Problem with "Reverb Feedback" on '68 DRRI

Discussion in 'Amp Tech Center' started by Record83, May 9, 2015.

  1. Record83

    Record83 Tele-Meister

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    Hey all, wondering if someone has had a similar issue...

    I purchased a new Fender '68 Deluxe Reverb Reissue Silverface amp over a year ago. Overall I like it, but have had some issues with it...

    Big issue right now is the Reverb itself. It will "feed back." I can even unplug my guitar, turn down the volumes and turn up the Reverb and the amp will slowly build to a loud feed-back type tone.

    This started to occur out of nowhere with another problem I had where a microphonic tube in V4 made the amp sound like it was going to blow up. I switched it with the 12AX7 in V5 (which just governs tremolo oscillation, no tone) and voila, it was fixed...however, I had this feedback problem even before I made the tube swap.

    I have to think I am not the only one with a DRRI that's gone bananas. Any info would be most appreciated. Thanks...
     
  2. jazzguitar

    jazzguitar Tele-Afflicted

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    That is an issue not specific to the reissue amps. I've seen that with vintage originals too.

    It is in the nature of the reverb circuit that there is an internal feedback loop operating just a little below the threshold of oscillation (i.e. it howls generating the howl itself).

    In some amps the reverb tank gets stuck inside the bag that is screwed to the cab bottom. Try to get it unstuck. Sometimes the frame holding the actual reverb springs, which is suspended with shock-suspension springs to the larger metal housing, gets stuck or one of the springs gets loose. In all such cases the speaker can cause the reverb to howl.

    In other cases, when the reverb howls even with no volume from the speaker, it is not as easy to find a solution. You already tried different tubes swaps, maybe not enough (do you have spares?)

    I have found in some cases only increasing the 3M3 reverb signal attenuation resistor in the circuit helps.

    Hope that helps ...
     
  3. clintj

    clintj Friend of Leo's

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    My kit built Super Reverb does it with the reverb on 10. Try placing a piece of cardboard under the tank, or even wrapping it in bubble wrap in the vinyl bag.
     
  4. Record83

    Record83 Tele-Meister

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    Thanks guys...

    I am going to try and mess around with the tank first and see if that is the issue.
     
  5. BBill64

    BBill64 Tele-Afflicted

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    My Vaporizer does this when I put it on certain surfaces. Putting it on a chair seems to help avoid feedback.
     
  6. Cleeve

    Cleeve Tele-Holic

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    Problem with "Reverb Feedback" on '68 DRRI

    I got my vaporizer's reverb to quit feeding back by sticking some thick self-adhesive weatherstripping foam as a sort of mattress underneath the tank, then taping the flange to the weatherstrip with more weatherstrip.
    I shall attempt a picture: ImageUploadedByTDPRI1431464004.417526.jpg

    The foam feels like memory foam, leaves a dent for a while when something jabs it.
    I got it at Lowes, I think it's 3/4 by 3/8 inch

    The Vaporizer's tank was mounted by wood screws and kinda hard rubber grommets with brass inserts-
    also the poor thing has no shielding under the tank, so it hums like heck when the amp is on the floor with wiring in the floor.
     
  7. Record83

    Record83 Tele-Meister

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    Thank you for posting the pic and the advice. I really appreciate that.

    Based on a lot of opinions I am getting I will try to "isolate" the tank within the cab (because it doesn't happen when I remove the tank) and wrap in towels or bubble wrap as aforementioned. If that works, then I'll grab some weather stripping and go from there...

    ...or I will buy another amp, because that will solve this issue as well :lol:
     
  8. Cleeve

    Cleeve Tele-Holic

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    I would have cut the weatherstrip more neatly had I known i'd be leaving it in there.

    Mechanical reverb is tough- good speakers, light cab, high gain tubes and circuits all conspire to push the amp into feedback via the springs.
    This combined with having to adhere to the old schematic and less post-assembly tweaking and testing has us busting out the weatherstrip.

    I've seen this in old amps as well though. Designing reverb springs inside of the speaker box is pretty absurd really, like expecting the guitar to not feed back while turned up and leaning on the powered amp.. But it usually works anyway!
     
  9. Wally

    Wally Telefied Ad Free Member

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    Have you checked out V3? This is the reverb driver tube.
     
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