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WD,
You should definitely get the AC heater voltage down a few volts on the 35Z4. Try a resistor in series with the dropper cap. Maybe start with a 22 ohm and see which way it needs to go. (Make it 2 watt or higher rating.) Heating won't be much of an issue.
The 1V DC tube filaments want to see no higher than 50 ma. So the DC filament dropper should be delivering 50 ma. to the string. Since it's series, always go by the current, not voltage per se.
The video won't play on my rig (iPad). But what you describe sounds like 'tunable hum'. That's when the received signal is getting 'modulated' at the powerline rate. It can be caused by a coupla things..
1. An open antenna coil causing the converter tube's grid to 'float', thus making it hypersensetive to ambient powerline fields.
2. The received signal being re-radiated by house wiring, usually due to power diodes in some piece of equipment switching the re-radiated signal on and off.
Try disconnecting (not just turning off) all appliances and electronic gear one-by-one and see if the problem clears.
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