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Shocking Ampeg SB12 story! Amp repair.

Discussion in 'Amps [BG]' started by TheEnthBass, Nov 17, 2013.

  1. TheEnthBass

    TheEnthBass

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    As in it is shocking me if I'm in contact with something grounded, a mic, a pole in a basement (not that kind of pole;), a steel support beam). This thing still has the two prong power cord on it. I'm guessing that is the bulk of the issue. I've heard about "death caps" on old fenders, could this be the case here, a cap not functioning properly and discharging to the chassis? Does replacing the power cord remove that cap? I'm very interested in learning how to do some of this myself. Is this a tough or dangerous job? Anyone know these amps well enough to talk me through it? In a less related matter, I'm looking for training by correspondence for tube amp repair or design. Any leads there?
  2. B-string

    B-string Supporting Member

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    There should be a polarity switch on the back, switch it to the other position. If you still get shocked then the "death cap" is bad (open). Yes installing a three wire cord will eliminate it and be much safer.
  3. beans-on-toast

    beans-on-toast

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    Yes, the amp did have eventually have a polarity switch but not on the early models.

    Try starting with this for some basic information. There's other SB-12 related information in the wiki if you poke around.

    There are a lot of how to tube amp books and videos. Check out www.tubesandmore.com . More stuff on YouTube.
  4. Mr. Foxen

    Mr. Foxen

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    Disclosures:
    Amp tinkerer at Ampstack
    Take it to a tech and get a proper earthed cord sorted. All the other stuff is kind of secondary to that. Once it has a proper earth, you can fiddle with it with much less chance of sparking yourself up more than is educational (as opposed to fatal).
  5. TheEnthBass

    TheEnthBass

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    cool thanks guys.
  6. TheEnthBass

    TheEnthBass

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    btw for anyone still interested, there is no polarity switch on this amp. I popped it open to peak around and found a screw hanging around rattling trough the electronics which might explain an occasional loss of power that was happening, thought it might also explain the jolts, then i caught a little zap. So while that screw could have been causing a short there is definitely still another issue. getting a power cable, building a discharge stick for caps and reading up a little.
  7. B-string

    B-string Supporting Member

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    There was always polarity switch (could just show "On-Off-On") when it was made that I have ever known with a 2 prong cord. If you have an odd ball somehow unplug the cord, turn it upside down and plug it back in.
  8. Pilgrim

    Pilgrim Supporting Member

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    There is no justification in today's world for NOT having a three-prong grounded AC cord installed on any older amp.

    I heartily endorse this idea...and it could save your life.
  9. lexington125

    lexington125

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    My early (7868) SB-12 does not have a polarity switch. On/off, volume, bass & treble; that's it. Oh, and the ridiculous speaker signal through the cabinet latches. But none of the additional features of the later SB-12s
  10. B-string

    B-string Supporting Member

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    In that case you would have to reverse the plug with the two wire "death" cord :)
  11. beans-on-toast

    beans-on-toast

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    You must have a very early model. Just wondering, is your amp attached directly to the lid without the four vibration mounts. And does it have a power tube balance pot on the back?

    The reason I ask is that the first revision had no mounts and the balance pot. The pot was used to match the power tubes. At some point they added the vibration mounts and removed the balance pot. They added a heater hum balance pot and of course the polarity switch that you mentioned. This was in place when the 6L6GC power tubes were used. I'm trying to figure out when the power tube balance pot was removed and when the isolation mounts appeared.
  12. Mr. Foxen

    Mr. Foxen

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    Also, check the fuses, if there is a screw shorting things, should have blown a fuse.
  13. SunnBass

    SunnBass All these blankets saved my life. Supporting Member

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    It's super simple to put a three prong on your amp.
    I was having the same problem with my '72 Sunn Model T.
    Changed the plug to a three prong and presto, no more shocks.
  14. TheEnthBass

    TheEnthBass

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    It has 7868 tubes. Don't recall seeing a balance for the tubes, not sure about vibration mounts or where they would be... I need to poke around a bit more I guess. Makes sense to flip polarity by flipping the cord, I will give that a twirl... Still planning to install a grounded cord when I find some time. It also has an original warranty sticker inside that gives the year so I can take a look and get specifics on the year and if there is additional model info or a serial number that might give a bit more info. Sounds like the same model as lexington125. All electrocution aside I grabbed this thing for 150 at a flea market and other that that and the handle being in rough shape it was a great buy and i look forward to getting it in better shape. Lots of good buys on fliptops.net so planning on grabbing the handle and a new logo which also broke. Just grabbed power tubes and patched up the tolex, cleaned up the latches that also carry signal, got the light working again, a little wear and tear on the tolex but I like that so def not replacing that.
  15. Bill Hughes

    Bill Hughes

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    That so-callled "death cap" (0.047 uf) was within the law for the period. Nobody ever died from that, it is within limits and is a special type of recognized part.

    The new limit I think is 0.01uf.

    I believe the SB12 had a two position ON OFF ON for polarity, but I really dont remember.

    The SB12 was cathode bias as I recall and I dont rememder a bias balance pot, but I do think that a heater hum balance was employed.

    Many class "D" amps have similar caps to meet EMC issues.

    Bill
  16. beans-on-toast

    beans-on-toast

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    If the death cap is healthy it does what it is supposed to do. Today they would use a Type Y or Type XY safety cap which is designed to fail in a way (it won't short) that won't leave you open to being zapped.
  17. Bill Hughes

    Bill Hughes

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    We used "Y" caps back then. If the polarity switch (if no switch plug reversal), you will be exposed to a shock of 2.13mA at 120VAC for 0.047uF. Thats not lethal. If you install a grounded, 3 wire cord, then you will have none. All Ampeg products were eventually UL listed.

    I have always been of the opinion that the line bypass cap is the way to go. I have seen house outlets wired wrong and that would cause three wire cords to be an issue. That so called death cap could really be youy last line of defense!

    Bill
  18. RobJ

    RobJ Supporting Member

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    My '65 SB-12 has no polarity switch, just on/off, no visible balance adjustment mechanism and no shock mounts. The speaker still connects through the latches and I see no reason to change that. I love that amp.
  19. Bill Hughes

    Bill Hughes

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    I believe that there was a line bypass cap employed, probably un-switched. That was 49 years ago! That is a 1964 product!
  20. TheEnthBass

    TheEnthBass

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    looked around stoned more. no vibration mounts, amp is just screwed to the lid. no polarity switch, no balance pot. warranty card says sold in June 66 so i'm guessing 65 or earlier. I've got pix of the guts and a cap that has leaked something. not sure if i put photo link from FB if it can be seen here. The speaker was replaced with a radioshack model in the 70s wondering what speaker folks would reccomend

    https://m.facebook.com/?_rdr#!/phot...notif_t=photo_album_comment&__user=1124754437

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