I recently pulled the Sovtek 12AX7 out of my Hartke GT60c and tried some other tubes in it. A Tungsol 12AX7 sounded the best so I left it in there. From time to time there is a big pop from the speaker when the amp is on, but it only happens when I'm not playing and the amp is sitting idle. It only does this when a Tungsol is in there, and it did it with another Tungsol I tried. I had Sovteks, EH, and Chinese tubes in there for testing and it never popped with them. It's not really causing a problem because it doesn't do it when I'm playing and it only happens about once or twice a day.

Is this normal for Tungsol 12AX7s ... are other people seeing this with Tungsol preamp tubes?
is this happening while in Standby?

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are you sure that its the tubes? i had the same problem but it turned out to be my input jack on my guitar
How much wood could a woodchuck chuck if a woodchuck could chuck norris

A lot
This amp is hybrid (tube preamp, SS power amp) and doesn't have standby. It never happened with the original Sovtek tubes, and it never happened once in several days when I was testing some Chinese and Electroharmonix tubes. It only happens with the Tungsol tubes, and I tried two different Tungsols. I guess they could be from the same batch, but they weren't bought at the same time or from the same place.

So the only common denominator seems to be that the amp pops when Tungsols are in there, and it only does it when the amp is powered on and idle.