Tubes Static/Grounding Issue??

[blubliss]

OK, so I have had for a long time now issues with some sort of interference in my system which manifests as a background high-pitch noise that is fatiguing after awhile. I have tried many things to assuage the problem because its source has been unknown. Thought it might just be digital harshness, the headphones, etc.

Anyway, I got some bendix 2C51 from luvdunhill recently and popped them in to my SDS-XLR (gotta live with what I got ) and when music started there was an intermittent static noise in both tubes. No music, no static. I thought it might be bad tubes. Anyway, this morning, for whatever reason, I decided to cup my hand around the tube (between driver & power tube), and lo and behold, the static ceased. I was not touching the case or tube, but if I touched the case it still worked. But only if i cupped my hand around the tube, not if I touched the case in other places.

Is this some sort of interference between the power and driver tubes (2C51 & 6BL7GTA)? I have not noticed this with the other driver tubes, 6SN7, but it may be real low. I have noticed interference of a high-pitched nature as I mentioned before.

Now I had noticed some time back that I could no longer listen to single-ended phones on my SDS-XLR with the APL as source, major hum/static. If I used an adapter and went RCA into the amp, problem solved. I was told that this was due to the grounding schemes of the amp and source. The APL is transformer-coupled XLR output and the SDS-XLR is who knows what.

My thought now is that this tube issue is also related to ground issues. One other strange thing I noticed is that from time to time when I would disconnect my ICs and then put them back on the sound seemed to improve, that is, the interference was much less. But, over a short time it came back.

Could all of this be related to ground issues? What could I do to rectify? Any other info you need to help? Thanks.

[dsavitsk]

Pictures would help (particularly around the connections to the tube sockets), but it sounds like a combination of things. There could be some grounding issues and any competent tube tech should be able to fix those. Additionally, the tubes could be oscillating due to poor construction. Oscillation can manifest itself in a lot of different ways, from static, to hash, to squealing, to microphonics, to ringing. 6sn7's are not known for a propensity to oscillate, but any tube can, given the right (or wrong) environment. Really, you'll need a scope to know if this is the case, but if there aren't measures to quell oscillation taken, then adding them is as good a test as any. This means adding grid stopers, and in extreme situations not likely here, plate and catjode stoppers as well. The tubes could also be picking up interference of some sort which would be cured with some shielding.

As a first go, try making shields out of aluminum foil and wrapping them around the tubes. Literally, this means wrapping a tube of aluminum foil around the tubes. Be sure they are grounded to the chassis -- probably touching the screw that holds the socket on will be good.

edit: ah, you are using those dumb tube adapters? Almost certainly oscillation. Could be other stuff, too, but those were a bad idea from day 1.

[blubliss]

As a first go, try making shields out of aluminum foil and wrapping them around the tubes. Literally, this means wrapping a tube of aluminum foil around the tubes. Be sure they are grounded to the chassis -- probably touching the screw that holds the socket on will be good.

edit: ah, you are using those dumb tube adapters? Almost certainly oscillation. Could be other stuff, too, but those were a bad idea from day 1.

Shields was an idea I had too. First I decided to prop up some ERS paper between the driver and power tubes, this worked. What does that mean?

So the adapters may be the issue?

[dsavitsk]

Shields was an idea I had too. First I decided to prop up some ERS paper between the driver and power tubes, this worked. What does that mean?

If adding a shield solved your problems, then it means you need a shield. No shame in that. Shielded sockets are not uncommon:

Here's my phono stage with shields

So the adapters may be the issue?

If shields solve your problems, then it sounds like they aren't in this case. But yes, they can be a problem. Search a tube forum for "grid stoppers" for more info.

[nikongod]

So the adapters may be the issue?

They can make oscillations or noise worse.

It dosnt sound like the amp is running properly with 6SN7 as gain. Thats what the amp was designed around; if its not right on its best tube noting else stands a chance. I would take it into a shop.