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.

@tomcy6 

Thanks. This is essentially what I read. This seems to agree with what I read on Head Fi, that using a GT or early tube in something that was designed for the later GTA/GTB types, could cause problems. What I read made it sound like you could damage your component, where this says that you may just ruin a tube. Some of these early tubes bring some pretty silly money and the gear is certainly not cheap. I'd hate to ruin either one. 

Using a GTA or GTB tube in an older design piece of gear that was designed around the GT types is safe but this is not my situation. Still, tons of people have talked about putting the older GT's, W's etc. in modern components and they end up being their favorites. Most notable is probably the Tung Sol BG RP, seems to me the 40's or maybe early 50's. Can't remember but I'm guessing these are the older GT type. I have not taken that plunge yet, if you find them they generally run $200 each or more. So far, I'm getting plenty of enjoyment from tubes that have been $50 each or less.

There are several tube rolling threads several years back about the very preamp that i use, as well as it's other iterations. The tube compliments are the same in all of them, simply saying 6sn7 but always supplied new with modern versions, which I can only assume match directly with GTA and GTB types with 4.5 to 5w output. Nearly everyone agrees that the best tubes were the oldest, the ones made in the 40's that would have the lower output of 2.5w. None stated that their gear or the tube failed. I have seen a number of reported failures of the old Russian versions with metal base Melz 1578 and 6N8S, which are highly coveted but not without controversy. Many complain about noise which seems mostly attributed to bad solder (sometimes able to be repaired) and some have complained about early failures. Most of these tubes are 70's and 80's but there are versions back to the 50's as well. All speak of great SQ, if all in working order. I have interest in them but am hesitant as well. I have plenty of US made tubes to fool around with for now.

I have several of the older types but have not used any yet. I am enjoying several different varieties of GTA/GTB's but am looking forward to hearing the older ones that everyone raves about. I'd hate to think that they could be unsafe to use (for the gear) or that I would ruin the tubes themselves. Has anyone experienced any of these problems?

@immatthewj 

Thanks for this. I have read all of this stuff repeatedly. You see some minor discrepancies here and there but the bulk of it is the same. W is the nomenclature given to the industrial versions (ruggedized) for heavy use. W's are popular. JAN is joint Army Navy and usually means something similar to the W, but I also read that the military versions are not always built to a higher standard. The VT231's are WWII, made for military and may or may not be any different than the civilian varieties. They do generally bring more money.

So..........cut to the chase, I guess. Have you used any early versions W or GT in your SLP 05? What did you think of them and did you experience any problems?

As for me, I have enjoyed Sylvania Chrome Dome GTB's (red letter) in the front and Raytheon GTB's in the back. This combination is pretty enjoyable. I have a pair of Raytheon VT231's on the way as someone (decooney maybe?) spoke highly of them in the rear position of their SLP 98. 

Pulling the EH tubes out after the first near 100 hours, has been enlightening. I didn't hate them but they just don't bring what the old tubes do. The goldpins were not bad in the buffer spot but after replacing them with the Raytheons, I'm not looking back. I'm a terrible reviewer. I simply don't have the descriptive words of a paid reviewer. Their "colorful" terms often leave me wondering just WTF they mean anyway. 

Yesterday, I switched the Sylvania pair out for an RCA pair of clear top GTB's. The Raytheons are still in the rear (buffer spot, I believe). This turned out to be a very pleasant combination as well. Both have good bass with Sylvania being maybe ever so slightly stronger there. Both with good detail, again Sylvania maybe with a slight edge but there is something about the RCA pair that is quite pleasant to listen to. I do have several others to try. 

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.

@immatthewj 

I think VT231's should be the same specs as GT's or W's as they were made for the military in WWII. GTA's came out around 1952 and GTB's in 1954, if I remember right. Anyway, I have a few pairs of VT231's, just haven't tried any yet. My RCA JAN 231's are the smoke glass not the black. I believe they are also labeled 6SN7GT. I also have a pair of RCA GT's in smoke glass that look slightly different than the VT version. I buy cheap singles when I find them, building a collection the hard way. I enjoy the chase though. At my age, I'm easily entertained.

I have a National Union pair of black glass GT's. I plan to try them today if they are a close match. Also bought separately, one came as a pair with a Sylvania GTB, the other I found by itself and it's a Westinghouse label but the exact same black glass tube. 

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.