strange reverb hum ab763.amp has been sent twice to tech

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by espinete, Sep 4, 2008.

  1. espinete

    espinete Member

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    technician resoldered circuit,changed resistances,caps,etc.,.it's been twice there. i've tried at home different tubes,replaced rca cable and took reverb tank outside the cabinet with no results.
    now i see some new things:
    1.hum starts when turned reverb on.(well,i knew that already)
    2.if i remove reverb rca cable from the amp(keep conected through tank)it sounds like an amp at very loud volume with a disconnected jack(instrument side)touching the ground.you know,very noisy.
    3.i've plugged the rca cable in but the other way round and hum stops.wish this last point it were a clue
     
  2. espinete

    espinete Member

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    i need some help..should i change reverb transformer?
     
  3. OlAndrew

    OlAndrew Member

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    It would help to know if this is a new problem that just popped up, an ongoing, or a new (to you) amp.

    I'd test things in order....There's a cable, may be a twisted pair, from the transformer to the tank, and an RCA cable from the tank to the reverb recovery circuit, (usually the grid of a triode, maybe through some volume or tone stuff). Both will be plugged into RCA jacks on the tank.

    Unplug the input..the xfrmr to tank cable. Still hum? Problem is not in xfrmr.
    If it doesn't hum, its still likely that there's a grounding issue.

    With the input cable unplugged, try attaching a pair of earphones to the xfrmr cable, and play a bit. Whaddaya hear? IF you get music or the tone your injecting if you're using a signal generator, the transformer is probably good. You'll get some distortion, maybe, it'll be a pretty bad impedence match, but recognizable. No music or tone, or a bad hum here, and you know the problem is in the xfrmr or the feed circuit...tube or tube socket, coupling cap, bypass cap or around there.

    Now, we've got the input disconnected, and we've still got hum? Disconnect the output cable at the amp end. Should get silence, or normal background noise. Hum now means trouble in the recovery circuit.

    If its quiet now, the problem is in the cable or the tank. Trouble is, its hard to tell...try a new RCA cable. If it still hums, problem is in the tank grounding. TAnks are available with both, only one (either end) or no jacks grounded, and the wrong things grounded will give you hum.

    I'll usually replace the stock jacks with insulated ones, and experiment with grounding until I get a nice quiet setup. You can't usually tell from the schematic how the grounding should be setup, and even if its indicated, schematics are usually inaccurate in this area.

    IF you do the guess-and-try and solve it, write on the tank what you did.

    Of course, this is really just a WAG, I don't have the thing in front of me.

    Good hunting...
     
  4. espinete

    espinete Member

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    thanks very much ,i will send your message to my tech by e-mail.it is only hum on the reverb.when it's turned down amp sounds perfectly.i've always had this problem in my amp.i bought the super reverb with no reverb tank,so i had to buy a tank and put everything together.since the first time i heard the amp reverb hum was there
     
  5. AR-305

    AR-305 Member

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    When you bought your new reverb tank are you sure you purchased a tank with the proper impedance for your tube driven reverb and not a tank intended for SS IC driven reverb?
     
  6. OlAndrew

    OlAndrew Member

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    Ha! I'd just about bet its a grounding issue, probably on the jacks on the tank. I've bounced off of that one more than once.
     
  7. espinete

    espinete Member

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    i don't understand what SS IC means.
    i just bought the reverb tank accutronics 4ab3c1b.
    my tech said he had same exact model tank.i saw same code 4ab3c1b.now it works pefectly.no problems at all.hum stopped..
    a(2nd digit)=8 ohms imput impeance
    b(3rd digit)= 2250 ohms ouput impedance

    http://www.accutronicsreverb.com/Type4.htm

    could be rca female conections from the tank.my tech said could be also caps inside rverb tank.no idea,but i have to repair it or change it for a new one(i bought it four months ago.)i wish can replace it
     
    Last edited: Sep 7, 2008
  8. mbratch

    mbratch Member

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    Solid state integrated circuit. It does not apply to your case.
    That's the right model for your amp.
    There are no caps inside the reverb tank. The problem is not likely the tank. He probably said caps in the reverb circuit.
     
