EL3, and AL1 on Ebay 2016
4654 (similar to EL34)
Due to this unusual socket, very beautiful NOS tubes can be found with Au8 socket at reasonable prices, even tubes like EL3n which are small EL34, they are low cost. There is the "A" series which is 4Volts, and the "E" series which is 6.3 Volts. Some people that do not know what they are talking about, dislike that socket. Their talk is very useful, because thanks to this, we can find beautiful NOS tubes for quite low prices still. When you go for the 4V heaters, you can find such beautiful tubes like AL1, which can be triode connected, and you have a top class driver tube, of appearance you never get any other way. (AL1 is a directly heated pentode, and the mother of EL34). Or how about 4654, this is very close to EL34 with a top cap. It is true, sometimes very old sockets in old radios can be broken, but NOS Au8 sockets can still be found for just a few Euro, and Yamamoto offers even a gold plated ceramic socket. Furthermore, this socket has the MAJOR advantage you can access the contacts when you remove the tube, for cleaning or inspection. Also, this socket pulls in the tube by itself. Did you know that? Such a nice feature. It is amazing, I write this text now in 2016, and the market for cheap NOS tubes has dried up. What you can get still nicely is Russian NOS, which is not pin compatible to American NOS, such as 6N2P, a finest version of 12AX7. When you make the step to using the Au8 socket, you can an unbelievable choice of beautiful tubes, which the crowds "does not like", however most of those opinion makers, know the tubes only by pictures. So better listen to people who really know about tubes. Are you tired of paying a fortune for NOS 5U4G? So what's the problem with using AZ4? Are you tired of searching for crazy priced EM34, which may not even be very bright? So why not use electrical identical EM4 for half the price, at find real NOS easily? Or have you looked at El3n, which is an early (lower power) version of EL34. And all those wonderful driver tubes that cost only 20 Euro, or sometimes I see then go for 5 Euro only. Mint quality NOS that is.
This, and a lot more is the hidden world of the Au8 socket tubes, but you do need to concentrate a little bit on this. There is nobody presenting you on a silver plate, how to replace those in existing Octal schematics, and if this is your main problem, perhaps you should not use them. So it is going to stay a nice, hidden area for people who have electronics skills, and pay very little for those nice NOS tubes. Though I have to say, they are getting discovered a little bit. So days when AZ4 cost only 3 Euro are gone.
Have you every thought of using the AZ4 rectifier? In case you did not know? This is similar like 274B. Just 4V heater and Au8 socket of course. 274B.?! Perhaps now it rings a bell. On Ebay you find NOS AZ4, from 30...60 Euro. For prices much lower than WE274B, and you can easily get then as NOS, or when you don't mind used tubes, with a bit of luck, you find used mesh wire versions, for 50...100 Euro.
Au8 NOS Mesh wired versions are rare, and prices depend a lot on the brand, and condition, and can go up to 250 Euro.
There is a optical lovely Tungsram Version AZ4 around, it looks much nicer than any USA 5U4G.
Yamonoto uses uses AZ4 in his "All European Tube" amplifier.
Here is somebody else, who spend a lot of effort and time on Au8 socket tubes:
Link to DHT-Rob website