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I just bought a pair of NOS RCA Mil-Spec JAN CRC VT-231 6SN7GT tubes and one of the tubes emit a small hum. What do I do to fix it? Or, is this common to new tubes and will eventually disappear?
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"Humility is the true mark of genius. Just get used to it."
-Anonymous
Follow Ups:
My personal preference is to avoid a hum or noise maker except for some microphonia when tapped. I fear it is a harbinger of creating another problem. Is there any other remedy in terms of return etc. If not can you get another pair. Do you only want RCA JAN VT-231 6SN7GTs? Are they grey, graphite or clear glass?
I fixed the problem by using super-fine sandpaper on the contact pins and tapping the tube with a pencil.
I did want the RCAs because they are great for vocals. They are gray.
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"Humility is the true mark of genius. Just get used to it."
-Anonymous
nt
"I know just enough to get into trouble. But not enough to get out of it."
That's why some circuits use DC for heaters. It eliminates the hum, but requires some circuit re-design it implement it if not designed that way.
Scruffy,Another possibility for the hum could be due to leakage in the 6SN7 you identified as emitting a small amount of hum. Such a tube can work perfectly without any hum in some circuits (such as driver applications) whereas in other circuits while otherwise functioning properly will cause some hum (such as in a gain stage).
Thus if you have such a tube don't assume it is bad and thus dump it. Instead be aware there are applications you can use it that the leakage does not induce the hum.
Per a well-known tube store: You cannot correlate leakage with wear as many new tubes have leakage and obviously no wear. Where leakage is a serious problem relates primarily to applications higher than the audio frequency range.
Edits: 02/08/17
Strange story but true. My Sherwood, when I put new linestage tubes it hums for about 10 hours then the hum starts to disappear.
run it for a few hours and see if the hum diminishes. If not see if the hum follows a particular tube or try putting back your old tubes to see if it's not the amp humming.
I identified the tube and I placed it into output stage instead of the voltage stage. That eliminated the hum, but I really got these tubes for the voltage stage of my preamp.
~~~~~~~~~~~~~~~~~~~~~~~~~~~
"Humility is the true mark of genius. Just get used to it."
-Anonymous
There's only so much you can do in hopes that it is not defective.Try cleaning the tube pins. Also clean and tighten the socket claws in the voltage stage.
If that doesn't work maybe it does need some hours on it as suggested.
So leave it in the output stage for awhile and then try switching back.
The hum is coming through the speakers?
If not, and it's mechanical, can you supreme the hum by putting some light pressure on the tube?
Cheers!
Jonesy
"I know just enough to get into trouble. But not enough to get out of it."
Edits: 02/07/17
~~~~~~~~~~~~~~~~~~~~~~~~~~~
"Humility is the true mark of genius. Just get used to it."
-Anonymous
In addition to what Jonesy suggests also have a look at the solder connections to be sure they are good. Reheat / reflow if there is doubt.
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