I have just created a tube by making a circle -
then with the first cycle selected - in read hold alt on MAC - and click on the circle edge line not any of the handles and you can pull off the second circle adjusting the size of the 2nd cycle dictates the tube thickness, I then extruded to the tube -
hope this helps also see here:
http://zaxwerks.com/movies/media/42_Invig5DrawingHighlights1.mp4
cheers
bob