Is there a way to SAFELY plug in the output of a guitar amp to the line in port on a computer. (so I can use the workstation) Mine's just an HP desktop.  I could mic it of course but just wondering about the other route as that would isolate the sound from other mics in the studio. There are effects on the amp I want to use. Reading the ads for direct boxes MAKES it sound like that's the answer but someone on a recording forum said no way!The output of a Guitar pickup is higher tan  most microphones, but not a  a lot.
I am supposing that you mean a direct output from the pickup, the guitar itself does not have any power, not even a battery.

When you really bang  on all strings, the output goes up to about 1 volt RMS for a short time and then decays down to a level of 10 0millivolts. The peak might damage the input of a microphone channel. But most guitars have volume controls,. At half open the level is down two orders of magnitude. (That is Geek speak saying it is way down.)
So yeah, you can use either line of mic inputs, but keep the guitar's volume control about half way. Or less.

Note: A problem with instruments is grounding and the kind of connectors USED. The common connectors do not have grounding protection. You could  blow out a preamp just by plugging in a connector. Ask other musicians about this. Or come back here. I am not a musician, but an old audio pro from long ago.


Sorry I was talking not about the guitar but the guitar amplifier.  After reading more I see there are Direct boxes made capable of handling speaker voltages. In this case the amp is capble of 75 watts.Right. Some amps pump over 50 VOLTS into the speakers. The will fry the input of the motherboard.  I supposed you meant right out of the instrument.

Best results are if you use an internal 'bridge' connection. Have a tech SOLDER a connector to the volume control of the amp. That level is 'line'  and often about two volts or less. He can add a simple resistor network to cut down the voltage more, if needed.