a digression: Fender Vibro-Champ

This doesn't directly concern my amp design; but I have based quite a bit of the amp design on the Fender amp circuit.  Anyway, here's a related project, taking a Fender Vibro-Champ amp, and re-purposing the 12AX7 tube from the tremolo circuit, to become a second gain stage.  This frees up two pots, which are re-purposed as the missing "mid" knob for the tone-stack, and the gain control for the second stage (the original volume knob now behaves as a master-volume).

One could use a pull-switch pot for the channel switching, but in my case, I want to re-use the same components as much as possible (i.e., the original pots), and I already have a non-original switch on the back for disabling the negative feedback; I can re-purpose this switch as the channel switch ("2nd stage").  Or, one could drill a hole in the front panel; presumably this would be the least desirable option for most people.

*** Edit: scratch that above, I'm now thinking the best plan may be to put a switch in place of the seldom-useful second input jack.
(Notice that in my own input section, which in many ways copies Fender, I have a second input jack, but the resistor ratios are different, to make a bigger difference between the two jacks.  The Fender is only a divide-by-2, i.e., -3dB.)

(If it's not obvious, these modifications turn the Champ preamp into something very close to the "F+" gain stage, in my amp design.  This project is both a way to make my Champ more versatile, and a way to prototype the "F+" circuit before I actually begin construction of the amp.)