ARC LS25 mkII adjustment

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.

Gmuffley,

I see. Do you know what to look for to confirm the trimmers are set up correctly? I am the 2nd owner of this preamp and want to make sure everything is set up right. I suppose you are to measure at some point on the board and use the trimmer to set it to a specifice voltage. Do you know the specifics?

There are no voltage readings on the ARC schematic, but you could check the voltage drop across the 1 K source resistor; it should be approximately equal. Unless you know someone fooled with it, I would not worry.

Jazzgene, Did you change the adjustment?