Forgot to add... Source follower output has another good feature, it isolates our triode and the load (capacitive cable + low input resistance of the amp). If we have rel. high input amp (say, >100k), we can try anode output, too. Then our triode has load Rccs//Ramp - not so high anymore.
Our DN2540 facing upward (in TO-220 case) has leads (from left to right) - G,D,S. Note that D is connected at the tab. We must use a little heatsink (see the picture, hosted by "ImageShack" :-)), and thermally conductive insulator. Better not to mount heatsink on the chasis - safety and negative influence of the stray capacitance.
We`d use two series connected cheap LED, cathode (short lead) goes to ground.