Recently I did something I never said I'd do -- build an amp. Nerve wracking, but I'm really pleased with the result! Handpainted faceplate, of course. Check out my blog for a pictoral, and I'll eventually post a build document when I complete it.
Some info about the changes from the original champ circuit:
-More power filtering.
-5AR4 recifier for more headroom. The preamp voltage is also higher than a champ, which gives a bit more dynamic range and makes overdriving the preamp harder.
-Elevated heater supply for hum reduction. (Thanks to davent on the forums for the suggestion, and Merlin Blencowe for describing it so well on his site.)
-Treble cut/boost control (524-1.9K cut, and a "boost" above ~1.5K, a little past noon works out to be flat). Nothing extreme, but good for taming particularly bright instruments or lots of harmonics. Based on Mark Hammer's Stupidly Wonderful Tone Control.
-Active mid boost control - a variable cathode bypass cap on the second gain stage applies additional gain to frequencies above 188Hz. Really good for tightening up the bass when overdriving the amp, or for bassy guitars (like my Sheraton). This was inspired by the Tone King "Mid Bite" control, but it's accomplished differently.
-Negative feedback switching: lifted (no NF); standard NF close to the original champ's value; and something I've never seen before, a capacitor in series with the negative feedback produces a bass boost. The Mids pot reacts slightly differently in the various NF settings, which gives the amp a surprising amount of versatility. Thanks to Uncle Doug here on YouTube for describing negative feedback so well.