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Crazy sounds--Fender amp

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  • #1

    Crazy sounds--Fender amp

    This is a hot rod deluxe - replaced a plate resistor and the amp came back to life (it had very low volume) , but now... it makes this VERY , VERY loud "chirping/feedback" sound. this can be "adjusted " by turning the presence control. Does it with volumes off . Seems to go away as you turn down the presence .

    sometimes it wont do it--the amp works but theres a strange distortion/ high end sound in both channels.

    Its something after the preamp--the amp works out of the fx send .

    Please watch the video ...

    its not the tubes , and all voltages look ok / bias ok. Just checked all solder joints too .


    HPIM6509.mp4 video by solrizzo | Photobucket
    Tags: None
  • #2
    Not checked out the vid yet, but the cause may be a bad B+ cap, C31,33 or 35, (guess C35 at the phase splitter node as most likely) allowing a positive feedback path between stages via the B+.
    Pete
    My band:- http://www.youtube.com/user/RedwingBand

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    • #3
      Filter cap,most likely in the preamp stage.There was comments about this issue in a Blues Jr in another recent thread.I have seen it in many Hot Rods,Devilles and Deluxes.I would change them all.

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      • #4
        Did you disconnect the output traqnsformer wires while doing this? And any chance they are on the wrong posts now? The blue and brown ones?
        Education is what you're left with after you have forgotten what you have learned.

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        • #5
          no enzo--

          right now ive got a new cap paralelled with the 1st 22u filter and the amp is acting normal.

          also--someone replaced the plastic input jacks with metal ones--they do have insulating washers , but i had them out--if one of the metal jacks was touching ground---would that cause some oscillations ?

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          • #6
            ANything is possible, but more likely using grounded jacks at the input would cause hum rather than oscillation.

            If one filter cap (decoupling cap) has opened or dried up, I'd agree with stokes, just replace the lot of them.
            Education is what you're left with after you have forgotten what you have learned.

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            • #7
              Originally posted by Valvehead View Post
              no enzo--

              right now ive got a new cap paralelled with the 1st 22u filter and the amp is acting normal.

              also--someone replaced the plastic input jacks with metal ones--they do have insulating washers , but i had them out--if one of the metal jacks was touching ground---would that cause some oscillations ?
              You found the problem.Like I've said,many times,I've seen this problem a lot in various Devilles,Deluxes,Blues Jr's.I've seen it in a couple amps that were less than a year old,thats why I think they got a bad batch of caps.

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              • #8
                Thanks guys--------- playing the amp now and all is good

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