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I'm no expert on these but I do know they have an overly complicated filament supply regulation scheme. If I recall correctly it's a switched duty cycle that controls the average voltage going to the filament, complete with sense lines I believe. There's something wrong with the sensing circuit on mine because it's output voltage changes as a CRT's filament warms up. If your hearing is real good it's possible you're hearing the pulse width modulation frequency changing.
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