ARC LS25 mkII adjustment

Tube preamps usually don't have bias adjustments. Most tube power amps have "output" tubes and they need biasing.

I have the LS-25 mkII and have swapped tubes many times without any problems. As Mofimadness stated, preamp tubes don't need to be biased.

Actually preamp tubes, in many cases, should be adjusted. However, biasing preamp tubes is not a simple task and it requires changing resisters. I believe the differences we hear when rolling tubes can often be how the tube is affected by the current bias setting. This is why it is always better to replace preamp tubes with tubes selected by the manufacturer.

The trimmers you see are for the constant current source in the FET voltage gain stage. Do NOT adjust these when you switch tubes. Did you know that all the gain in that model is provided by the solid state FETs? The tubes are used as cathode followers and do not provide any gain. They are there so that ARC can market it as a tube line stage.