Well, that can be a number of things. It could be a cold solder joint somewhere. It could also be the extra gain pushing the tube into self oscillation. I also can’t quite tell from the picture where you have the twisted black and white wires that cut across the amp. Those could be creating an impedance or cross-talk issue.
Often, when I have an issue like this. I use a set of chop-sticks with one hand behind my back and probe all the solder joints and slightly move wires to see if that quiets it a bit.
But we know that voltages are a bit high, so my guess is that it’s going into self-oscillation when dimed.
Also suspect might be the wire that goes from the tube to the tone stack. How do you have that wired?