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The tube that has its own filament supply probably has a DC bias voltage (like around +150 volts) applied to it. This is done when the cathode is at a high DC voltage, so that the cathode-to-filament voltage difference doesn't exceed the tube specs. A seperate filament supply is needed as the regular supply usually has one side grounded and putting high DC voltage on it would cause a short.
Look at the photofact and see what the cathode voltages are on that tube; you will probably find that they are high. Also you can probably find the bias voltage connection for that particular fil. winding.
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