6SN7 tubes and variants question.

@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.

If the grid of the tube opened up (which can happen due to poor solder joints in the base of the tube, which is a lot more common these days with Russian and Chinese power tubes) the tube can go into runaway. 

@billpete , I am sure that there were some in that massive collection of used pairs that were bestowed upon me, but I will have to go back and check to see which were which. My benefactor mentioned the Sylvanias that he sent me, so those for sure, but I cannot say I remember what sonic impact they had on me when I put them in the preamp.  However, I can absolutely tell you that none of those tubes gave me any problems whatsoever.

But he three pairs of tubes that made the biggest impact on my ears were:

1) a pair of black glass RCA VT231s.  The detail that they brought out in the music I was listening to at the time was incredible (to me).  Imaging was enhanced significantly.

2) in a later batch he sent me, there were a pair of ancient National Union 6F8Gs that I am running right now because although I do not have the vocabulary to describe what it is I am hearing, they have a seductive effect on me.  After expressing my thoughts in an email with Mike, he referred to them as The Velvet Glove.  They do require adapters to use them with.

3) in that same batch as above, he also put in a pair of '52 Sylvania Bad Boys and he actually shipped them in their original packing box that the Navy had them in.  They made me think of the RCA VT231s I alluded to above, but on steroids.  I was infatuated for quite some time with them before I tried out the National Unions.  I emailed Mike to express my thoughts on those, and he called them The Iron Fist.

Getting back to that first batch I was talking about, there were also a pair of TungSol VT231s that I also liked (more than most of the others), but the RCA VT231s captured my heart and ears (at the time) with the incredible detail they brought out.  I do not know if those TungSols were the GTBs he was referring to in the email (I am thinking not), but I will have to go back and check.

 My RCA JAN 231's are the smoke glass not the black.

@billpete, I think that it is quite possible that we are talking about the same tube, but I am mistakenly calling them "black glass."

I'm waiting for a pair of Sylvania VT231's that I found a few days ago. They are also labeled 6SN7GT. Not sure if they are the bad boy or not. I have to study that a little more to understand which is what. I think the BB's are silver bottom if I remember right and I don't remember what this pair looks like. Seems to me they are silver bottom but the seller never called them bad boys so maybe not? I am still kind of anxious to hear them. 

I know that there have been some threads on identifying the Sylvania "bad boys", I think if you plug that nomenclature in as a search engine in the 'search discussions' box, you should get some hits.  Regardless, you jumped in with both feet on tube rolling with that SLP98--you have my respect!  I am looking forward to your follow-ups.