periodic chirp (or squeak?) every ~12 seconds

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. 
 
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
 
Paul Birkeland said:
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! 
Click to expand...

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!

Deke609 said:
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".
Click to expand...

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.

 
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.
 
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.
 
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?
 
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.
 
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?
 
Do you know anybody who has a mini Honda generator to provide you with power? Desperate measures! Not sure if you have taken the Crack to another location for a cross check that way.
 
Another cheap experiment: wrap the wooden base a few times in aluminum foil, and tuck the last 1/4 inch of the last corner of the foil wrap under a corner of the metal chassis plate (but NOT at the corner with the IEC inlet). This will form a shield of sorts that can drain to house ground via the chassis plate.

If the foil experiment helps, you could buy some copper film tape and line the inside of the base, including the narrow lip/shelf of the base that supports the chassis.

cheers, Derek
 
Actually, the jellyroll pan was a pretty good idea. I grabbed a heavy duty 9x13" one, and if I hold it over the Crack like an umbrella (tilted just a bit toward the west south west) it seems to block the chirping almost entirely! I can only assume this means the signal is coming from the mothership locked in geosynchronous orbit somewhere up in that vicinity, but I'm much more interested in enjoying my amp again than in deciphering whatever message they're trying to send me ;)

I'm going to try wrapping the base in foil too, like you suggest Derek. If that works just as well, the copper film tape will be an elegant solution. If not, gonna buy a few more jellyroll pans and break out the hacksaw to build an enclosure for the amp to live in.

Really appreciate all of the help and suggestions in tracking this thing down (and more importantly, ideas for how to block it)! Can't tell you how much the chirps have been driving me mad!
 
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