Amp Screeches on High Notes! Help!

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Creamy, Jul 11, 2006.


  1. Creamy

    Creamy Member

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    I have a Rivera era Fender Twin that recently started to add some high pitched screech, especially when playing on the b and e strings above the 12th fret. At first I though it was the speakers but after connecting to an external cab, it looks like its not. The speakers in the Twin are real tranparent (JBLs), and this seems to reveal the screech more than the other speakers I tried, so I am still scratching my head, wondering if it may also be them. The noise is a recent phenomenon that doesn't always reveal itself but when it does the sound is a shrill overtone on top of the original note. Any ideas on what causing this or how to diagnose it further?
     
  2. tonezoneonline

    tonezoneonline Member

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    Check your preamp tubes for microphonics.
     
  3. brad347

    brad347 Member

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    parasitic oscillations can cause a 'cricket' sound on top of your note. Maybe a wire got moved around inside the amp to just the wrong place...
     
  4. swanproamps

    swanproamps Member

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    I agree with Brad. Higher gain amps can be very touchy with too much gain.
    Wiring placement and shielding is critical. Check for loose mechanical connections, preamp signal leads(check shielding), make sure the output transformer secondary wiring is away from power tube input wiring. You can also eliminate the preamp tubes by swapping with a known good one, one at a time.
    I had a 100 watt Marshall one time that squealed at high volume. Turned out to be a loose brass screw that connected the circuit board stanchions to the chassis. The circuit board has excess capacitance running around on it and the mounting screws help shunt that to ground. If need be you can shield (to ground) the input area with light gauge shielding.

    Remember, if your not trained be careful inside of a chassis, there are deadly voltages present. If your not sure get someone who knows how to diagnosis and repair safely.

    Bob
     
  5. Creamy

    Creamy Member

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    Thanks for the feedback. It looks like the easiest place to start is with checking for microphonics, so I'll do that first. I hope it's that simple. If not, I have a poke around inside that amp.
     
  6. donnyjaguar

    donnyjaguar Member

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    Usually if you're running an external cabinet the microphonics will be much lesser. If that's the case then you're on the right track. I'm thinking its HF oscillation though. Disconnecting the NFB loop will prove my theory. As for corrective action it could be a simply matter of reorienting the internal leads or you may have to put in a bypass cap on your NFB resistor. The reason this happens is because the phase of the higher frequency signals is undergoing a change through the output transformer. This is a common problem and not necessarily indicative of low quality iron. Some amplifiers combat this by routing a second NFB point from the plate of one output tube. Its difficult, well impossible to be honest, to optimize this without an oscilloscope and some experiementation.
     
  7. tonezoneonline

    tonezoneonline Member

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    I agree with donnyjaguar but mentioned a microphonlc because it's an easy place to start and rule out.If that is the problem and you don't rule it out you can be chasing oscillations for days.
     

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