High pitched noise alternating if I move amp or come close

[TestNamePleaseIgnore]

Hi,

First-time poster and DIYer so be gentle

I completed my crack and speedball. All resistance checks passed, all voltage checks passed except some were slightly higher than recommended. OA and OB were around 110V but the manual says it could be due to tube tolerance so I thought it was fine.

Problem is that there is a high pitched buzz sound coming from the amp. It is a constant noise no matter if there is an input connected or where the volume pot is. The weird thing is that the volume of this buzz changes if I move the amp around, but it is always there. Also, if I move my hand close to the amp the volume changes. If I play around and cover the tube with my hand (without actually touching it) I can get the volume down to inaudible levels. This does not seem to work with other objects like plastic, metal or cardboard.

Now I've double and triple checked all my solder points. I've checked the transformer mounting. I've tried 2 different 12AU7 tubes. I've tried different power cables. I've tried a different wall point. I've tried switching off ALL electronics in the area including my phone and router to eliminate any noise from it.

This buzz was there before I installed the speedball, but I thought that is just how the amp performs. I thought the speedball will have "cleaner background" and remove this buzz, that is why I installed it before posting.

Any help will be greatly appreciated!

[Paul Birkeland]

No, there shouldn't be any buzz.  Buzz that is just there no matter what you do is generally a loose 220uF filter capacitor letting power supply noise through.

If the nature of the noise changes when you touch the top plate, I would check the solder joint where the black wires all meet on the headphone jack (it's easy to solder that joint an accidentally still have one loose wire there), and if you've painted the bottom side of the panel, then the paint on the bottom needs to be scraped off under the #8 screw by the power entry module and under the terminal strip at terminal 3.

How were the rest of your voltages? 

110V at 1 and 5 is a little high, and may actually indicate that you have a faulty solder joint elsewhere in the amp.

-PB

[TestNamePleaseIgnore]

As I was remeasuring now, with my negative lead connected to terminal 12, it made quite a large spark when I touched IA with the positive lead. It left a black burn mark at IA. Luckily didn't get a shock, but I got a fright and decided to rather unplug the power and post some pics first. I did manage to measure:
OA: 110,2V
AB: 106,9V

I did check the solder at the joint and it seems fine to me. I did not paint the panel at all.

Sorry but I kept getting "request to large" errors on this form so I decided to share my google album
photos[dot]app[dot]goo[dot]gl/euR3KR8yz4OwPy3Q2

[Paul Birkeland]

Please remove the Speedball from your Crack before we proceed.  The Speedball will only complicate the debugging on an amp that wasn't previously working.  This is thoroughly covered on page 10 in the Speedball manual.

The meter would cause a spark like that if you accidentally had it set to read resistance instead of voltage.

[TestNamePleaseIgnore]

Sorry, but I did not know that it was not working as it should.  The buzz is not that loud. At normal listening volumes, the music covers it and I don't hear it anymore.

I can attempt to remove it.

[Paul Birkeland]

You can also try tightening all of the hardware significantly, but when you are moving the amp around and the noise is coming and going, you're likely just disturbing a loose connection.

If all Cracks had annoying buzz at 0 volume, that's all you'd read about on here!

[TestNamePleaseIgnore]

Yeah, I should have expected better.

It's making this buzz when I move my hands around the amp. If I cover the 12AU7 without touching it, it changes. If I bump or tap on it, the sound does not change. If I move it to a different physical location (up/down/left/right) it changes.

This is why I don't think it is bad soldering or a loose connection but rather some external noise like a router/cellphone etc. I did try switching everything off but no luck.

[Paul Birkeland]

All of that sounds like loose hardware or poor grounding in the amp.  With the volume pot all the way down, external noise sources will have a very hard time entering the amplifier.

[Doc B.]

That noise level varying with proximity of your hand is exactly what one would expect from a bad ground connection. You are a sort of antenna for electromagnetic interference. The floating ground is not bleeding off the EMI, but rather letting it into the circuit.

[TestNamePleaseIgnore]

I went through this guide and all seems to check out: https://bottlehead.com/smf/index.php?topic=4812.0

I've measured my mains again and saw that and I am getting 236V AC but wired my crack in the "225 < X < 235" config. I did that cause officially my country is supposed to have 230V and I thought that it could be my meter being inaccurate or something. Could this be the cause?

[Paul Birkeland]

No.  Do you have grounded outlets in your dwelling?

[TestNamePleaseIgnore]

Think I found the issue. I've measured the VAC over neutral and ground and saw it was ~0.3V. And if I move my hand near the amp it fluctuates to around 5V. So I think the issue is that my outlet not being correctly grounded. I'll test other outlets and report back.

[TestNamePleaseIgnore]

So I've tested my wall outlet and it is fine.
Live <-> Neutral ~ 230V
Live <-> Earth ~ 230V
Neutral <-> Earth ~ 0V

Quick question? Why is the red wire connected to N and black to L? That is AFAIK the wrong way around?

[Paul Birkeland]

So I've tested my wall outlet and it is fine.
Live <-> Neutral ~ 230V
Live <-> Earth ~ 230V
Neutral <-> Earth ~ 0V

Quick question? Why is the red wire connected to N and black to L? That is AFAIK the wrong way around?

Traditionally, power transformers had a primary with two black wires that was wound for the voltage where it was to be used.  Though we would love to carry over this tradition, wiring the incoming AC with all black wire makes it a bit harder to debug and much harder to photograph for the manual, so we used red for one of the connections.  In North America, the hot wire in home AC mains is black and the neutral is white.  Red tends to contrast a bit better against black, so we used red instead of white.  There isn't any "wrong way around" in this situation.


Can you pull your power plug out of the wall and measure the DC resistance between the ground pin on your AC plug and terminal 3 on the Crack?  Have you tightened all of the hardware?

[TestNamePleaseIgnore]

Thanks. Just had me confused for a moment as we use red for hot and black for neutral in SA.

Ground to terminal 3 is 0.8 ohm. Double checked all screws.