I have a guitar amp that works on 110 V and someone accidentally plugged it in a 220 V socket. I want to replace the fuse (160 mA) but my local store's lowest is a 250 mA. Can I actually use this? Thanks!
Widening of protective envelope for any circuit is NOT good. the fuse is there to protect the circuit in case of overcurrent. Overcurrent does not mean shortcircuit, it means that the circuit is design at certain current level, in your case according to original fuse is 160mA. everything above that is overcurrent event and a fuse must deal with it to protect the rest of the circuit. even tough it seems like not a large current 250mA in respect to 160mA is increase of 56,2%. It is like driving 95 in 60 km/h zone. just put this in perspective and you will see that even though you can use any fuse to make the circuit work only the one specified for this circuit will be proper.
Well, its a gamble your taking.
If you choose to use a larger fuse than the device had originally, you run a real risk of it not doing its job. The 250mA fuse will let the guitar amp pull about 15 Watts more from the mains than the 110 mA fuse will. If you are lucky, the circuit of the amp can handle those additional 15W = (110V * (0.25A-0.11A)) in case of a fault, but if the amp is built at a low budget, its unlikely that the componetns inside will survive such a fault event. Also mind that there is a difference between fast-blow and slow-blow fuses! You will want to get the same type as you had.
If there is a short at the transformer secondary:
Losses in transformer windings due to resistance are \$ R I^2 \$. Current is squared, which means 240mA will cause 2.4 times more heating in the transformer than 160mA. If the transformer is small and/or "cost-optimized" it might not manage to draw enough current to blow the bigger fuse. In that case the thermal fuse on the windings will blow, which is usually impossible to replace, and if there is no thermal fuse, it will start a fire.