What do you think might be the cause?I am also the proud owner of a '66 Vibrolux Reverb. I have a hunch that the problem you encounter has nothing to do with a potentially faulty tube. However, I could be totally wrong AND it's still worthwhile learning how to change and bias your power tubes.
With that said, you could theoretically change the power tubes and BEFORE biasing run the amp for a few minutes to see if you encounter the same problem. If you do, it isn't the tubes and you don't need to change/bias them.