Well, I swapped the power tubes and the buzz didn't move channels, so I guess its not them. Could it be the 12AT7 tube?
I also checked terminals 8 & 10. I have continuity between the volume attenuator & T8 &T10. I also reheated all the solder joints at T8 and T10 as well as anywhere else that connected to ground, so I don't think it's a cold solder joint.
Again, the buzz doesn't change with the volume. It's steady no matter how loud I turn it up.
I posted some pictures of my build in case that helps. Any other likely culprits to look for?