One other rare but completely frustrating issue that masquerades as a bad AC ripple condenser is the ol' heater to cathode short in the 1st audio tube or phase inverter. B+ looks clean, yet there's a hummmmmmmmmmm that just won't quit. Sometimes a tap of the tube will blip the hummmmmmmmmm and that's a clue, other times the set plays fine until it warms up for five minutes or so. Then the hummmmmmmmmmmm starts to sneak into the audio.
Freeze spray can be a useful diagnostic tool but never, ever let it splash on the glass of a hot tube....... Instant stress fracture followed by extreme gassiness.