@billpete , I don’t know if any of this will be helpful or not, but just in case I will paste some text from some emails that a tube guru from another forum sent me as he took me under his wing and blessed me with several pairs of vintage 6sn7s.
The W in WGTA is the designation for "ruggedized" construction. Thicker mica supports, extra support rods, smaller bottle ... all supposed to reduce the effect of physical shock/vibrations. The GT is the designation for the bottle shape, and the A (and B for that matter) designates uprated plate voltage capability. The B further designates the serial heater/filament connection capability.
and
Enjoy the ride, we’ll sort out the money. The RCA’s are an interesting tube. No highs, no lows, but boy do they do midrange and 3D. Absolutely not the tube for rock ’n roll. Small ensemble acoustic jazz, or acoustic vocal, or chamber music is their forte, IMO. The TungSol GTB’s (one of those is marked A and one is B, but they are the exact same) and the Sylvania GTA’s are okay tubes. The TS are kind of bass heavy for my taste, and I could probably live with them or the Syl GTAs if I didn’t know there was better out there. I found out that the only difference between the GTA and GTB is that the GTB is capable of having the filaments connected in serial in the case of using multiple 6SN7s in a circuit. Furthermore, TungSol built all their 6SN7s at that point in time (late 1950’s) to the GTB spec, and then labeled them as A or B depending on what spec the client order called for. The GTB always works in a circuit needing GTB, and it will also work perfectly in a circuit where there’s no requirement for the filaments to be connected in serial. Why make two kinds when one will do both duties?
And, for what it may be worth, I have rolled pairs of both the GTAs and the GTBs in the balanced input sockets of my SLP05 with no ill effects.