nothing changed from the original diagram, this seems the same as the '82 model.
    I'm wandering if this model is prone to noise, as i've always found old AC30's to be noise free. The top cut control completely cuts the noise; channel volumes dont really change it until full up, when the input noise takes over.
    Tried different valves;- no change. Meter probe on grids cuts noise. Dropped the reverb bypass resistor down to 1M, no change. Will continue to faff about with it, but if anyone has an insight into this i shall be eager to hear.
    Ta Alan
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  • #2
    I'm not familiar with these amps (could you post a schem?).
    Generally spoken I'd say if you can turn down the noise with the cut control the problem is BEFORE the cut control.

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    • #3
      Sorry, its the '78 model as shown here:-http://www.schematicheaven.com/voxamps/ac301978.pdf

      Changed the PI grid 1M reference resistors and coupling caps, no change.
      Meter on pin 7 (grid 2), stopped noise, but when i removed the probe got low frequency oscillations that would only stop by turning off the amp.
      Grrrr

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      • #4
        Might be oscillations.
        Some brown Fenders had a cap in between the plates of the PI to prevent that.
        See here: http://www.schematicheaven.com/fende...6g11_schem.pdf
        You could try to put a 100pF between the plates of your PI and see if it changes to the better.

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        • #5
          Done a more thorough investigation and the noise is coming from the 3M3 reverb bypass resistor and seems to be a byproduct of shoehorning in the reverb circuit in the power amp compartment and all the consequent extra wiring.
          Has anyone tried getting rid of this resistor and running the reverb into the grounded side of the phase splitter? This would seem to be the best thing i can come up with as a workaround.

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          • #6
            Could be a bad resistor, try replacing it.
            "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
              Resistor has been replaced- twice!
              Trouble is it's not bad enough to warrant the time i'm spending on it, but having had an AC30 in the past i know it's more than i would expect; mine however was one without the reverb.
              Feel that i'm now starting to embark on a redesign job rather than a repair.
              The guy needs it tomorrow so it's going back together the now and will see how he gets on.

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