Ever heard an OPT actually make vibration noises??

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by pfrischmann, Oct 22, 2003.


  1. pfrischmann

    pfrischmann Member

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    I was working on this same amp with a Dr.z attenuator on it to see what it sounds like dimed. I had the attenuator maxed then going to an 8 ohn cab.

    As I hit certain notes the OPT would start making these notes come out of it like it was a really crappy speaker. Please note the chassis was not on top of the cab. I'm asssuming this is bad.

    However, the amp sounded the same after. This amp is styled after a Vibrolux and has 423 volts on the plates of a pair of RCA 6L6gc's. The builder has them biased at 53ma (according to my bias king, which seems to have a 60 cycle hum)

    It's a shame this is never easier.

    Any opinions??

    Thanks,
    Paul
     
  2. John Phillips

    John Phillips Member

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    Yes...

    I had exactly this problem with a '59 Tweed Champ I bought - cheap, because it sounded awful when cranked, and the previous owner knew it wasn't the speaker because they had tried replacing it, along with just about everything else... (luckily they kept all the original parts). I was optimistic and guessed I could find the problem.

    I nailed it when I had the amp connected to a dummy load. The OT was buzzing extremely loudly, almost like a little radio speaker. Some of the core plates had come loose. I cured it simply by re-crimping the mounting bracket tightly and adding more varnish.

    There's definitely something loose in that OT. Whether a plate or a winding I don't know, or if it's serious. Does it affect the sound through the speaker when it's used normally?
     
  3. reaiken

    reaiken Member

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    At full volume into a dummy load, all transformers will have some amount of "singing" (it's called magnetostriction, by the way). Most tranny vendors will tighten the laminations down real well and then "varnish" the entire assembly to keep it from getting moisture in and to keep it as quiet as possible. If you hear an inordinate amount of noise, you probably have a loose screw on the lam stack.

    There is also a remote possibility that the tranny is arcing over inside causing a noise.

    Randall Aiken
     
  4. pfrischmann

    pfrischmann Member

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    Wow,
    Thanks. How do you guys learn this stuff?
     
  5. John Phillips

    John Phillips Member

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    A curiosity about all things electrical since I was a kid and getting on for 20 years experience as an amp repairer, mostly... :)

    I did study electronics at university too, and although some of the theory is very useful when asking 'why'-type questions, nothing beats practical experience (and a good memory) when it comes to this sort of repair problem.

    It took me a while to find the fault with that Champ, even though it was so obvious when I did. But it won't the next time. And I knew I would be able to fix it... tube amps are not actually complicated.
     
  6. pfrischmann

    pfrischmann Member

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    I was thinking about a couple of the things you guys were saying and hope will you indulge a fem more questions.

    First "Magnetostriction" Great word!! It sounds like a Holdsworth ablum.

    I noticed when I tap the transformer, I can get the singing to stop. Is this something I can track down or should I just leave it, or send it to the designer? THe OPT looks like a hand wound wax laminate job like a tweed fender would have. It is not encased in anything.

    Any of this strike you as odd (below). I'm new to the bias king and question whether mine is working correctly 53ma seems really high? I would think it would be more like 35ma?

    "This amp is styled after a Vibrolux and has 423 volts on the plates of a pair of RCA 6L6gc's. The builder has them biased at 53ma (according to my bias king, which seems to have a 60 cycle hum)"

    Thanks again,
    The builder is a little difficult to reach so I'm trying to make sure I don't have anything serious on my hands before I send it back. It's used and will cost me a lot to have him look at it. He does not like looking at his used stuff.

    Thanks,
    Paul
     
  7. Mark Kane

    Mark Kane Silver Supporting Member

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    Paul, did you try biasing them at say 30 to 35mA. I would guess the humming would go way down there.
     
  8. Mark Kane

    Mark Kane Silver Supporting Member

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    Paul, is this the amp you had at the jam last weekend that everyone said was underpowered sounding. You may have a bad cap in the bias supply or a bad screen resistor or something like that. I've seen screen resistors read fine when cold and then open up when you got juice on them. I wouldn't bother sending it back to NY, just have someone local look at it. Unless the OT is bad it's probably a tube or a bad small part.
     
  9. pfrischmann

    pfrischmann Member

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    Thanks Mark,
    Yes, it's the same one. Good thought about the Bias supply etc.

    When I mentioned the humming, I was refering to the Bias king. It actually has a humm inside it. There is no humming through the amp which I'd expect there to be at 53ma. The bias king may be broken.

    Sometimes it just aint' easy...

    Thanks,
    Paul
     

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