Michael, excellent point all.

Years back I used Lambda regulated H.V. supplies in the workplace, but
only for critical application in experimental physics apparatus where
the accuracy and stability of the voltage had an important impact on
our research. (I still own a comercially produced 0-5 Kv regulated
supply, whose use is largely limited to powering photomultiplier tubes
used in nuclear research applications.)

Like you, I don't recall voltage regulation ever playing a significant
role in vacuum tube electronics, except for those instance that you
mention using VR tubes (as in my Hallicrafters SX-71 receiver to
stabilize the local oscillator). Still, stabilizing that sucker
required much more effort than simpy a VR tube!

Sadly, I gave away the original Variac adjustable power supply to a
student that had greater need for it than myslef. Then too, very little
of the vacuum tube work I do today requires more than a few mils which
My Heatkit IP-17 supplies.

Kindest regards, Harry C.