Tail -f | grep > output.txt

hi guys,

I perform a sort of monitoring. I have a server running and with
I monitor the log-file for recent specific entries. This is ok and works fine.

Now, in addition I want to have my search results not posted into the shell but into a file. I tried:
works but has no "grep" statement.

Would be very nice if you could help me out.
thx in advance