  9. espinete

    espinete Member

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    thank you guys,now reverb tank is doing the same again.this is driving me mad because my tech said he has no idea of what's happening.i bought a few tubes from internet and he said i could wait till i check these tubes out.
    my super reverb now has a ss rectifier tube but before had a jj gz34 and reverb effect sounded with the same hum.

    i've just compared both tanks now and and both two have the same level of noise.i promise you that i went to my tech and the reverb tank from srs webstore sounded with hum and the new one that we tested not.
    sorry about the others messages,
    can you help me?
    is that possible,not noise and a couple of days later starting to make that same hum? it is true but i can't believe it,that's what it did happen.

    you now i will try to understand things you could tell me but i think there is nothing i can do by my own.i will send all information to my technician ..

    thanks for your patience

    thank you.it's my falt,don't think my tech said caps.i can't remember exactly what he said about the small red pieces conected to cables inside tank.sorry about that :jo
     
    Last edited: Sep 8, 2008
  10. donnyjaguar

    donnyjaguar Member

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    Are you sure the power cord isn't routed near the reverb tank or return lead to the amplifier?

    If I run my one amp with the power cord in the bottom it does this.
     
  11. espinete

    espinete Member

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    i think i understand you. you mean main lead(ac power cord cable),don't you?i always keep power cable as far as i can from rest of the cables(instrument,etc.,..) thanks
     
  12. espinete

    espinete Member

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    other things i can say about my amp.
    i noticed there is a blue glow on power tubes when standby switch is on.
    power transformer is a new m.magnetics that i got from weber site,same as the ot wich is also a m.magnetics multitap tone clone series transformer.
    my rca cable is a normal rca cord from my hi fi equipment working perfectly.
    vibrato effect is disconnected
     
  13. skipm45

    skipm45 Member

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  14. shuie

    shuie Member

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    Maybe try swapping the blue/brown wires that are on pin 3 of the power tubes. Im not a tech, so that suggestion is worth what you paid for it :)
     
  15. drgonzoguitar

    drgonzoguitar Member

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    Please don't do that! If you were getting parasitic oscillations (high whistling sounds), that would be a case for switching the primary leads.
     
  16. espinete

    espinete Member

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    good news for me.you're gonna kill me..i put the reverb tank about 40 inch away from the amp with a long rca cable and it's all right now.now the problem is how to insulate the tank.
    so i supose,after this experience,that is impossible to own a super reverb head cabinet with chassis and reverb tank inside together..sorry but it's not my falt.my tech put it this way into the head cab,and he was thinkin' like you in a much more complicated solution to fix the hum problem produced by the reverb tank.
    so now i'll ask people who own super reverbs in two parts because the ev speakers size(like SRV)
    thanks for your support and sorry about this stupid mistake
     
  17. Jahn

    Jahn Listens to Johnny Marr, plays like John Denver Supporting Member

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    Want to hear something strange? Every amp I've owned with an onboard reverb has hummed, most likely due to a very bad grounding/wiring problem in my building. I mean, it's not even at usable volumes before that hum kicks in - try that with the onboard tremolo and the hiss-hiss-hiss will drive you crazy.

    That's why all of my amps do not have onboard reverb and I rely on a Verbzilla (a Sole-mate didn't hum either, i guess any pedal 'verb is safe...actually, not the Holy Grail, that hummed too).
     
  18. mbratch

    mbratch Member

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    I can see how having the tank close to the amp chassis might cause the hum, but I don't see why it has to be 40 in away to make it stop. How close can you get it before it hums? Fender made head amps with reverb, but they had a little height to them (12 inches or so).
     
  19. booj

    booj Member

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    It sounds to me like a connection problem. Make sure your RCA plugs and jacks at all connections are clean. If you have heard the problem go away, then come back it's most likely the connecting cords. Even new cords go bad sometimes.
     
  20. Papajuice

    Papajuice Silver Supporting Member

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    Had the same problem with an Orange Rockerverb, one of the solder joint on the center connection in the RCA plug was hitting the side and grounding making a nasty hum. It sound like the same kind of problem.
     

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