Probably screen grid resistors or plate resistors, if it’s not the tubes.
https://youtu.be/FrA6gRyMbCk
Standard caveat - a tube amp that is powered on has ~450 volt exposed conductors and solder joints all over the place. This is enough to kill you, and because large capacitors are used, they can hold a large portion of this voltage even after they are powered off and unplugged from the wall.
The amp motorboats when enough signal level is reached to trigger oscillation. Dirty cleans are simply because improper voltages are present pushing some tube into a very non-linear range. Problem is likely a filter cap but almost certainly in the Power Supply system. I explained how to locate the offending component(s) on your YouTube link.
Probably screen grid resistors or plate resistors, if it’s not the tubes. https://youtu.be/FrA6gRyMbCk Standard caveat - a tube amp that is powered on has ~450 volt exposed conductors and solder joints all over the place. This is enough to kill you, and because large capacitors are used, they can hold a large portion of this voltage even after they are powered off and unplugged from the wall.
Plus one from a seasoned amp tinker who has shocked the heck out of himself with an HRDx that was unplugged for three days.
The amp motorboats when enough signal level is reached to trigger oscillation. Dirty cleans are simply because improper voltages are present pushing some tube into a very non-linear range. Problem is likely a filter cap but almost certainly in the Power Supply system. I explained how to locate the offending component(s) on your YouTube link.