The next linestage I tried was a variant of the 6SN7 linestage that was featured in Vacuum Tube Valley. The schematic of the active portion is here, but pick up a back issue for the full article.
I generalized the schematic, and made up a small PC board for ease of construction. The board will accept any 8DB pinout tube such as 6SN7, 6SL7, 6BX7 etc.
My power supply was somewhat different - allowing for the use of junkbox parts.
Sonically, the 6SN7 is enjoyable and very pleasant. However after a few weeks of listening, I still prefer the 2C51/407A linestage
Ok, so back to the 2C51/407A. I purchased a case of 34vct transformers surplus (read - cheap!) and made up a small PC board with LM7820 regulator for the 407A filament. I also revised the 2C51/407A circuit board to accept either type with a simple jumper change.
This latest revision includes the regulated 250vdc supply based on the 6GV8/ECL85 (see the main page for details), a stepped attenuator I picked up off that "auction place" and both the revised 2C51/407A circuit board and new filament supply. I tried to use the same sized chassis, and things got a bit tight! Output coupling caps are some nice vintage 4uf Spragues that I picked up a recent NNETG (Northern New England Tube Group) Yankee Swap
My initial impression is positive - the regulator circuit is dead quiet, and there are some subtle improvement in soundstage.
Stay tuned... I have this box of 26's and 27's just sitting here, and then there's this pile of 12B4's....