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.)