7N7 to 6SN7 adapter
Take a 7N7, plug it into this adapter, and you have a 100% compatible 6SN7. Electrical parameters are 100% identical. So tube at the inside is the same, for 6SN7 and 7N7. The Loctal base was an improvement, since it needs no soldering of the pins during production. Also the metal base is very nice since it provides shielding. When talking about 6SN7, some metal base tubes excists, and people pay crazy prices for it on Ebay. Well, with those Loctal based tubes, you find it included for free.
||When you buy the adapter, the 7N7 tube is not included. We sell those separately from Prime Quality stock. This is the Sylvania "Angled plate" which is amongst the top 10 best 6SN7 - 7N7 ever made.
Prime Quality (*) NOS 6SN7 are now very hard to find. As a tube dealer, you always have to collect them in small quantities. Rather one by one even. The times where you could find factory packs with 50pcs of one kind are definitely gone now, and sure this makes it extremely hard to find two tubes of an optical pair, same date code etc, and not to forget same electrical data. For this we only have the Toshiba 6SN7-GTB for sale, which is a specialty, and worth to check it. A nice alternative, is use 7N7 which costs less than 6SN7. The 7N7 is electrically fully identical to 6SN7, apart from the Loctal base, and the pin out is different.
For the question of GT, GTA, GTB etc, refer to the 7N7 you take. So a 7N7-GTB will become a 6SN7-GTB. However do not pay much attention to this, for HiFi this doesn't matter. It was only for tube televisions where you needed specifically a GTB, if the TV was specified this way. Today, you can use it to get an indication of the tube age. Generally speaking no suffix tubes are the oldest. Then comes GT, then comes GTA, and GTB is the youngest.
Prime Quality (*) means untouched factory packed boxes of 50 or 100 pcs, and inside are individual boxed tubes, crispy like new. Definitely this can not be found with 6SN7 any more.
NOS quality is of 7N7 is far better, and still costs less than 6SN7! It does not mean 7N7 is a better tube, just good 6SN7 NOS for decent prices is simply gone, but 7N7 you can still find. Though the situation is not like 10 years ago, when you could get them for 1$ on Ebay. That is over now. Still we have the situation of low price, good availability, and high quality for NOS 7N7. So take advantage of it now, and not wait until it is too late. I write this in 2017, and NOS prices go up 20% every year, and they are doing so since 15 years now. (Yes, that many indeed). 7N7 this tube is discovered only "a little bit", but it is coming.