v2 preamp

Here's the schematic for the version 2 preamp.  This uses 4 12AX7 tubes, as opposed to 3 in the first version.  The extra stages are used for the "boost" stage in the input section, and for two inverting NFB stages in the fx loop return, allowing this to be non-inverting and to have stable high gain (10 x 10 == 100 as shown; values for this gain and for the ratio of division in the send may be changed as I develop this circuit in prototype form).

The 5 relays will be controlled by 5 switches on the lower back panel.  Each will be 3-position: "on", "off", and "ftsw".  The footswitch will still be the same 2-button unit, with stereo (3-conductor) 1/4" connecting cable.  "nfb", "boost", and "M-drv" will be assignable to button 1; "aeq" and "fx" will be assignable to button 2.  Thus, the fx loop will now be footswitchable, unlike in version 1; however, all the gain functions will be on button 1, so unlike in version 1, it won't be possible to have three choices of gain level from the footswitch (in practice, with the version 1 prototype, I've found that I seldom really make use of all three gain choices dynamically, but I frequently wish that the fx loop was controllable from the footswitch).  The new switching arrangement is a compromise to maintain the basic simplicity of the circuit while giving much more flexibility than the version 1 footswitch could provide.

The other switching functions, i.e., "voice", "scoop", "brt", and "cold", will be controlled from switches on the upper back panel (no footswitch control).