Re: Getting a Blues Junior on the 3rd, recommend me some mods
You'll find lots of info searching here too. There was a thread less than a month ago with various opinions about the Blues Jr.
Shortcuts for everyone:
http://mywebpages.comcast.net/machrone/bluesjunior.htm]Bill Machrone's Blues Jr. Page
Mark Huss's cathode follower mod
The best "beginners mod" IMHO is a speaker swap. A Weber Blue Dog or a variety of other speakers will give you much better tone, and a slant toward British sound.
Trying different power tubes, the EL-84's, is worthwhile, but the biggest bang for the buck is sampling different 12AX7's in the V1 spot. If you go with Eurotubes, they'll sell you JJ's. (no offense intended Hoss) Not bad tubes at all - they may turn out to be your favorites - but you could try Shuguang (Chinese) and Tung Sol 12AX7's among many choices, and various power tube sets. (Ei's are my current favorite, but YMMV and the JJ's I've got are 2nd place in my book.)
Try a new speaker first, then the first preamp tube, then power tubes. If you still aren't satisfied and you know how to work on an amp without electrocuting yourself or terminally torching the circuit, then you might open it up. My Blues Jr. is still stock inside, but the first things I'd try would be the bias mod and tweaking the tone stack.
Just my semi-educated opinion, but Bill Machrone's recommended tone stack values won't get you a British tone. The cathode follower mod and standard Marshall resistor and cap values should get you much closer. Note: the cathode follower mod is possible because half of V2 is unused in a stock Blues Jr.
Have fun!
Chip