Right/Left channel balance issues? Pre-amp

I did have the same problem. It took some time to figure out that it was the potentiometer that were unbalanced. I sent it back to seller, he put in a new one and it sings again. It is very frustrating with all the waiting and uncertainty, but it is probably fixable. Do not accept having a new and unbalanced component.

Assuming this was never ever an issue prior to getting this new piece, and with it you have a noticeable disparity in the balance of power to each side, I got to throw in with Musicophile.

Send it back. Bummer... drag city... BUT I've yet to exp run in periods wherein the balance between loudspeakers was off and playing it more fixed things. Run in fixes only tonality, extension, and refinement... not balance.

One thing too... turn it on and once warmed up while at the LP... Stand up. Sound different? No? Then turn around & face the other way. sound different now? No? Then it ain't your hearing. Don't laugh. Our ears do change.

Openings to the sides of a room too can cause suck outs and balance issues.

My point being... if it's never been an issue with other gear... it remains it is now... get another, or have it fixed, but don't accept that problem. Might just be the tube but it needs be repaired.
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I would definitely be looking for a different preamp. Not only should it match precisely, but it is desirable to have a balance control to compensate for the irregularities found in many if not most recordings. I like to listen in a dead center position if possible. I really value the ability to get that right and it would significantly reduce my enjoyment of the music if I couldn't.

Have you tried a new, (different) tube? If the preamp uses just (1) tube, it may have mismatched sections within the tube.

I do not have another tube to try.... I have done A LOT of testing and it is the pre amp for sure but the tube could be the root problem.

It sucks because I really like the piece. I am waiting for a reply from the manufacturer too see what they say. I might just try and exchange it for the same piece at the dealer. I hope it is not a design flaw and I just got a bad apple.