Switch it with the other one and see if that behaves like the tube that is flickering.

Maybe you should call Steve.

These are not expensive.  Just by yourself a pair of new ones.  These NOS tubes are generally very sturdy (Sylvania, RCA, etc.).

If you were to experience the same issue with one of the new ones, then call Steve.

I guess my main concern was "is this going to hurt my amp?"

Roggae,

If the particular tube is faulty, I would stop using it altogether.  Just replace that pair and see if the problem goes away.  This would not make anything worse.  If the problem persists with the replacement, stop the amp and call Steve.

It does not matter.  My stock ones are larger than any 6V6 tubes I've tried so far (Coke shaped, with wide shoulders).  

The important thing is to use the same spec tube.  These come in different shapes and sizes, but internally are all equivalent and should do the job.  

They may sound slightly different than your originals, in which case if you prefer that sound, just buy those same ones. Technically, there are no differences.

RR,

My point was that the Op could easily and inexpensibly replace them and find out what the real issue is with his amp.

It is well documented that, in general, these voltage regulator tube types last for a long time and are very robust.

Having said that, better reputed vintage NOS types 'may' even sound better and may last longer.  The Op will always have the prerogative of choice.

Thanks!

Roggae,

One of my Ob3a's does something similar just not as often or obvious.  Since I love them and it hasn't affected the sound negatively they've stayed in place.  Pick up a couple back up pairs and tube swap to see if there is an audible change.  I suggest NOS RCA's.

JD