How mod'd do you have to do before it's a new patent?

[selloutrr]

I'm curious to see how far you have to change an amp before it's no longer a trade mark or patented piece of work. I reason i'm asking is if you think about it. Fender started as an RCA tube amp project out of a recieving manual. and marshalls are basically fenders with a couple mod's. Mesa Boogie started as a fender....

just wondering what others are thinking...