Not sure how/why I avoided laying this one out for so long. The Azabache is a great sounding overdrive that emulates classic Fender amps. Check out the ROG article on the circuit for more info, schematic and sound demos. Adjust the trimmers so there's 6v on the drain of Q1, and 5v on the drains of Q2-4. You should be able to squeeze it into a 1590B.