A lot of posts on FDD20 and 6N7 family tubes on the last pages and rightfully so. To my ears the FDD20's are the best and they run very well with stock Russian powers. Indeed, they brings out a lot of detail in both the mids and treble but that is served on a basis with increadibly 'physical' bass. Very nice for rock too. Euphonic? Yes. Too euphonic? No, bring it on!
To answer your question one or two pages ago Howie13: yes, with a 12V supply for FDD20 you are fully equipped to work with 12SN7's too. They are electrically identical to 6SN7 except for the heater current.
I decided on a 'lab power supply' so I can even use 25V tubes, or 3,2V, or whatever, and tune the exact heater voltage around the recommended value. I previously reported I needed some 13.5 volts to run the FDD20, but that turns out to be an artifact. The output voltage cables supplied with my lab power supply are so thin that the resulting resistance lead to a significant voltage drop (and hot cables..) The voltage at the tube end was in fact a normal 12.5V. Replaced by thicker cables, all is normal now. When I change powers to 6AS7G or 5998, I need a bit more voltage to be fully distortion free with FDD20. So, I really like the flexibility to raise or lower the heater voltage with this kind of power supply.
The normal 6.3 volt tubes can also benefit from an external power supply. My Elise is very finicky with the entire class of 6N7G tubes (sometimes distortion, sometimes not), I know of several others that had problems with this class of tubes and gave up on them. But when I run them externally heated all is well and they sound fantastic. Apart from an external power supply, you need one more part to run tubes that way. Xu Ling has 'socket saver' adapters for 6SN7 with external power leads (similar to the FDD20 adapters). You can use these in combination with ANY tube (+ the regular adapter that goes with that type of course). I've been doing that for the last few days and now EVERY 6N7G I put in works. Hmmm, interesting... One more experiment: the same is true when I put one of the powers tubes in the externally heated adapter, taking load from Elises heater supply. My conclusion at the moment is therefore that in MY Elise the heater power of my Elise is challenged a bit too much with 6N7G's in (two of them draw 1,8A versus 1.2A for the stock 6SN7). I bet the same problem will arise with eg Mullard ECC31, which draws even more (1A each if I recall correctly). I don't have these thought...
Long again, sorry. Basically my message to Elise owners who'se amp refuses to work with 6N7G and ECC31 is: buy an external power supply + these 6SN7 socket savers with external power leads and you are good to go. In fact: one will probably do (you need to supply only one power tube or drive tube with external power and it works). If you are among the lucky ones where Elise runs them 'as is' without any tinkering : you are among the MOST Lucky ones, others like me are the plain lucky ones
Howie13: if you want to buy Fivre 'horned' 6A6 (labelled ultron) NOS at a low price, have a look at my post 8238 in this thread. In fact, I have too many so you can also contact me for a pair.