[MrFixr55]

Interesting issue. This is a rather mature design, the basis for all transformer powered AA5s with the exception that the 42 Cathode (and the cathode of some of the other tubes) is referenced to Chassis ground, and the bias is from a "C-" developed between Chassis and the return to the power supply through R47.

Are the tone control caps, C38 and C39 new? if not replace with 630V mylars or polys. If this does not fix it, power up, warm up and power down with one tube out, power down, install the tube, and pull the previous stage tube starting with the 42, then the 75, then the 78, then the 6A7. There is likely going to be no noise when the 42 is out. If there is noise with the 42 in and the 75 out, troubeshoot the power supply and all components associated with the output stage (transformer, caps, resistors, etc.) sounds like a spark when the magnetic field of the output transformer or field coil collapses. I am not really thinking arcing but it is possible, possibly in the field coil. Temporarily replace the field coil style speaker with a PM speaker, a resistor and bigger caps. If this solves, then the field coil is at issue.

Good Luck.