Royer Tube Mic Mod - I can't reccomend this one enough. I have built a total of 10 of these using Cinemag and Jensen transformers, Peluso capsules, stock capsules, AKG capsules, and am always pleased with them. Highly customizable, great sound, and a very technically easy build. Email me if you have any questions.
Akai Tube Preamps- Another highly reccomend mod. This one was a bit harder and you should know how to discharge capacitors as this runs at high voltage. What you get, though, is a true high voltage tube preamp with 2 channels on EF86 tubes (same as Nuemann used in the 1960s) and 2 on 12AX7 tubes. These are my go to preamps. In blind test these were consistently the winners.
Shure M67- Simple cheap mod. I use these for mic'ing the snares and sometimes the bass drum.
Missle Control Overdrive - I built this out of some extra CV4502 tubes I had. The link used 5840 subminitare tubes, but I decided to just loosely base the pedal off it with some slight changes. Great tube distortion sound.
D&R Compressor - cool, vintage sound. I went with the Ross version. This is a great resource for schematics on effects pedals and lots of DIY help.
Viking RA72 - a simple conversation of this to a 12ax7 ribbon mic preamp...I left most of the original inside and think I may be able to add a 2nd channel. Still tinkering with this guy.