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Im new to the tube amp world. I have a 59 bassman on the way and I was looking at mods and tube changes. Please dont kill me here....what is rebiasing? How do you rebias? Like I said before, I have no knowledge of tube amps. Just solid state.

There's a 90's Reissue Bassman 4-10, and then there's the newer Bassman LTD.
The reissue has a set bias, so all you need to do is use Fender labeled 6L6 tubes, and pop them in.

The LTD, has an internal bias adjustment pot, so if you're a qualified tech, you can set the exact mA running through the tube, which is sort of like setting the idle on a carburetor, so the tubes are running at their optimum.
On new cars with an electronic ignition, you don't have to mess with anything. That's the equivalent scenario with an amp with a pre-set bias.

If your amp is new, just play it. If it's been a couple years since those tubes were replaced, just buy some Fender labeled 6L6's at Sam Ash and pop'em in. The bias point is usually pretty close to perfect anyway, so the new tubes will be a definite improvement.

thanks