With power supplies, it is wise to replace with same or slightly higher values. Never go lower. Actually you can go lower, but you must have a bit of experience with situations in which this is acceptable. With AC power supplies, it is wise to never go above 40 MFD unless higher values were originally used. Using values higher than 40 MFD can sometimes overload rectifier tubes during turn-on of the receiver, especially if the receiver has just been turned off (like a second ago). One rather sensitive rectifier tube is the 6X5. The 80 is less sensitive due to its filament cooling off almost as soon as power is turned off. With AC/DC type receivers (no power transformer, tube filaments strung in series), you can go higher, say as high as 60 MFD. I've gone as high as 80 MFD with the 35Z5 rectifier, but I don't recommend this. Such large condensers can overload rectifiers when the receiver is turned off and then back on when still warmed up.

Thomas