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!

hot-rodding the Champ: overview

These are my modifications to the Fender Champ, to boost the gain and generally dirty it up a bit.  First, a collection of simple and small changes, to make slight improvements to the sound, and to add a "mid" EQ knob; later I'll cover the most significant mod, where I add another gain stage.
(My Vibro-Champ was built in 1978.  A similar circuit was used for many years, before and after this model year.)

The Champ as it starts has a unique tone, due to its single-ended 6V6 power amp and small open-back speaker.  However, Fender in a way worked against the natural tendency of this amp to contribute non-linear colouration to the sound: they added negative feedback.  So this is my first mod: remove the 2700 Ohm NFB resistor (just clip one lead so you save the vintage component, as a suggestion).  I originally fitted a switch on the back to turn NFB on and off (and even 3-way, to select different resistor values).  But I found the difference was pretty subtle, and always no-NFB was slightly preferable to my ears, so now I just leave it disconnected.

A second change which I make, both to loosen-up the sonic behaviour, and to reduce the current loading on the amp and tubes, is to insert a "sag" resistor in the high-voltage power line, just after the rectifier tube and before the first electrolytic cap: on this amp I used 1 kOhm, 5 Watts.  These amps were designed back when line voltage was closer to 110VAC, contrasted with today's 120VAC.  Plus, hot components such as cathode resistors always seem to drift in the direction of lower resistance over time, thus leading to more current and more heat.  A "sag" resistor mellows-out such a hot-running amp, and should thus extend the life of the old components.  (Some people run old amps on a variac, but this will also drop the heater voltage; as tubes age, they might actually need more heater voltage, not less, to approximate their original performance.)

Circuit-wise, these mods could be made to any Champ of the right broad vintage (AA764).  But I've used a Vibro-Champ and I recommend this, because then you have extra knobs and an extra tube available when you get rid of the tremolo circuit; so this can all be done without drilling new holes.

Assuming Vibro-Champ, then, one of the available knobs re-purposed from the trem is 25k, and this can be inserted between the bottom contact of the "bass" pot and ground, replacing the fixed 15k resistor (wire the pot for maximum resistance at full clockwise).  This pot then becomes the missing "mid" EQ knob which most other Fender amps provide.

As long as I'm re-purposing knobs and thus invalidating the names printed on the front panel, I felt I might as well go whole-hog, and physically re-arrange the knobs in an order that makes more sense to me; otherwise, we'd be looking at a knob ordering like "treble, bass, mid", which I find yucky (sorry, Mesa Boogie and others!).  The remaining knob from the trem (3M) becomes the 2nd stage gain control.  So the new knob roster looks like this:

volume, bass, mid, treble, gain2

I created a PostScript/PDF image to provide text for these new knob assignments.  The PDF file can be printed, cut into a thin strip of paper, and can be stuck to the Champ panel with Scotch tape.  (Vibro-Champ knob spacing is 1.25 inches.)

(Important!  I almost forgot!  The actual first mod I do to any amp of this kind, even before first power-up in many cases, is to remove the so-called "death cap", a capacitor which many tube amplifiers have running between one of the AC power line inputs, and chassis-ground.  This is supposed to reduce hum or something?  But the ways it can go wrong should be obvious.  Clip it out and solve hum problems with other means.)