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Blowing speakers... but why?

Discussion in 'Amp Tech Center' started by MrCoolGuy, Nov 21, 2020.

  1. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

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    I'm confused... I have built a few 5F6A's as well as some others. I currently have my own and the one I'm working on. I just finished the new one, but it immediately blew one or two of my speakers. I had it plugged into my old 5F6A cabinet (Two WGS Vet 10's(20watt) and two Weber CTC10's(25watt)) to test it out after completion. Light bulb limiter went well, voltages look great. Sounded good so I turned it up, as you do. Speakers blow. Not sure which one(or more) yet... . This cabinet sounds great with other 5F6A chassis or heads... so I'm not sure what happened. So I plugged the chassis into a hot rod deluxe cabinet (same output power as a 5F6A). I turned it up slowly.
    And before you can even get to any type of overdrive, it starts pushing the speaker too hard ... so I stopped. Any ideas?
     
  2. Richie Cunningham

    Richie Cunningham Tele-Holic

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    Is it transformer-coupled?
     
  3. schmee

    schmee Doctor of Teleocity Silver Supporter

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    That's very strange, got stray voltage on the "speaker out" somehow? Did it sound right?
    BTW: If any of the Webers blew they recone their own for $25 I think....
     
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  4. galaxiex

    galaxiex Tele-Afflicted

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    I'd suspect a mis-wire somewhere that puts DC on the speaker out.
    Maybe something in the feedback network?

    Before you blow another speaker, test for DC on the output.
     
  5. dogmeat

    dogmeat Friend of Leo's

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    yeah, pretty much has to be the NFB loop
     
  6. Richie Cunningham

    Richie Cunningham Tele-Holic

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    I was going to say this, but is it still possible to get DC at the output if there is an OT?
     
  7. galaxiex

    galaxiex Tele-Afflicted

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    Look at a schematic.
    The feedback is wired directly to the speaker jack.
    If something is mis-wired or maybe shorted in the feedback network, it's "possible" to have DC at the speaker.
     
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  8. galaxiex

    galaxiex Tele-Afflicted

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    From robrob site...

    [​IMG]
     
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  9. trobbins

    trobbins TDPRI Member

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    So it is a new build that will need faultfinding.

    It could be as simple as swapped feedback polarity, or it could be some unanticipated feedback coupling. Either way the output signal could have a large oscillation, which may not be audible.

    Perhaps disconnect the feedback resistor, and only connect a much more powerful and rugged speaker, or a dummy power resistor.

    If you haven't 'heard' the oscillation so far, and the speaker cone isn't showing signs of gross movement, then you probably have to get a scope to 'see' the waveform reaching the speaker.
     
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  10. Dacious

    Dacious Poster Extraordinaire

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    There's no other sounds or tone before the speaker blows?

    I would have thought DC would be audible.

    The other thought I had - it's not oscillating?

    If you can pass a signal maybe measure current and voltage at the speaker terminals.
     
    Last edited: Nov 22, 2020
  11. trobbins

    trobbins TDPRI Member

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    It would have to be a very gross wiring mistake to get DC on the speaker (such as the output transformer primary is being accidentally connected to the speaker secondary).
     
  12. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

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    16060697309281877562676718400409.jpg No nothing like that. The only thing is is that the nfb resistor is the wrong value for 4ohm tap... (4 ohm being the one i would use and the lowest value offered).
    The nfb resistor being used is a 27k, which would be correct for a 2 ohm tap.
    But I thought it would be ok until I get the correct one in there. The amp sounds fine up until it blew the speaker...
     
  13. MrCoolGuy

    MrCoolGuy Tele-Meister Silver Supporter

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    No, there's nothing connected to the switch Lugs of the output jacks. They just have old solder on them.
     
  14. King Fan

    King Fan Poster Extraordinaire

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    Have you tried lifting the NFB? Leaving aside DC, whether likely or not, I'm wondering about gross (and sometimes inaudible) oscillation as mentioned by @trobbins .

    And it's a drag. Blowing speakers isn't like blowing fuses, for like $1 each. Do you have/can you get a scope? Dummy load? How about other ways to test/analyze the output?
     
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  15. galaxiex

    galaxiex Tele-Afflicted

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    Think about that.... ;)

    DC is a flat line.... Zero frequency.... I'm "pretty sure" you can't "hear" DC.

    What you might notice is the speaker cone would move either in or out depending on the polarity of the DC component.

    Touch a 9V battery to a speaker terminals sometime.
    Watch the cone move, you might hear a slight pop when you touch the battery, but you won't hear any "sound" from the DC.
    Reverse the 9V and the cone will move the other way.
    Don't worry, a 9V batt can't hurt your speaker if you only connect it for a few seconds.
    It's a useful test to determine speaker polarity.

    But yeah... it was a stretch to consider DC at the speaker on such an amp.
    As already mentioned, it would have to have some gross wiring error for that.

    DC on the speaker is/can be, a common fault on SS amps, due to a shorted output device or capacitor.
    DC on a speaker from an amp fault will kill a speaker fast!

    .
     
  16. Dacious

    Dacious Poster Extraordinaire

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    Yes, but if it's got DC and AC it might 'look' to the speaker like a square wave. Which is usually the SS speaker popper.

    I'm just spitballing. Maybe scope it.

    I'd go ultrasonic or subsonic oscillation, which might be visible at the plates as current.


    I worked on a Laney AOR100 that kept toasting one EL34. The culprit was a 12AX7.

    All volumes turned down, everything good. All voltages and currents bang on. Moment the channel gain went up if master was up, it ran away. Always smoked the same tube, which took out the ht fuse.


    You could just hear this high pitched whine for a second
     
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  17. trobbins

    trobbins TDPRI Member

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    I use a pair of Celestion G12K-85's for initial amp testing - something like that can be cheap secondhand as many OEM speakers get removed from big quad cabs when an owner sees someone else's tantalising youtube video.
     
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  18. cyclopean

    cyclopean Poster Extraordinaire

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    are you running drum machines and synths through it?

    is it lower powered than your guitar amps?
     
  19. Phrygian77

    Phrygian77 Friend of Leo's Gold Supporter

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    Have you scoped the output?
     
  20. NTC

    NTC Tele-Meister

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    Have you put a voltmeter set to DC on the output? That will settle this quickly.
     
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