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ARC LS25 mkII adjustment

Does anyone know the procedure to adjust the LS25 mkII preamp? I am considering swapping tubes with new ones and would like to know how to adjust the 2 trimmers that looks like sets the voltage for the tube bias.

I don't have any documentation on where to measure and what to set to. Anyone have any experience with this preamp?
jazzgene

Showing 2 responses by gmuffley

gmuffley
132 posts
 
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
132 posts
 
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.
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