Currently using a couple of interesting preamps with some rather obscure valves in the line stage...
6EW7 and 8608
Both can be bought for very little and sound very impressive.
Untitled by
The Biglebowski, on Flickr
Untitled by
The Biglebowski, on Flickr
I'm thinking that at least the top one is a DIY one that started life as something else... the chassis clearly shows "E55L" next to the socket... this is utterly different from the 6EW7, the first is a pentode (and very similar to the 8608! but different pin out etc) and the latter a dual triode.
They are very interesting valves and vastly better than the usual suspects.
Unfortunately hard to obtain and pretty expensive.. which is the huge problem with many/all of the best valves.
The likes of ECC81/2/3 etc are toys compared to these!
The last gasp of valve technology was the frame grid valve (and the Nuvistor) and many types were developed for industrial, professional and military applications but also some for TV. They have much better characteristics than conventional valves. The only one that springs to mind as a commonly available valve used in audio is the ECC88/6DJ8/6922 and it's parameters make it my favourite triode. It is what I used in my DIY amp project mentioned above and is what makes my circuit an order of magnitude better in many regards compared to the Croft design in terms of measured performance.
There are many types around which are much better still than the ECC88 but unfortunately not common.
And therein lies the rub... There are valves around which as I said are an order of magnitude better than the usual suspects but maybe £30 - £100 (some a hell of a lot more!), often available only in small quantities, you usually can't just get them from Watford Valves/Hot Rox etc etc and a suspicion I have is that after all these years many of the best in terms of microphonics, noise, matching etc will have already been selected and sold off, leaving a bit of a lottery as to what you get.... not to bad if it were modern ECC83's @ £12 each but if they are rare and £90 - £160 each...
Russia made many superb frame grid valves for military use and at one time they were dirt cheap, like say £3 each, but as mainly DIY'ers (and a very few manufacturers) got wind of them their value went up and even these are now often say £25 - £50 these days. There's still a few bargains to be had though.