A friend of mine recently gave me a Samson C-que8 headphone amp, but he doesn't have the power supply. It requires a 19V 1A AC input. I found two power supplies laying around: one rated at 12V 1.2A AC output (correct amperage, wrong voltage) and one rated at 18V 0.5A AC output (more or less the right voltage, but too low of current.) I'm guessing that the 12V one shouldn't be used, but I don't know about the 18V one. The amp supports 4 headphones and 2 inputs, and I'm only going to be using 1 headphone and 1 input- so I might only use a fraction of the rated 1A. Do you think it's safe to use a 0.5A power supply?
Too many amperes is never, ever a problem. A PSU doesn't dump out all its amperes all the time - that would be a huge waste of energy! It basically only provides the current asked for.
18v is close enough to 19v for it to function - it is probably close enough for the physical plug to fit as well.
.5 amperes is not enough power - potentially.
Too little amperes can be a problem, but almost never causes any harm. If you have an amplifier connected to a power supply that is inadequate, you'll be able to test it quite easily!
Hook it up to some speakers, play music, and slowly increase the volume. Eventually the amplifier will cut out - and that is the amount of amplification it can provide with the given amperes. Reduce the volume below this breaking point and it should kick right back on.
EDIT: AMPs to Ampere or Amplifier to avoid confusion haha.