The Ampeg J20 is a very cool amp from Ampeg that is very closely modeled after the old brown Fender Deluxe, the 6G3 circuit. It is not a clone, there are some differences, but it is remarkably close, and it does have the great "bias vary" tremelo, as used in the 6G3 Deluxe and also in amps like the Princeton Reverb. The amp is hand wired using a turret board, no pcb in this bad boy, and even comes with an alnico magnet speaker, which is nice enough at low volume but it seems to have issues when pushed hard. I'll be looking to upgrade the speaker at some point, but for some lo volume gigs I've been using the amp for it's been working fine, so I'm not in a rush to change it out.
In any case, as nice as the amp is, there are a couple of things that you can do to make it even better.
On this page are links to my J20 mods document as well as a schematic for the amp showing the mods. The doc's are in pdf format, so you will need Adobe Acrobat Reader to open them. If you don't already have it you can download it here: download
Once I get some video's or clips of the amp available I'll post links to those as well.
Cheers, and as always, if you need any clarification on these mods shoot me an email. Also, there's not a lot needed in the way of parts, but should you need any I can help there as well, again, shoot me an email and I'll hook you up.