High Frequency Noise

[Haakonl]

Hi, I have been struggling with high-frequency noise for quite some time, and even after researching many of the topics on the forum here, I still haven't found a solution.

My setup is as follows:
I have a Crack w/ Speedball 1.1 modded with Cree diodes and film caps. I have also installed ground breaker diodes between the chassis and the ground tab of the IEC power entry module to reduce low-frequency noise. I use it with a Tung Sol 5998 and a Ken-Rad 6SN7GT VT-231.

The noise is after the input, as it is still there with the Crack wholly disconnected. Turning the volume knob has no effect either. I have also noticed that the amp is quite microphonic around the power tube.

Here are some of the steps I have tried:

• Measured all voltages, looks good
• Tried different power outlets
• Tried using it in another home (though still located in the same city) to see if it was due to but some antennas or something, still no change
• Turned off all cell phones nearby
• Tried with other tubes
• Looked for cold solder joints and reheated a bunch of them with more flux

It would be great if someone could help me with this!

 

[Paul Birkeland]

What does the noise sound like?  Are you positive it isn't present with a regular old 6080?  Is it present in both channels?

 

[Haakonl]

Hi Paul,

I tried to record the noise, but neither my phone nor computer microphone would pick it up, but I found a video on Youtube with a similar sound. I don't have permission on this forum to post URLs, but it is called "How To Stop The Electrical Noise From Studio Monitors!" by Omoi Senpai, and the noise is shown in the first couple of seconds of the video. My amp sounds quite like the noise in the video, though with an even higher pitch and without the white noise in the background.

I tried again yesterday with the 6080, and the noise is still there.

The noise is present in both channels, though it is louder in the right one.

Thanks for the help!

 

[Doc B]

Looked for cold solder joints and reheated a bunch of them with more flux

If a bad joint is causing the problem the most reliable technique for reflow is to add just a bit of rosin core solder and only add it if the joint needs a bit more solder. Not sure if this is what you mean. If you are adding only flux to the joint when reflowing - adding more flux alone to a joint often makes things worse because the problem can be a cold joint that already has excess flux in between the wire and the terminal or pad.

It seems that we see people posting about using separate flux a lot lately. I'm guessing there is a YouTube out there suggesting this. It is not a good way to do our kits, point to point, or thru hole soldering in general. Rosin core solder has all the flux needed.

 

[Paul Birkeland]

That kind of noise is almost certainly environmental.  Have you tried the Crack in a different location?  There may be some device in close proximity to the Crack that is spewing out that noise and it is simply being picked up by the Crack itself.  This would be even more likely if the noise is present with nothing plugged into the inputs and the volume pot turned all the way down.

 

[2wo]

Are you using an adapter with the 6sn7? Maybe back to the 12au7 temporarily...John

 

[Haakonl]

If a bad joint is causing the problem the most reliable technique for reflow is to add just a bit of rosin core solder and only add it if the joint needs a bit more solder. Not sure if this is what you mean. If you are adding only flux to the joint when reflowing - adding more flux alone to a joint often makes things worse because the problem can be a cold joint that already has excess flux in between the wire and the terminal or pad.

It seems that we see people posting about using separate flux a lot lately. I'm guessing there is a YouTube out there suggesting this. It is not a good way to do our kits, point to point, or thru hole soldering in general. Rosin core solder has all the flux needed.

Yeah, maybe I was not clear enough; I have mostly just added some rosin core solder, like you said, though in some cases, I have just added flux depending on the situation. Will take that into account moving forward, though! Most of my experience is from SMD soldering, where I learned that technique. I didn't know it was not transferable to THT.

That kind of noise is almost certainly environmental.  Have you tried the Crack in a different location?  There may be some device in close proximity to the Crack that is spewing out that noise and it is simply being picked up by the Crack itself.  This would be even more likely if the noise is present with nothing plugged into the inputs and the volume pot turned all the way down.

