periodic chirp (or squeak?) every ~12 seconds

[ludddite]

You mentioned pigeons, and the first thing that popped into my head is that one of your neighbors might have a bird x or some other type of pest control running. I don’t know how you would confirm this other than talking with them.  Just a thought.

[Deluk]

This is a bit off the wall and I can't be too specific but I have read of a using a small AM portable radio tuned to a non station position with the volume up full and use it as a receiver to pic up the signals. Chase the noise. Anyone want to comment?

[jmacinnes]

Waited for my partner to be away, and then spent a couple of days running up and down stairs flipping circuits. Here's what I'm learning:

More evidence that the chirps are caused by interference. Location within the house (upstairs, specifically) is the only variable that seems to change anything. There are 2 circuits upstairs (lets just call them A and B).
• chirps when the amp is plugged into A and the breaker for B is switched off.
• chirps when the amp is plugged into B and the breaker for A is switched off.
• no chirps when the amp is plugged into a different circuit downstairs (C) but along the same wall of the house.

The source of the interference isn't on the house's electrical
• chirps when the amp is plugged into A (and to the best of my knowledge, is the only thing plugged into A), and I switched off every other breaker in the house, one by one

Next thoughts are:
• there's an obsolete phone line and coax cable running up the outside of the house. There shouldn't be signal going through either, but worth ripping down anyway?
• try to see if I can rent an RF spectrum analyzer or something to at least get a clue on the source based on what frequencies the interference is coming in on? I have an AM tuner next to the amp, and I cycled through the frequency range on it, but never heard anything resembling the chirps.
• talk to neighbors about any pest control thing.
• Too uninformed to know if this is foolish, but is it worth building a small faraday cage out of some screen mesh to place the amp in? Or does the fact that I'd still have to run power through the cage totally negate any effect it would have? Grasping at straws...

[Paul Birkeland]

On your beaker panel (I'm going to assume you are in North America), are the breakers for A and B next to each other?  If they are not, are they on the same side of the panel?  How many breakers are between them?

It's still possible that A and B are running off the same half of the feed and C is on the other half, in which case this is still a fixable problem.

[jmacinnes]

Oh interesting, yes in fact, A and B are on the same side (left), with no breakers between them, and C is on the other side. Would it be helpful to know if there are chirps with the amped plugged in to any circuit on the left side? Looks like there are 2 other breakers on that side that power outlets in other (non-upstairs) rooms in the house.

[Paul Birkeland]

The way panels generally work is that two breakers that are right next to each other will be on separate branches of the main feed.  This lets you slot in a 240V breaker to feed a water heater, range, dryer, etc. and conveniently get 240V wherever you put a double wide breaker.

Since A and B are right next to each other and you have quiet operation elsewhere in the house, this is good news in a way!  It does still suggest that there is a source of noise causing these issues that has not yet been determined, but it isn't pollution coming from the power company at least. 

[Deke609]

To rule out the possibility that external interference is just particularly bad in your upstairs, you could run an extension cord from line "C" to upstairs and plug the crack in, putting the crack in one of the spots where you previously got the chirps on line "A" or "B".

cheers, Derek

[jmacinnes]

The way panels generally work is that two breakers that are right next to each other will be on separate branches of the main feed.  This lets you slot in a 240V breaker to feed a water heater, range, dryer, etc. and conveniently get 240V wherever you put a double wide breaker.

Since A and B are right next to each other and you have quiet operation elsewhere in the house, this is good news in a way! 

Ah, was hopeful there for a second that moving the breaker to the other side might fix it. But happy to be ruling out potential causes all the same!

To rule out the possibility that external interference is just particularly bad in your upstairs, you could run an extension cord from line "C" to upstairs and plug the crack in, putting the crack in one of the spots where you previously got the chirps on line "A" or "B".

Thanks for this suggestion -- I tested this over the weekend, and I think the evidence is pointing toward external interference. I plugged a 100' extension cord into "C". When I'm downstairs, no chirping. When I go upstairs, I get the chirps. Moreover, it varies in volume based on my location upstairs. Not a huge space (2 rooms and a hallway, ~300 sq ft total), but I can hear the chirping anywhere upstairs, and it is:

• quietest when I put the amp on the ground
• louder at waist/desk level
• loudest of all when I hold the amp over my head and place it near the south wall.

The south wall has 2 old phone lines (no signal, as far as I know) and a fiber optic line coming into the house from the street. However, those lines first hit the house below the upstairs, and my sense was that fiber optic wasn't likely to emit interference due to light and all that (true?). I've read about interference in the context of a nearby wireless device, where you hear intermittent noise as the device is transmitting/receiving. So I'm baffled about what a source of interference could be that is causing such a regular, consistent pulse, and a strong signal that's detectable over a wide area of the house.

[Paul Birkeland]

Hmm, what WiFi enabled devices are up there?  Any cell data enabled devices? Anything unusual in the attic?

[jmacinnes]

Typical wifi and cell data devices near the amp (phone, laptop, etc), but I've tested shutting everything down (these devices, and also all other devices throughout the whole house) and it didn't have any effect. In my last place, I kept the same devices next to the amp and never had an issue.

No attic, but a few months ago I redid the closet in the office, and saw an old galvanized pipe of unknown purpose running vertically in the south wall between the drywall and the siding. That's the only odd thing I can think of.

[Paul Birkeland]

You can also try fitting an aluminum tube shield over the 9 pin socket.  The add on ones will take the place of the socket retainer ring that comes with the stock kit.

[jmacinnes]

Awesome, thanks, I'll give that a shot! So something like this? https://www.thetubestore.com/belton-tube-shield-for-9-pin-tubes

In the meantime, can I DIY one out of cardboard wrapped in aluminum foil to test?

[Paul Birkeland]

Yes, that will fit.

You can just use foil, you don't really need the cardboard.  Just make sure the foil touches the chassis plate well.

[jmacinnes]

9-pin tube shield arrived and installed, but no difference, unfortunately.

Assuming there's nothing else plugged in to circuit "A" that could be causing the interference (I still hear the chirps when "A" is the only breaker switched on in the whole house), seems like the interference is coming from something outside my house (could that also be why it's loudest near the outer wall and higher toward the roof?). As far as I could tell the amp was the only thing plugged into "A" when I was testing, but I'll flip all of the breakers off except "A" and check the meter outside just to be sure.

If it is coming from something outside my house, seems like my only option is to shield the amp somehow. Are there any additional shielding options beyond the shield for the 9-pin that I should try out?

[Paul Joppa]

You can try a jellyroll pan - hold it close (5 inches?) to the amp and between the source - a crude direction finder!