Instead of over biasng one tube, why don't you try and under bias the other tube to balance out your system?
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First off, are they all the same type/brand of tubes? If different brands mixed, or a tube/tubes getting tired can do something like this.
Next, if they are all the same and good (tubes) , try switching the left and right source inputs and let and right speaker output from amp. If the opposite speaker sounds louder/better, then something is going on at your amp, not your ears. Maybe a resistor or something in your amps bias could be out of spec, if your tubes all match and are good.
Different brand of tubes can sound different when biased the same. Changing the bias to a higher setting might not be good trying to get them to sound the same.
If you bias tubes out of spec, you could hurt your amp to. Overheat and damage transformers or other parts, not just tubes.
You are not solving the problem, you are just covering it up. Swap the right and left side tubes. If the problem switches sides then the problem is with the tubes. Replace them with matched pairs. If the problem does not move then the problem could be with the receiver. Has your receiver been restored at all? If not there may be a problem with it. It is almost 50 years old and some of the components may no longer be in spec or not working properly. I would have it looked over be an expert in vintage electronics. By increasing the bias you are not only putting more stress on the tubes but on the rest of the receiver as well.
The output tubes are about a year old.. and the amp was properly restored by a well known professional.. however, he is over on the other coast.. so I am reluctant to send it over.
My system has never sounded better than it does now.. so it's hard to want to change anything. The bias adjustment I made was about a quarter of a turn.. nothing too extreme.. just a fine tuning to my ears.
I am just suggesting the question..
trust your voltmeter or your ears?
By what I've read so far I assume that there is no provision to balance the tubes in the PP pair. After a year of use one tube could be weaker than the other creating a situation where one tube will hog more of the current. You should check the individual bias of each tube to see if they are still well matched. Running a weak tube hard is a recipe arching and failure. I've had a tube (6550) fail under those conditions and short the grid to the plate. Fortunately it only smoked the grid stopper resistor, though it was a 5 buck tantalum. As previously mentioned it could also be a weak input or driver tube. A swap of all left and right channel tubes except the output tubes will confirm this. Then start swapping back the individual pairs to find the culprit.