Just to expand a bit (pun
), adding the cap reduces the HV comensurately with the width increase. It also reduces cathode current a tad in the H out. tube. So it's a trade-off, no free lunch. Much higher than 100pf, the HV drops to where you start getting blooming.
If you recall, this was discussed near the end of the thread on your little Majestic 'Mighty Monarch' set. It had
not quite enuff width to completely fill the screen. Using a damper cap was discussed. And since the set also had had issues with the HV rectifier arcing, it woulda benefited from slightly reduced HV also.
I didn't know then if the damper cap trick work on a pre-standardization setup, so it was never tried. But we now know it will work.
We usta keep 6KV disc ceramic caps of 47, 68, 82 and 100 pf for just this purpose. Also a 120, but never ended up using it that I recall.
4KV caps would be OK for just B&W sets, but 6KV covered color sets as well. (Certain Admiral color sets had horrendous issues with HV regulation and H out. tube overcurrent. It took some serious juggling of H out. sceengrid voltage and damper cap values to keep the the cathode current within reason.