Here's a simple MOSFET overdrive from Lovepedal. Should be a quick and easy build in a 1590B. The board is designed to fit vertically in the enclosure with this kind of pot soldered on alternating sides of the board (Intense and Volume on the component side, Tone on the solder side). Oh, and the electrolytic caps are just lying flat, but not axial types. Schematic, notes and even a 1590A layout here. My layout below is so symmetrical I had to get a fabricated version on store... Here's the description from Lovepedal:
Reminiscent of the mid 60's Blackface Fender era. Specifically a Blackface Super on 6.
Plug the Super 6 in to a ANY clean amp to nail that mid-60's American amp tone. This unit will turn a small amp into a much larger sounding amp instantly.
With an onboard vintage tone stack to die for, you now have ULTIMATE control over ANY guitar / amp / effects combination.