3/4 amp vs. 1/2 amp

Ok to use 3/4 amp in place of required 1/2 amp or the other way around
If your talking about fuses then using a 1/2 amp instead of a 3/4 will have it blow sooner and might be ok till you could get the right one. Using a 3/4 for a 1/2 could have it blow later, after it has already done some serious damage to your amp, preamp etc.
The rule that "bigger is better" doesn't apply to fuses. The purpose of a fuse is for it to blow before something downstream does.

Using a smaller one means that you may be starving the component from getting the current it needs. It won't hurt the component itself, but it will restrain its performance.

Use the size fuse the amp designer intended.
Uru and Mark are right. But using a too-small-value fuse could cause the fuse to blow when there is nothing electrically wrong.

Using a too-large-value fuse defeats the safety protection of a fuse. In the event of a malfunction, fire can result.

As a result, use the properly indicated value fuse.

One exception: I had purchased an cheap Chinese tube amp that used a transformer that could be used for either 110V or 220V. It had a 2A fuse for 220V operation, which blew immediately. For 110V, it needed a 4A fuse, because when you halve the voltage, you double the current to get the same power. That fixed it. But this only applies really in this case, as there was no documentation.