Couple of questions about cap values

[nee]

Greetings!

What would be the effect of substituting slightly different value caps?

Specifically:

1. Using 47uf instead of 40uf in a power supply, or 22uf instead of 20uf. Better to use a larger value than a smaller one?

2. Using a 350V rated cap rather than a 25V rated cap elsewhere in the circuit. (The BOM calls for a 25uf/25V so I'm guessing it's cathode bypass)

Thanks!

[Ears]

1) In most non critical situations substituting a close value cap is fine.
electrolytics are frequently + - 20% in any case. But how critical depends upon the application.

2) Some sources claim electrolytic performance is compromised when operating at DC levels well below rated value. I've read dissenting opinion as well, but I prefer the first opinion as I assume the dielectric is unlikely to be properly formed and maintained at low DC voltages. But be assured you won't fry an over-rated component.

[Phil_S]

1) In most non critical situations substituting a close value cap is fine.
electrolytics are frequently + - 20% in any case. But how critical depends upon the application.

As Ears says, in a guitar amp, using +/-20% of spec probably isn't a big deal. You can always try it and see. If there is too much ripple, too stiff, not stiff enough, you can always make a change. The one place to be careful is filtering for a tube rectifier. Always check the spec'd limit for the particular rectifier tube and don't go over it.

I can tell you that I once replace a bunch of old, tired 20mf caps with 32mf. This stiffened up the power supply and improved both the bass response and the general "crispness" of the tone. In that particular amp, I deemed it a very good improvement. I has a solid state rectifier.

2) Some sources claim electrolytic performance is compromised when operating at DC levels well below rated value. I've read dissenting opinion as well, but I prefer the first opinion as I assume the dielectric is unlikely to be properly formed and maintained at low DC voltages. But be assured you won't fry an over-rated component.

As a practical matter, using a 50v where 25v is called for is probably not an issue. I have no idea where the line is, but the notion of using a 350v rated cap for a cathode bypass is probably the wrong thing to do. Also, there is typically a meaningful price difference between a 25v cap and a 350v cap of the same value.