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.

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?

I know someone did swap and adjust the amp. I was told it was John Rutan at Audio Connection.

I do want to confirm it is all good.

Gmuffley,

R10 and R11? Or the 1k trimmer itself? I guess I can measure the voltage drop at RV1 and see if the 2 channels are similar.

I think John probably knows better than to change those adjustments.

If it was my preamp and I suspected adjustments have been changed I would ship it to Audio Research, so I know it is working properly.

I think a lot of audiophiles try equipment that isn't working properly or not broken in and decide they don't like it. These hybrid preamps take a few days of warm up to start sounding their best.

I don't know John Rutan but have heard good things about his business. I don't think he's a tech though. All this is what I heard from the previous owner. Not that I don't trust him but I just want to make sure the amp is working properly.

Found a schematic for the LS25 mkII. I think I got it now so I will check the voltage tomorrow. Right now, I am going to enjoy the Yanks bash the Rangers again!

Did a full voltage check. Had to tweak one of the trimmers a very small amount.

The preamp sounds really great.