the McGrath Mini amplifier

 

 A couple more things have changed in the version 2 design.  Updated schematic coming soon... For one thing, in the fx loop return circuit, I'm using two inverting stages with negative feedback, to multiply gains together and create a clean and stable 100x non-inverting gain.  In the first version of the amp, I have used negative feedback triode stages in several places, and I just used a single coupling capacitor, shared between the output and the feedback loop.  However, I noticed that Howard Dumble used separate branches with a capacitor for each one; I got to thinking, maybe he knows something here, maybe it's better to keep the branches more "isolated" from each other.  So in version 2, I drew the schematics using the two-capacitor topology.  But then I encountered more discussion of NFB triode stages in the Aiken Amps pages (a great resource for tube amp design, along with Rob Robinette and others).  Aiken seemed to imply that the single-capacito...

 

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...