Actually the V.T.V. article is extensive and it would be hard to summarize in a short space but they describe them, do the load lines for some, and suggest uses for them in circuits. It's an interesting and informative article; worth the price of admission. The Bob Carver nulling circuit originated when Bob Carver of Carver Electronics;(he no longer has any relationship with the company is the last info I have,), decided the whole tube revolution was hype, that tube circuits were nothing but tone controls and that he could devise a circuit that would enable a SS amp to sound exactly like a tube amp with the cooresponding tube response. He installed this circuit in a SS amp and provided an adjustment access from outside the chassis so the owner could have that"Tube Sound". It was supposed to introduce so-called tube distortions and non-linearities that would mimic the typical tube response. Then you could turn a screw and adjust and cancel anything not seen in the original signal; ie. "Null" the output to zero distortion to return to SS mode. That was his proof that tubes degraded the sound. I don't have a schematic so I don't know how he accomplished this. I was present at a demonstration of this innovation back in the 80's.