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SET Asylum Single Ended Triodes (SETs), the ultimate tube lovers dream. |
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SET Asylum Single Ended Triodes (SETs), the ultimate tube lovers dream. |
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In Reply to: RE: I'm Melting... What a world, what a world... posted by drlowmu on December 18, 2011 at 22:29:33:
Jeff makes a number of good points, especially about finding the cause of the problem. Most of the responses so far in this thread make the assumption that the transformer failed from excessive heat, and that the heat came from the current through the wire resistance.
There are other causes, and other potential problems, with power transformers.
First off (I have said this before) transformers have a limited magnetic headroom. If you use grain-oriented silicon steel laminations for example, you can according to the manufacturers run them as high as 16,500 gauss of magnetic flux. Run this hard, the core dissipates a fair amount of energy and gets hot; traditional designs try to match the heat generated in the core with the heat generated in the wires. If you run them even a little bit harder, the core temperature will rise quite rapidly because it is in saturation.
Running the core hard also leads to more vibration, hence mechanical buzzing - this comes from both magnetic forces on the current carrying wires, and also from magnetostriction - the metal itself distorts under the presence of its magnetic field. This vibration can accelerate abrasion of the wire insulation, and thus shorted internal turns followed by what appears to be heat death and loose smoke.
Running the core hard also leads to excessive radiated magnetic field, which often induces hum into other parts of the circuit. As you get into saturation, the core rejects more and more of the field, forcing it into the surrounding space.
If you use a transformer specified for a much higher current you will reduce the copper losses, as Jeff suggests. But you will not change the magnetic losses at all. To sensibly reduce the temperature, you should put a few more turns on each winding and reduce the flux.
Flux increases with voltage, so if your house power is at a higher voltage than the transformer specifies, you can easily exceed the design flux density. Flux density also increases linearly as the inverse of the frequency, so transformers rated to operate at 50Hz as well as at 60Hz will have a 20% margin of magnetic headroom. This is the important difference between the Hammond 3xx and 2xx series, and the reason the 3xx are physically larger for the same ratings. If the transformer runs 16500 gauss at 50Hz, then it will run a bit below 14000 gauss at 60Hz.
Transformers are complicated beasts, with a large number of important variables interacting with each other. I am convinced that you must respect all of them in order to get a good result. Not that it does me any good, but I feel the same way about single-issue politics!
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Follow Ups
- RE: I'm Melting... What a world, what a world... - Paul Joppa 18:29:36 12/19/11 (0)
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