No, I don't think that is normal.
Given that you've changed the tubes with no improvement, I'd have to say that unfortunately it will probably be difficult to isolate. I would think the most likely bet would be a capacitor or other component that has to handle high voltage, and is starting to break down under that stress.
But external sources would be possibilities as well, such as rfi/emi being picked up through the air or the power line when an appliance or other high current device elsewhere in the house cycles on or off.
Sorry I can't offer any more concrete suggestions.
Regards,
-- Al
Given that you've changed the tubes with no improvement, I'd have to say that unfortunately it will probably be difficult to isolate. I would think the most likely bet would be a capacitor or other component that has to handle high voltage, and is starting to break down under that stress.
But external sources would be possibilities as well, such as rfi/emi being picked up through the air or the power line when an appliance or other high current device elsewhere in the house cycles on or off.
Sorry I can't offer any more concrete suggestions.
Regards,
-- Al