Another odd positive side effect of valve rectifiers is the lack of reverse recovery. They therefore do not excite the leakage inductance/interwinding capacitance resonance of the mains transformer.
Regular solid state rectifiers generate reverse recovery transients like a champ, so every time each rectifier diode turns off, a burst of oscillation occurs via the above effect usually in the range 200kHz to 1MHz with a Q of 5-10.
Of course you can use more sophisticated soft recovery diodes, and/or use a C-RC snubber across the secondary winding of the transformer, which kills the oscillatory transient dead if you get the RC in particular correct.
There is a small circuit on DIY audio that enables these values to be determined by direct measurement
https://www.diyaudio.com/forums/powe...-test-jig.html
Craig