Light chirping/humming sound on new Crack + Speedball build

[michte]

Hi,

First of all I'd like to say that building the Crack was a blast and the sound it produces was well worth the wait and effort.
Unfortunately, there is still one minor issue that I'm dealing with on a daily basis. I get a light chirping sound, mainly on the left channel side. It is really soft, but enough to be noticed when no music is coming through or during very quiet songs. Of course, all my resistance and voltage checks matched the values indicated in the manual and my solders joints look OK.
The sound dwindles down to an almost imperceptible hum if I move the Crack far away from any possible disturbance (electro-magnetic, wifi, ...), But I can only listen to music in the bathroom for so long
The weird thing is that I can make it go away if I shield the 12AU7 with my hand with my fingers touching the aluminum plate. This leads me to think that the tube is picking up some interference (lots of wifi hotspots nearby). I must also add that I live in an apartment that doesn't provide me with ground enabled power sockets (these were not mandatory in all rooms until 1992), which also would explain why touching the aluminum plate would get rid of the interference.

I guess buying a tube shield wouldn't be worth it since these seems to need a ground connection. Is there anything I can try that would reduce this sound without me listening to music my hand around a hot vacuum tube ?

Thanks in advance !

PS : sorry if some sentences seem weird, I'm from France

[Doc B.]

The old time solution would be to try grounding the chassis to a metal water pipe. These days that may or may not actually ground the chassis depending upon what the pipe is made of and how it is connected to the system.

[kgoss]

Isn't the ground tied to the aluminum plate?  I don't have a crack myself but if that is the case the mounting screws would ground the tube shield. If this works it might be a good idea for all crack owners. I have seen a lot of posts about EMF by crack owners.

[Doc B.]

He said that his AC outlet is not grounded. That means the chassis floats with respect to earth ground since it is designed to be grounded through the third, ground wire in the power cord.

[kgoss]

Ah, I didn't catch that.

[btrancho]

The old time solution would be to try grounding the chassis to a metal water pipe. These days that may or may not actually ground the chassis depending upon what the pipe is made of and how it is connected to the system.

I think that Doc has suggested your best bet.  You don't say where your apartment is but in my Brooklyn pre-war (1920s) building all the plumbing is still cast iron and (more recently) copper.  They don't allow PVC in many cities because the fire departments are concerned about the toxic fumes during fires.  If you are in a 1922 building chances are that you have radiators and there's a good chance that they are still connected via good old cast iron.  I'd try running a wire to the nearest radiator.

I'm fortunate that my apartment was gut renovated in 2007 and has 300 amp service, including dedicated 20 amp AC outlets in each room.  But I still have grounding issues.

[michte]

Thanks for all your answers.
My flat is from 1990 and all the pipes are in-wall and the heating system is all electric based, so not really ideal in terms of possible grounding alternatives. I guess I'll just move my setup as much as I can from any possible interference. I'll keep you posted if I find anything interesting.

[Paul Joppa]

From the description, it seems that merely shielding the 12AU7 and connecting the shield to the chassis will be adequate. You can replace the tube socket with one that accepts a standard tube shield, though that will be a lot of difficult work. You can find copper tape at electronic supply places and wrap the tube - but first, solder a short piece of wire to one end of the tape. Then you can wrap the wire around the mounting screw of the socket to complete the chassis connection. Kitchen aluminum foil is impossible to solder to, but would make an adequate test of whether this approach works.

I have never tried it, but you might be able to find a copper pipe of suitable diameter to fit over the tube, and solder a wire to it.

[michte]

Wouldn't something like this be easy to install ?
http://www.tubesandmore.com/products/P-SS9-325
It seems I would only need to remove the socket holder and replace it with the one provided with the shield. Or am I missing something (like if the socket itself is wired differently to accommodate the shield) ?
Also, what would be the best material for the shield ?

[Grainger49]

I believe the base of the tube socket, the part with the tube pins, has the connecting metal on it.  I'm saying I think the whole socket needs to be replaced.

But it is inexpensive to try this.  So order one up and give it a try.

[Paul Birkeland]

Yes, you can remove the retainer ring on the 9 pin socket and replace it with that shield.

I would still look for a good ground in your apartment, it would be very unusual for a building from 1990 to have ungrounded outlets.

[Doc B.]

I hate to be Doc Downer here, but I don't know if the shield will have enough effect without a ground reference. But it sure won't hurt anything to try, and if it works it sure wouldn't be the first time I was wrong.

[michte]

Apparently, the heaters are connected with a ground socket, so I'd just need to get an electrician to put a grounded socket right next to it to solve this. I'll definitely keep you posted about the shield efficiency.
Thanks for the help

[michte]

Hello everyone,

Sorry to dig up this old thread but I forgot to post the result of installing a tube shield. I did buy one off ebay and installed it. Short story : it didn't really change anything and potentially made things worse. But I also had the opportunity to test the amp in a "clean" environment : recent electric installation (ie good ground connection) and few wifi networks around. The result was like night and day, the amp was dead silent and I could finally listen to quiet tracks without being bothered by buzzes and chirps in the background (and it was glorious).

I will sadly have to shelve my crack while I'm in my current flat since I can't reproduce these conditions at the moment. Nevertheless, I don't regret building the thing and I'm sure I'll get to enjoy it fully soon enough.

Cheers

[Paul Birkeland]

If you've isolated it to the power line, you can look into getting some line filtration.   

-PB