I'm using that chip for a similar project.  The cleanest sound I was able to achieve was using the +15v side of a regulated bipolar supply (Rod Elliot's board).  It sounded great, but the regulator got too hot to touch so I shut it down.  Velleman makes a really cheap 1a regulated, adjustable supply that I'd like to try in the future, but for now I have 4X9v batteries in series/parallel for a bit more than 18v.  It still breaks up too soon for my taste, but I put a 50k pot between the guitar and the input and that cleans it up at the expense of some volume.  I'm using an 8 ohm, 8-inch Peavey speaker (I think I like my Radio Shack closeout/ceiling PA 8" better).  No preamp, yet.  Glad to see someone else using this chip/board; keep us posted.

Like I say, this is a great forum.  Enzo, you're obviously right and I'm a dork about the parallel resistor. I can live with 25-35 watts as long as the speaker can.  And, the idea of fixing a cooling fan pretty much blows my mind (no pun intended).  I always thought of them as disposable components (they're certainly cheap enough).  Thanks for the good ideas.

Thanks for the prompt reply; this is a great forum.  I have the fan ordered.  It's VERY small (40mm square X 10mm) but it's in there for a reason, right?  Hypothetically, what would happen if I put a 50W, 8-ohm resistor in parallel with the speaker?  It seems to me that the chip would be back to rated power and the new speaker would have some power-handling help.  I wouldn't do that since the amp sounds terrific right now, but it's a thought experiment.

I'm working on a friend's Marshall AVT50 combo.  2 main problems: The speaker is bad (a Celestion G12-412 AVT, 4 ohm), and the cooling fan bearings are shot and it makes a lot of noise. My 2-pronged solution was to swap in a random 8-ohm 12-inch driver from an old organ, and disconnect the fan.  The speaker sounds surprisingly good.  My question is, with the new 8-ohm load, the power output from the chip drops from 50 watts to 25-ish, right?  That also reduces the heat it dissipates, and maybe makes the fan unnecessary, right?  Did I miss anything?