Interference in left channel

[Nignoog]

Dear Bottlehead gurus,

It's been a while since I have visited your hallowed forums of eternal knowledge and problem solving. Here's the scoop:

I had a few issues with my Crack a few years ago. I sent it in to be repaired and it came back working perfectly. Over the last few months I noticed some interference in the left channel. It's a faint ringing that is sporadic, almost like Morse code. I can't hear it when sound is playing, but when there is silence it's very apparent and annoying.

Here's the part that is bizarre. When I move it away from my computer and plug it in somewhere else, it's dead silent. And I recently relocated my computer to a room that previously had no issues when the Crack was plugged in. So it must be my computer right? Well, when I shut it down and literally unplug everything, the interference is still present. I have tried the following to no avail:
1. Different electrical sockets
2. Creating a "shield" by holding a large jellyroll pan around the Crack
3. Tried "shielding" the tubes with tin foil and a large metal bowl
4. Tried moving the Crack around my desk to find a spot that doesn't pick up the interference
5. Tried tightening all the screws

A few observances that may help the diagnosis:
1. When I hold the Crack in my hands while it's on, if I press firmly on the chassis, I can feel it vibrating slightly, or rather, pulsing with energy. I am assuming that's normal, but I never noticed it before.
2. I have had multiple rectifier tubes, two 6080's and one 6AS7G, have their left channel break. They don't fail, but when I tap on those tubes with a pencil, I can hear ringing/vibrations in the left channel of my headphones. Not sure if the Crack is actively disrupting/breaking tubes. The current 6AS7G I have in is fine right now.

I am at a total loss. I have scoured the forums for answers and feel like I have exhausted all logical and possible solutions. Help me Bottlehead gurus, you're my only hope.

[Paul Birkeland]

The morse code type sound is typically from a cell phone or wifi router that's too close to the amp.  Based on your description, I would be leaning toward your cell phone being the culprit.  It's also possible that you have some other LTE enabled device nearby causing the problem. 

Tapping on a tube and hearing noises doesn't indicate that anything is wrong with the tube.  Those aren't broken tubes.   That is a physical reaction inside the tube called microphony. 

[Nignoog]

Thanks for the quick reply Paul.

I have removed from the room all devices that emit or receive signals (including my phone, laptop, and watch) and the left channel interference still persists. I can't think of anything else in the room that could be transmitting an LTE signal. What else can I try to diagnose or ameliorate the interference?

[Paul Birkeland]

Try taking your amp to a different dwelling.  It's somewhat possible that you may also have noise riding on your powerline, so it's good to eliminate that from the list of possibilities.  Also be sure you aren't listening to the amp with nothing plugged in and the volume pot all the way up, as that is a recipe for picking up external noises and amplifying them.

[Deluk]

Have you done the diode mod on the power in socket? That will often clear up strange noises. It did for me.

[Nignoog]

@Paul

I will take my amp into my office and give it a whirl. I have found that certain sockets, or at least the rooms in which the sockets lie, in my apartment do not produce any noise. Is it possible that the noise riding on powerlines can be in certain areas but not others? Additionally, if I pick my amp up and move it around, it can effect the noise levels. Wondering again if noise in powerlines can change based on proximity.

And good to know about listening without anything plugged in. How much needs to be plugged in though? Just RCA cables? A DAC too?

@Deluk

I saw that solution but want to exhaust other options first. I'd hate to perform surgery before ruling out it's just a "common cold."

[Paul Birkeland]

Just turn the volume pot all the way down when you do your tests.  If the reception of this noise changes when you move the Crack around where you are now, that does not indicate pollution in the powerline, but rather a wireless device spewing out a lot of noise.

[Nignoog]

Paul,

I took my Crack to work and plugged it into a surge protector and wall outlet. Dead silent. Based on your suggestions, it sounds like half of my apartment is polluted with wireless interference and the other half is not. Aside from relocating my amp, is there anything I can do to further diagnose or fix the issue?

[Deke609]

This would be my next step:

@Deluk

I saw that solution but want to exhaust other options first. I'd hate to perform surgery before ruling out it's just a "common cold."

It's barely surgery.  It only involves removing about 1-2" of bare wire and replacing it with two reverse-paralleled diodes.  All in, it's maybe 10 minutes of work, 9 of which is spent getting the soldering iron setup up and hot.  Plus, the area involved is readily accessible - very low risk of accidentally melting something with the barrel of the soldering iron. 

cheers, Derek

[Nignoog]

Based on the recommendations shared in this thread, I think it's time for me to add the diode modification to my Crack.

Based on what I read in the thread about the modification, I searched Amazon for diodes that will work. Before I order, can someone please confirm this will be adequate:

https://www.amazon.com/FAIRCHILD-SEMICONDUCTOR-1N4001-STANDARD-pieces/dp/B00LQPV2KA/ref=sr_1_4?dchild=1&keywords=1N4001+diode&qid=1607459465&s=industrial&sr=1-4

And if there a better diode, for ease of installation or better performance, would you let me know or post a link please so I can get those instead.

And thank you to everyone who has helped me and commented on this thread!

[Paul Birkeland]

Yes, you can use those diodes.

[Nignoog]

I have ordered the diodes and a solder wick. Based on the picture and what I can glean from the thread on this modification, is it simply a matter of removing the buss wire from the chassis ground and the 3-prong electrical ground and then soldering in two, twisted diodes between the two grounds? And the other half of the buss wire remains connected between the chassis ground and the connection labeled as #13, right?

[Paul Birkeland]

Yes, that is correct.

[Deke609]

And remember to "reverse parallel" the diodes - i.e., have the marked bands of the diodes at opposite ends before twisting the leads together.

[Nignoog]

Thank you for the tip! I see that in the picture now. I would not have done it had you not mentioned. You saved me a serious headache!