Faint high pitch sound on left channel only (resolved)

[Big Bamboo]

A few months ago, I completed my stock crack build. All the resistance and voltages were spot on. I posted my build in the gallery. Using Hd 600 phones, I had a nearly silent background with pot turned down and very little noise with full volume…certainly not an issue at listening levels. So, I was happy.

But, very rarely, I did perceive a very faint “pop” in the left channel only. It was a soft pop which sounded like making a popping sound with your mouth. I played around trying to trouble shoot it but was frustrated because of how rarely it occurred. I dismissed it as being unimportant and decided to proceed with the speedball.

The upgrade went “by the book” with resistance and voltages OK

Small board
OA 60-90 (mine 73.9 v)
IA  170-270 (mine 181.9 v)
B-A/B 0 (mine 0v)
IB 170-270 (mine 182.8v)
OB 60-90 (mine 77.9v)

Large board
OA 75-100 (mine 106.4v)
OB 75-100 (mine 103.9v)
G  0  (mine 0v)
B+ 170-195 (mine 182.3v)

Having finished the speedball, I now notice that even with the pot turned all the way down, I can detect a very faint high pitch noise from the left channel only. It is so faint that sometimes, I am not sure if I am still hearing it. But, when I switch off the amp, you can tell that it disappears. If I increase the volume, the noise in the left channel does not change until I get to really high volumes, when, depending on location and what it's hooked up to,  I pick up some background noise on both channels. I was not concerned about the noise at high volumes, since it exceeds listening volumes and is external to the amp.

If I dead short the inputs, the faint high pitch noise in the left channel does not change. Dead shorting the inputs does completely eliminate the noise on both channels at high volumes.

So, since dead shorting the inputs doesn’t eliminate the left channel low volume  noise,  I assume it is internal to the amp.
It is not very loud, so perhaps I am being too picky, but if the right channel is dead silent, why isn’t the left the same?


Any thoughts  are welcome.

[Paul Birkeland]

You could have a 12AU7 or 6080 with one half that has a little bit of noise, maybe what you're hearing is just a bit of tube hiss?

It's certainly worth borrowing a spare set of tubes from someone close by to at least eliminate the tubes as a potential source of noise.

Another possibility is tube pin oxidation, which can result in noise similar to what you're hearing.

-PB

[Big Bamboo]

Thanks so much for your input.

I do not have access to other tubes but will consider buying a second set if need be.
I will try Removing the tubes and do the dremmel, Q tip , polishing compound trick followed by some deoxit to see if that makes a difference. I will report back on those results.

To be clear, i would not characterize this as a hiss. It is very high pitched. It reminds me of two things:

1. An audiometry test where they send a really faint high frequency sound to one ear.

2. It also reminds me of one of the rooms in our house where using a light dimmer causes the light to emit a very high frequency  squeal that i can hear, but my wife cannot. She has some high frequency hearing losss.

[Doc B.]

That kind of oscillation is most often caused by dirty tube pin connections.

[Big Bamboo]

I polished the pins on the tubes as well as treated them and the sockets with Deoxit.
That seems to have done the trick.

Just curious......Is there any physical explanation as to why suboptimal contact at the pins cause this.

Thanks everyone for such great support!