old univox(?) crackle & bias question

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Devin, Feb 22, 2012.

  1. Devin

    Devin Low Voltage Supporting Member

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    I have a 12" combo i love an use as my bedroom amp. I was told it was from the 50's but it looks early 60's to me, it's a fugly mess cosmetically. Ocassionally i take it out to gigs and rehersals but it really isnt loud enough for that. I purchased/found a compliment of replacement NOS tubes for it because i want the amp to last. I installed them maybe 4 months ago.... sounded instantly louder with more headroom and better high end. I was happy.

    But now im noticing that it makes a sort of static like fart when hitting it hard with chords or od pedals. Not badly enough to notice all the time it just seems to pop out of nowhere and sounds like crackle/distortion, but very "seperate" from the tone of the amp itself. I think the amp may be biased cold for the tube(s) i have in it now. does this make sense?
     
  2. zenas

    zenas Member

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    I'd almost bet that amp is cathode biased. If it is you should be fine. The noise could be a tube or socket. A bad solder joint or cap.
    Has this thing ever been serviced ? If not it's time.
     
  3. Devin

    Devin Low Voltage Supporting Member

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    No, never been serviced. This is something im hesitant about. I have only taken two amps in for service in my life and i have an itching suspicion that tube amp service might be the biggest scam in the country. I drop off a twin reverb for a blackface job, costs $100, sounds exactly the same. I drop off an ampeg that shuts off sporadically costs me $75 only to find out the problem is back a month later. I think my local place is just charging me high dollar rent for space to let my amp sit in the shop.
     
    Last edited: Feb 22, 2012
  4. Devin

    Devin Low Voltage Supporting Member

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  5. Devin

    Devin Low Voltage Supporting Member

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    ok, i guess i will take it in. No choice.
     
  6. epluribus

    epluribus Member

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    Never seen a Univox like that, but that doesn't mean it isn't one. Looks more like an old Estes or Panramic or something. Might also be a movie projector system or a powered extension for a really old stereo. Cool, whatever it is.

    Identifying it: Obviously look for nameplates, stickers inside, that sort of thing. Pix of the back and the chassis will help. Also, if you grab the speaker code off the back of the mounting ring or basket that'll help too, along with a size and the type magnet (very likely AlNiCo). What tubes was it running?

    Could very well be a torn or punctured speaker cone...which can be fixed. May also be a loose part inside...does it also happen if you move the amp? For that matter, I trust you checked to see if the jack is solid.

    Safety note...two-prong cord? Death cap still there? Even more important...got two transformers in the back? If it's an old record player extension, it may only have one that's wired to the speakers, meaning there's no power transformer and therefore no protection from wall voltage being applied to your strings.

    If you're not really familiar with safe working technique and working on amps in general, a trip to the tech is highly recommended. He'll take a look at doing a three-prong cord, he'll clip that death cap, and he'll rightly recommend you replace all your power filter caps to protect the amp. Also, he'll check to see if it has a power transformer at all, and if not, will likely install an isolation transformer. All great things to be done for an old amp by a tech.

    Cool amp, plz post more pix.

    --Ray
     
  7. zenas

    zenas Member

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    Dev...in I hear you ! I started doing my own amps because of the same thing.
    Bring amp to shop, wait a good long week, pay through the nose and amp still not fixed.

    But that amp should be looked at by a good tech. Or learn how to do it yourself.
     
  8. Devin

    Devin Low Voltage Supporting Member

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    [​IMG]

    check this out!

    has accordian, guitar and mic/vox inputs
     
  9. epluribus

    epluribus Member

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    I trust you drained those caps before doing anything else with this chassis? The voltage they store up are far more than enough to kill you, even if the amp has been unplugged for a long time.

    --Ray
     
  10. Devin

    Devin Low Voltage Supporting Member

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    this is an old pic, from the day after i got the amp. All i did is take the back off. Tried to remove some grime.

    Is just touching one of the exposed leads from a cap enough to really hurt you?
     

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