6SN7 tube GTA vrs GTB


I would like to know the difference between the designation GTA and GTB.

Thanks
fordmod
It has been a journey for me to search for the "perfect" 6SN7s. I have 3 amps that use 6SN7s, and I tend to use GTAs and GTBs for driver tubes only. I have the following tubes for the gain stage and my summary is as follows:

Sylvania VT231s, bottom getters: clean, fast, accurate, and airy. My choice for classical music, but a bit thin for vocal music.

RCA VT231s, smoke glass: rich harmonics and sweet sounding, but a bit slow. It is great for vocal music, but not for complex classical.

Raytheon 6SN7GTs: nice sounding, but a bit too sweet for me

RCA 5692 Redbase: fast, rich, dynamic. It is excellent for Jazz and vocal, but not as good as Sylvania VT231s for classical music.

CBS 5692 Brown base: a tuned down version of RCA Redbase.

Ken-Rad clear glass: a bit too euphonic for me

Ken-Rad black glass: strong bass, but does not have the airy magic

Sylvania 6SN7GTs chrome top (not to be confused with GTA chrome top): my current overall choice, they are as fast and airy as Sylvania VT231 bottom getters, but have more sweetness.
I use Sylvania GT's and the ones known as (Bad Boys)in my pre. I can not find better.
(http://home.att.net/~chimeraone/6sn7sound.html) I've personally verified the findings of this site. That's why the TungSol JAN CTL round plates, and Sylvania JAN CHS/6SN7W(tall bottle) tubes have found a home in my system. None can deliver the music with less harm than they.
Rodman-

I've been wanting to try some Tung Sol round plates..I use the Pope Holland's now. Thoughts?