Could be the setting of the antiskate. Could be other things, but I have forgotten them. Good luck.
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Does this consistently happen on every record? It could be your cartridge alignment, tracking force, or antiskate as mentioned above. It could even be an issue with the cartidge itself. You may want to look at the documentation for your cartridge and arm to make sure everything is set up correctly. I should probably don my flame retardant suit before saying this, but this is one reason why I love having a preamp with balance controls.
If swapping tubes doesn't produce results then check cartridge alignment. See if you can find the image by moving to the left from your listening position. If it seems to be there but a foot or so off center you should be able to adjust this back to center. Find the azimuth adjustment on your tonearm, try making small adjustments and then listen for the change. You should be able to pull the image to the center this way.
It could be the tubes. try to switch the tubes and if it moves then you can isolate the weak tube.
It could also be the cartridge. Get a tester and put it on 100ohms both channels should have almost the same reading if one channel is drastically way off then the cart's coil is ready to open. before you know it one channel will be dead.