I've got a Sonic 1 that was made in 1970, so the info Rich Brier has on his site isn't totally accurate. The earlier Sonic 1 was 40 watts and used a pair of EL34's. Later on they changed it to 6550's, AND they also changed the power and output transformer, so it is a 60W amp. The later ones are most likely all the 6550 type with 60W, although if you put EL34's in it your power would go down to 50W or so probably. You can tell which one you have by the size of the transformers. The bigger ones will be the 60W 6550 version. You would have to compare yours to a 200S or something to tell. You amp is exactly the same as a 200S if it has the bigger transformers. If that is the case, then it was intended as a bass amp, however, it works fine as a guitar amp too.
I got mine for $40 in need of lots of repairs. I modified it with a true ground buss, discrete caps instead of a can cap, a ground lift switch, and bigger filter caps in the first two stages for more and tighter bass. I use the amp for bass and it sounds MUCH better after my mods. I had some labels made up to make it into a 200S because the faceplate was all rubbed off on mine, and thats pretty much what it is anyway as the circuit is identical.
If you're using yours for guitar, I would think that the EL34's would sound better. The 6550's won't give you any grit until more than half way up the dial, and the quality of the distortion is harder than with the EL34's. Make sure you have good tubes in it, and make sure all electrolytic caps are replaced with new ones if they are over 10 years old.
