2nd gain stage

By sacrificing the Vibro-Champ tremolo circuit (interesting effect but only occasionally useful to me (ever?), and obtainable through other devices if needed), two knobs on the front panel are made available, and the second 12AX7(A) tube is made available, for a second gain (overdrive) stage.

With some compromised component values, I've managed to design the 2nd stage circuit making use of many re-used passive components from the tremolo circuit.  This is convenient because the re-used capacitors have high-voltage ratings, and the resistors are of higher wattage ratings, both not so readily available (i.e., not immediately at-hand in my low-voltage parts bins).  The components attached to the plates of the tubes (pins #1 and #6) in particular need high voltage and wattage ratings.  I contributed some new resistors, but only in places where 1/8W or 1/10W would suffice.

The two triode sections operate in series, with an R-C network in between them which generally increases the treble response.

The first section is configured for high gain.  The second section is configured as a "cold clipper", with a relatively large cathode resistance.

The 3 MegOhm pot from the trem circuit provides a gain control ("gain2") ahead of this circuit.

A DPDT switch inserts the 2nd gain stage circuit in between the "treble" pot wiper, and the "volume" pot top end; or when disabled, the switch connects "treble" to "volume" in the original way, and switches the input to the "gain2" pot top end, to ground (to minimize incidental noise in the clean channel).

To avoid having to drill new holes in the panel (or having to purchase non-original pull-switch pots!), I remove the second "instrument" (input) jack, and fit the DPDT switch into that hole with a washer.  Internally, I connect the two 68k resistors in parallel, so that the remaining input jack sees a series resistance of 34k into the first tube grid (and 1M to ground), as originally designed.

It would be best to mount the passive components for the 2nd gain stage on a small circuit board.  I do not recommend the technique I used for my own (constantly-evolving) prototype here, aka "flying leads".  But with caution and care, I make it work quite reliably.

As mentioned, I am continually playing with the component values of this 2nd stage, but I'll document my latest preferences as of this writing.  The tones I am obtaining (with my single-coil Squier Mini-Strat) are not heavily overdriven, but the circuit adds a nice bite and grit to the otherwise almost Puritanically-clean Champ.  I'd compare the distortion to that which one hears from the Vox AC-30.  Most modern guitarists would want something upstream to add more overdrive, at least some of the time.  But the tone is very nice, a much-more-rewarding semi-clean tone than the raw Champ tone.

Try it, and let me know what you think!