are the bias probes for tube amp any good? are they a waste of money or does it make biasing your amp doable at home?

Bias probes are nice tools, but you can do almost the exact same thing with a couple of high quality 1 ohm resistors on the cathodes. Just lift the ground connectrions and install the resistors there.

The main difference is that the cathode resistors will show you the plate current AND the screen grid current. You have to subtract the screen current in your head.

For more info, see this link: http://www.duncanamps.com/technical/lvbias.html

Check out the rest of the site and the tools as well! Excelent web page!

Take care,
-Peter

You might also want to read the Tube Amp FAQ at GEO (http://www.geofex.com) as well as the build-it article on making your own bias probe/bias meter, also at GEO.