O.K. this is not a great start. From this and other sites other mods included swapping out the tubes (with very different results both good and bad), adding a Weber speaker, ways to reduce hum, fixing the fizziness of the tone by replacing/fixing the foil inside the amp.
Changing the transformer (the Mercury mod) is a tad radical for me and besides, that is never coming up as a problem. Absolutely NOBODY is complaining about how this little sucker can power multiple drivers with the original transformer. The mods I'm looking for are more about getting rid of the fartyness and boxy sounds.
You can lift the ground end of the R19 resistor and it bypasses the tone stack effectively doubling the volume. I put it on a switch. I also blew my output tranny, so I have a hammond 125cse int here and it sounds extra chimey. The key is to use the 10k primary.
There are many cabs available. I might suggest that seeing as how this is a low cost amp, that a low cost cab which sounds good might be in line. I have tried the Epiphone 1x12 cab made for its Valve Jr. head and it is quite amazing for $129. I must add that I play harp, so am looking for different qualities of sound. What you want to hear is pretty subjective, and will vary from ear to ear.
See the similarity? The 180k/75k pair made by R20/R21 is essentially a standard 250k treble pot/control set to a fixed position. That resistor pair can easily be replaced by a 250k pot with the pot wiper being the junction of the two resistors. The 15k R19 component can also be turned into a midrange control as it is on the Vibrato channel of this Super reverb: http://www.ampwares.com/ffg/schem/su...b763_schem.gif
If you replace R19 with a 25k pot, you'll get a few advantages. First, you'll be able to dial in a nice midrange dip/scoop as the pot resistance is decreased. Second, by turning the resistance value of that pot up, you'll actually get a sort of rounded boost as the increased pot resistance reduces the amount of passive signal bleed. You could probably even get more boost with a 50k pot, though I imagine you'd need to tinker with the taper of the pot to get optimum dialability.
I was turned onto this by a 25 year-old article in POLYPHONY which showed how simply lifting the ground side of R19 (and its functional equivalents in other Fender amps) effectively cancelled the tonestack and all of the passive loss it introduces to produce tone shaping. With less passive loss, more signal hits the second half of the 12AX7 for some nice overdrive.
Finally, note that the classic fender "bright" switch is essentially a small value cap across the input and wiper lugs of the volume pot, as amply illustrated in the Super reverb schem. No reason why that couldn't be implemented in the Champion 600.
Bottom line: The Champion 600 makes a nice base for an easy to mod amp that can be turned into what is essentially a Blackface Champ for much less than the cost of one on the open market. Plugged into a bigger speaker, like a 10" or 12" in a different cab, I'll bet it makes a really nice amp.
I don't buy the run it through a bigger cab thing with this amp. It just doesn't sound good with my greenback 2x12, alnico blue dog 2x12 or a 2x10 with eminience alnicos. Maybe with an eq in front of it?
I've put a JJ 6v6 in it and changed the 12ax7 (I forget what's in it now) which improved it. But to me it seems to sound better through the stock speaker than the bigger cabs I've tried, but it definitely could sound better. I might try the weber out.
The best tones I've got from the champion 600 is when running a sansamp classic pedal in front of it. I'm betting my incoming Kingsley Jester will make it a lot better than it is hopefully.
As far as noodling in the living room volumes go, the champion 600 is a fun little toy but even at the those low volumes I get 3000% better tones with my valvetech hayseed and ultimate attenuator. The champion is a lot easier to carry around though especially when the wife tells you to get your **** back to the jam space.