Most of the time, if you had an old analog 'scope,you could pretty much "wing it" as they don't seem to be as fragile as DSOs.
Additionally, the really old Tektronix probes (the big,clunky ones) were designed for use with tube equipment, hence their voltage ratings are better, removing that source of worry.
As I mentioned before, some Oscilloscopes have a DC offset function, which allows you to view small ac signals superimposed upon large DC voltages, whilst in the "DC coupled" mode.
You can also do this with your 'scope in "A-B" mode, using two channels, with one being fed a DC voltage which becomes the "DC offset".
The problem here, is finding an adjustable high voltage supply which is beyond reproach as far as hum, etc are concerned---you don't want to be seeing imperfections in your "offset voltage"!
A way to use your "ac coupled" mode without,any threat to your coupling capacitor, is to make up a voltage divider, with a fairly high total resistance, but with the bottom, & smallest, resistance having a voltage drop across it well within the 'scope ratings.
Hang the voltage divider across the HT line, & look at the hum level on that lower voltage to your heart's content.
Obviously, the total resistance of the divider must be high enough not to draw too high a current, otherwise, your results will be in error.