Slight hiss in L channel

[EraserXIV]

I'm getting a slight "hiss" in the L channel and I was wondering what could possibly be causing this. It's very faint, but still noticeable when there is no music on, and slightly noticeable in passages of music that are very quiet. If I had to describe it I would say it sounds very slightly like white noise, but not as "sharp" and high frequency. Very similar to what you hear when you put a conch shell to your ear (some people refer to this as an "ocean sound").

The twisted pair cables are pretty loose and not as tightly wound as I would like them to be. Same goes for the braid from the RCA inputs. Could this introduce some interference that leads to what I'm hearing? Possibly noise from the transformer?

Any help would be appreciated. Thanks in advance.

[Paul Birkeland]

This could potentially be a little bit of tube noise, most likely in the 12AU7 if it is tube noise.

Tightening up the braid on the input wiring may help, as may tightening the twisted pair leaving power transformer terminals 4/5. 

Sometimes issues like this may be crunchy/cold solder joints, so keep an eye out for those also.  Is the static there with the volume pot all the way down?

[EraserXIV]

Yeah it is there even when the volume is turned all the way down. It seems independent of the volume knob. I did notice it more after switching my 12AU7, but I switched back to my original 12AU7 and also noticed it was there, though not as loud. If it is the tube is there any reason it's isolated to the L channel? Seems unlikely that 2 independent tubes would have the same issue.

[Paul Joppa]

Stock or Speedball?

[EraserXIV]

I have Speedball installed

[EraserXIV]

I just resoldered a couple "iffy" connections and the problem remained, but now I'm pretty confident that it is my new 12AU7. It is just too much of a difference between the new one and the old one. I think what may have also been confounding the results was the fact that I recently also got a new power tube. It appears that this new power tube has a "hum" in the L channel. This may be what I was hearing when I switched back to my old 12AU7 and still heard something. I guess I never really noticed the "hum" from the power tube with the old 12AU7 at first because it was faint and I wasn't really looking for it. Switching back to my old power tube and the old 12AU7 gets me back to a silent background. I guess these 2 new tubes are going back. Thanks for the help.

[Paul Joppa]

If you can, give them some time if they are actually new - say, 50 hours? - the hiss could be a cathode issue. And the hum could be an internal leakage that gets cleaned up by the getter. Not saying that's it for sure, just a possibility.

[EraserXIV]

Thanks for the information. The 12AU7 is a new production tube and the 6AS7G is supposedly NOS. I may give them a bit more time then.

Out of curiosity, since I don't know much about tubes, how would the hiss resolve itself if it is a cathode issue?

Also, does the fact that the hiss is independent of volume tell me anything about where exactly it may be coming from? (if it isn't actually the new tube)

[Grainger49]

Good information.  New production tubes need "running in."  Give them 50-100 Hrs.  Most often it goes away.  There are a number of posts about this.  You are not alone.