For the past year or more, a persistent intermittent (half second brittz every few tens of seconds) burst of RFI that would pollute the MW AM band was happening. Took a while to track down. Thought maybe a bad thermostat in the electric water heater, turn its breaker off, not it, still hear it. Try other breakers, seems one breaker cycling makes it go away for about 20 minutes. I eventually find that it's the power supply charger for my laptop. Seems when the laptop blanks its screen (screensaver and powersaver mode) it makes the RFI noise. Tickling the laptop mouse makes the screen light up and the RFI goes away. Or if I unplug either end of the power supply (mains side or the 19VDC output side). Okay, need to filter the power supply charger. I have a couple of those Cornell Doublieer powerline RFI filters (small metal box with 3 prong power plug and socket and RFI filter inside, and that looks to quiet the RFI down to almost nothing.

And for the longest time I figured it was faulty electrical wiring in the apartment building. I could see the lights flicker ever so slightly when the RFI bursts happened. That power supply must have been sucking down much amperage for brief periods of time. The filter is rated for 3 amps, but that's continuous. So I don't think it will get blown out...

Many switchmode power supplies are noisy, irrespective of what they are, in or doing. I have seen these with "slow blow" fuses in them due to the in rush current. The slow blow fuse will take about 10 sec to blow.

Its a fact of life that the higher the frequency, the less wire is needed and the smaller you can make the transformer. Unfortunately that High frequency and it's switching rate are the enemy and tube radio's are apt to pick up that RFI, as well a the crap generated by many compact Flouro's.

I have the added problem of large HV aerials which gather up RF from several transmitters around me and that rides on the mains. It has to be filtered out.

I normally use a 0.1 to 0.22 mfd cap across the mains and 0.047 or less Active to ground to filter out RF. These need to be mains rated caps, which you normally refer to as safety caps.

As it is a lightening prone area here (Granite) I also have Metal Oxide Varistors Neutral to Active, Active to Earth in the filters.

If a really large spike hits, this configuration is capable of taking out the Bench circuit breakers as well as tripping the RCD protection circuit.

We were hit recently and it blew out a Utility Transformer feeding next door. No device damage here.

Also make sure the tube radio has a non polarised cap say 0.047 (have one here with 0.25 mfd) decoupling the B+. Electrolytics are not good at Filtering RF riding on the mains.

Marc