I have just come across this; an appropriate dropping resistor is needed in series to compensate the voltage drop when the old selenium rectifier is replaced by modern diode. In this farm radio with a built-in selenium rectifier, he blew three audio output 3S4 tubes in a row because of the LT over voltage:
https://www.youtube.com/watch?v=FH7ZCtGjHow
Plus the usual silver mica migration disease.