What is the best way to drop out the entire background?

[MPArtist]

I have photos taken of fine-line pen and ink drawings. I would like to make the background completely transparent so they will print only the black lines. What is the best way to do that?

[Peru Bob]

Can you post one here?

[c.pfaffenbichler]

Personally I would prefer a Blend-if/Color Overlay-approach that maintains the composite image, because doing touchup on a Layer Mask can prove problematic in the long run. 

But photos are at times a poor substitute for a proper scan and may need some additional adjustments to get an even brightness across the whole image – so, as @Peru Bob already asked for, please post a sample image. 

[Earth Oliver]

you can either load a channel as a selection or copy the image into a layer mask and invert it.