I am looking to use the 1/4" stereo headphone out jack on my Crate GLX15 amp to patch to a line out to a mixer board. Do I have to do some signal conversion to match the normal line out signal? Or, can this be done with a simple "Y" cable to convert from Stereo to Mono? I am hoping that I can still use the speaker out function on the amp to drive a small cabinet so I can use the amp's output for a monitor at the same time. Can this be done?
I'm not sure what you want to do, because you are asking about connecting an output to an output. No, that usually cannot be done.
When it comes to your amp headphone output, often those are switched jacks, which turn off the speaker whenever you insert a headphone plug.
From an artistic standpoint, most of us like to get a little of the sound of the speaker into our signal, and sometimes a little room ambiance, too.
There are speaker level direct boxes which will allow you to take the speaker output of your amplifier and plug it into a mic level input.
There are several other ways to take the speaker level signal and pad it down so that it does not overload the inputs of a console or recorder. But to determine and price these would be dependent upon knowing the power output of the amp.
All in all, it looks easier and cheaper just to mic the amp.
Can you give us some more information?