It's common to use something in the range of 4.7uF to 10uF there. I think 5.6uF and 6.8uF are the most common.
If the amp has another mirrored half, measure the cap on the other half.
If the amp has another mirrored half, measure the cap on the other half.
Physical size doesn't mean much. You'd need to find one that's being used in the same type of circuit.
30Uf would be very high for that application.
30Uf would be very high for that application.