Thought I'd post an update. I decided not to mount a meter in the laser cabinet; more vibration than I wanted to subject the tube to. What I did do is install a 1/4" shorting-type switching phone jack in the tube extension cover plate, and wired the cathode of the tube through it. I then used the laser and a CAD-drawn template to mount a 50mA meter in a plastic box, and put a cable on it with a 1/4" stereo plug. A safety diode was installed across the jack contacts in case the meter circuit fails and it all works fine.

What might be more interesting is that even with the power dialed down to where the current reads 10 - 16mA, the squeal is still *mostly* (not always) there. I'm thinking something is oscillating or resonating in the tube power supply, so I'll probably change to a more expensive, "guaranteed stable" 40W unit, since it is is after all pretty much a 40W tube. :-(

I thought about putting a small series resistance in the tube cathode and using that I*R voltage to observe what's coming from the power supply with the scope, but I chickened out. I don't know how far the cathode is held above ground, (assuming it is) and I have no desire to zap the scope, myself or anything else in the room. (sounds of chicken clucking...)

Thanks to all that helped with this, I learned quite a bit about Chinese laser engravers while I was at it.

--jim