6SN7/6H8C/6F8G

I've been using the double triode 6SN7 tube for as long as I can remember, I have a collection of about 500 NOS 6SN7's , about the same of the single triode version the 6J5.
The older NOS tubes sound damn nice , but they are frequently microphonic, and although I don't think that is necessarily a bad thing, with a high gain preamp that can be a problem.

NOS 6SN7's 

For at least 10 years I've been using the Russian 6H8C version of the 6SN7, it is a well manufactured, sturdy version that sounds very good, and is not too microphonic.

Russian 6H8C

Another earlier version of the 6SN7, made around the 1930's is the 6F8G. This has a top cap for one of the grids, which is a bit inconvenient as a hole has to be made in the chassis for the top cap connection.  Although it uses the same octal socket as 6SN7, the pin connections are quite different.
But its a fine sounding tube, and I built a few preamps and power amps that used it.
I built one power amp with 6F8G and 807 , both using top caps which gives an "antique" look.
Recently there have been some convertors available on Ebay which allows one to use the 6F8G in place of 6SN7 without any re-wiring of circuit.
One of my Russian customers asked if it was worthwhile, so I bought a pair to give it a try.
They look kind of stupid, but there is something about the 1930's period tube sound  that is very nice, it's different rather than better, but it is a enjoyable alternative , and the 6F8G tubes are relatively inexpensive, or they were when I last bought some many years ago.
A fun experiment for those of you with 6SN7/6H8C preamps.