I have a radio where there is an electrolytic 20 mfd connected to the cathode of the output tube. Without having a schematic, would the use of a capacitor rated at 450 volts have an adverse effect on performance? I recall reading something about this recently but cannot find it.
Thanks

As long as it's 20 MFD that's okay. The over rate in voltage won't make any difference.

That's called the "cathode by-pass" capacitor.
There's usually only about 5-10volts DC on the cathode of a typical 50L6 output tube as an example. So a low voltage 20uf cap will be fine.
You can measure the voltage and then find a cap of about twice that voltage rating.
A 20uf 25v cap can be found at radio shack or anywhere.