Now that you mention it, the issue actually appeared when I moved to a different city, so you might be onto something here. I have, however, recently moved once again to a different place in the same city, and the noise is present at both places. Maybe it is related to some of my computer equipment? I'll try to bring the Crack to my workplace and check if it's still present there. If this is the issue, are there any places where I might add shielding or filtering to mitigate this in the amp?

Are you using an adapter with the 6sn7? Maybe back to the 12au7 temporarily...John

I have replaced the 12au7 socket with a 6sn7 one completely, so I will unfortunately not be able to do that. I have, however, tried switching out the Ken-Rad tube with a Raytheon one with no effect on the noise.

Once again, thank you so much for the help everyone!

 

[Haakonl]

So a minor update, I tried the Crack at work today, and there was no noise, so it is picking up something at home. I'll try to figure out what is making the noise at home, though the ideal solution would be to make the Crack to avoid picking up the noise.

 

[Paul Birkeland]

If you wrap aluminum foil around the outside of the 12AU7 and allow it to touch the chassis plate, does the noise go away?  (Do not let the foil touch any of the actual pins of the tube)

 

[Deluk]

I don't know if this will work with your noise chasing but if you have a small cheap transistor radio, tune it on AM to the noise between stations and move it around the room near whatever electricals and power points you have local to the crack.  It might pick up the buzz.

 

[Haakonl]

If you wrap aluminum foil around the outside of the 12AU7 and allow it to touch the chassis plate, does the noise go away?  (Do not let the foil touch any of the actual pins of the tube)

I tried this but with no effect. While trying it I noticed, to my great confusion, that the noise is greatly reduced when I hold my hand around the tube, especially at the front. I guess this means I should buy a Faraday cage for the tube, or is there any other way to remedy this? And if the solution is a Faraday cage, does anyone have any suggestions on where to buy ones that fit 6SN7?

I don't know if this will work with your noise chasing but if you have a small cheap transistor radio, tune it on AM to the noise between stations and move it around the room near whatever electricals and power points you have local to the crack.  It might pick up the buzz.

I unfortunately do not own an AM radio, though thanks for the tip, I will try to borrow one to test!

 

[Paul Birkeland]

Can you post some build photos?

 

[Haakonl]

Hi, sorry for my late reply; I have been through a very hectic exam period.

Yesterday I ended up disassembling and resoldering my entire kit. This did fix a few minor issues I had with the Crack. However, the high-pitched noise is still there, so I am sure the soldering is not the issue.

You can see build pictures here: https://imgur.com/a/liiFnuC (No idea why it is marked as adult content, haha). My Crack is quite cramped, so it was hard to get good pictures of everything, so please tell me if you would like more photos of specific places.

Again, thanks so much for the help!

 

[Paul Birkeland]

Are you positive this doesn't happen with a 6080?

To be honest, there are enough mods on this Crack that it's tough to offer much in the way of definitive help.  Did this noise start after one specific mod was installed?

 

[Haakonl]

Hi, I tested again with a 6080, and the noise is also present with that.

The noise only appeared after moving to a different city, so I am not sure whether a mod made it susceptible to noise or not, unfortunately.

 

[Paul Joppa]

I tried this but with no effect. While trying it I noticed, to my great confusion, that the noise is greatly reduced when I hold my hand around the tube, especially at the front. I guess this means I should buy a Faraday cage for the tube, or is there any other way to remedy this? And if the solution is a Faraday cage, does anyone have any suggestions on where to buy ones that fit 6SN7?

I unfortunately do not own an AM radio, though thanks for the tip, I will try to borrow one to test!

Octal tube shields can be hard to find, but they do exist. Here are some (ugly) examples:

https://www.ebay.com/itm/115832857135?chn=ps&mkevt=1&mkcid=28&srsltid=ASuE1wQuswwiLE149kSDCSmoO98mcFDghtanSPsXyMtNWipOzO_VoY7AWi4

 

[Haakonl]

Thanks for the suggestion! I will look into getting one and update you on the result.

 

[Doc B]

The solution might be to try a different 6SN7.