I was looking over my RCA Tube Handbook, the five volume set. I noticed that some of the data sheets (from the '50's & '60's) for very large, high-power transmitting tubes list data for the tubes used under some rather unusual conditions: SELF-RECTIFYING CLASS C OSCILLATOR OR POWER AMPLIFIER WITH SEPARATE, UNFILTERED, SINGLE-PHASE, FULL-WAVE POWER SUPPLY
Why would somebody run their output stage with anything else than fully filtered DC on the plate(s)?
What mode(s) could you run with unfiltered HV to the output stage? About the only mode I can imagine is CW and then only in an emergency.