Need help with diagnosing a Mojotone 18w build

[_ej_]

Check the connection at the wire that goes from the common of the impedance switch to where it connects to the output jack. I don't see solder there.

If you look in the second pic you can see it's soldered.

[danman]

I have had that same static type noise pop up in a couple of my builds. In every case, it was a cold solder joint on the power rail or tube cathode circuit. They can look good but still not make a solid connection. I take a can of compressed air, turn it upside down and apply a tiny blast of freezing spray to every solder joint. The amp must be on and connected to a speaker. Once you hit the bad solder joint with the freeze spray, the static will show up briefly through the speaker. Be sure to use the plastic extension nozzle and keep your hands away from any high voltage parts of the circuit while performing this test.