Schematic I drew up for my dual Maestro EP-2 preamp pedal. Basically two tube echoplex preamps in series with a switch to switch between running just one pre or both in series. :
12AU7 / 12AX7 Tube Preamplifier Power Supply Schematic
12AX7 Tube (valve) Preamplifier Kit
Carvin X1 Tube Guitar Preamp Pedal - Carvin Audio
Peppermill
High voltage tube guitar preamplifier with 12AX7 - H&G Amplifiers