I put my Crack together a little over a month ago, and it's been a great sounding amp that I've been loving. However, after a couple weeks of use, I've noticed that the Crack makes a faint 3-4kHz (or its harmonics, I can't tell without a mic) ringing noise. It happens sporadically and not necessarily for long periods of time. I can sometimes hear this noise in the headphones if the music is quiet enough.
I narrowed it down to the stock 6AS7 tube that came with the kit because if I gently grab the tube with a microfiber cloth, it stops. I can grab/push/tap anything else on the board, and nothing else will make it stop. I tried reseating the tube to no avail. I can unplug the RCA inputs, and headphones, and it will still do it (although the vibrations from unplugging can sometimes be enough to temporarily stop the noise). The anti-phase diodes on the ground help with the DAC ground loop noise, but not this.
Is this a bad tube? Is it just what the 6AS7 tube normally does? I'm guessing it's something in the tube vibrating at high frequency, and holding the tube dampens the vibrations. Would something like a Herbie's UltraSonic Rx dampener be useful in eliminating this kind of noise? Thanks for any input.
