I started with this R.G. Keen Schematic
and made a couple of minor changes for voicing. I also added a MOSFET buffer
to the output per R.G. Keen again.
Power supply uses a doubler per the Alembic original. The transformer
is from Antique Electronic Supply. It is capable of powering two 12A*7 type tubes. The doubled and filtered voltage under load is 300V. The filament supply is 12VDC regulated. The preamp has no hum, but has some hiss when volume is cranked.
I made two PCBs
, one for power supply which can also be configured for a conventional bridge rather than a doubler, and one for the preamp. All external connections are made using swaged and soldered turret terminals. This makes for an easy assembly that will fit in a 1U chassis.
The second tube socket is for a 12AU7 SRPP transformer output stage per the Gyraf G9 preamp design
but haven't needed it at this point due to how I am using it (driving the effect return in an amp). The output is hot and needs to be padded down.
I'll add the final schematic later but it is pretty close to the Keen schematic with the exception of the regulated filament supply and the MOSFET buffer.