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Blown Power Transformer?

Discussion in 'Shock Brother's DIY Amps' started by HBamps, Oct 2, 2020.

  1. HBamps

    HBamps Tele-Meister

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    Im headed to the hardware store to get stuff for a lightbulb limiter!
    Funny, I have never even heard of one until yesterday! haha.
     
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  2. charlie chitlin

    charlie chitlin Doctor of Teleocity Silver Supporter

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    I have a big 150 watt spotlight in mine.
    I spray painted it red because the damn thing had me seeing spots for 20 minutes!
     
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  3. sds1

    sds1 Tele-Afflicted

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    Feel bad we sent this guy out into the wild thinking he was gonna come home with an incandescent bulb. :cry:
     
  4. Nickfl

    Nickfl Friend of Leo's

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    True, I would guess that was the thinking behind Fender using a champ power transformer in the original Princeton. However, those were also supposedly prone to failure in the Princeton, but apparently fine in the single ended amps.
     
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  5. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    The tube sellers will be stocking NOS incandescent bulbs soon. hah.

    And we will buy them.:rolleyes:
     
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  6. tubeToaster

    tubeToaster TDPRI Member

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    Is a variac the only alternative to a light bulb limiter ?
     
  7. King Fan

    King Fan Poster Extraordinaire

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    Apples and oranges; in my view, an LBL is cheap, readily available, and has one *primary* purpose -- to detect shorts while protecting the amp from them. A variac, set right, may limit short-circuit currents and damage, but doesn't AFAIK actually detect 'em. And it's not cheap or as readily available. OTOH it does have many more advanced functions an LBL doesn't.

    TLDR version; every amp builder needs an LBL; only a few really *need* a variac, and if they do, it's for other purposes.
     
    Last edited: Oct 4, 2020
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  8. BobbyZ

    BobbyZ Doctor of Teleocity

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    Agree with @King Fan A LBL is a lot more useful than a veriac. A veriac changes voltage, a short at 50, 90 or a 120 volts is still a short and shorts are always bad.
    With the LBL a dead short just makes the bulb burn full bright, amps fuse doesn't blow and nothing starts smoking.
    The one or two times I used my veriac for firing up a new build or a rebuild, I put the LBL in line between it and the amp.
    Haven't used the veriac in years, really kind of unnecessary in my view.
     
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  9. HBamps

    HBamps Tele-Meister

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    So I built a limiter (a kind of fun project on it's own) and tested the amp. With everything 120 VAC disconnected the lightbulb was glowing BRIGHT. From there I was pretty certain the PT was a goner. I had an old Hammond 269DX laying around so I figured I'd try that. It's a 275 / 275 secondary so I was prepared for a lower B+.

    Anyway, hooked it just the 120 VAC, lightbulb barely lit.
    Hooked up the rectifier, barely lit.
    Hooked up B+, lit a little on start up.
    Hooked up heaters, same.

    That all seems as it should and voltages are right in line with the Weber 350 on the plate of the 6V6.

    I don't know what happened to the weber PT but it's the 4th one I have had experience of blowing. Maybe now I'll get the lesson to stay away from their PTs....
     
  10. sds1

    sds1 Tele-Afflicted

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    Sorry about the PT but I feel so much better about where you ended up. That's crazy about all the blown weber PTs...
     
  11. Wally

    Wally Telefied Ad Free Member

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    a meter will likely tell you what winding/s is/are problematic. Test the resistances of each winding...all primary and all secondary windings. Also, test each winding’s resistance to the frame of the transformer. If you find an open winding....infinite resistance....you have found a problem. If you find continuity between any of the secondaries or between any winding and the frame, you have found a problem.
     
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