I have the aforementioned Hammond amp which is the "reverberation" amp for an L-100 series organ. As such it has a 12BH7 tube to drive the reverb spring and a choke to act as a high-boost for the reverb drive as well (it's the plate circuit load, so its impedance goes up with frequency and so therefore does the voltage gain). I was thinking to sell this but the more I think about it (and nobody's buying it) maybe I would like to try my hand at adapting this to my own nefarious purposes. My own background is suitable. I have a double-E degree from a noted university, recently involved in the admissions scandal, but I had nothing to do with that your honor! My electronics textbook's first 8 chapters are all about tubes but we skipped over that part because it was no longer relevant (cough). I know all about ohm's law and have a nice scope and the workbench is here somewhere under all this crap. But I don't have a good "feel" for tubes in spite of reading all sorts of stuff about them over the years. I'm thinking about how to get started with this project. Let me sketch out my initial ideas and you tell me where I'm mistaken. Step #1: Get the power supply/rectifier going. That section is not shown in the schematic above, but you can see that B+ over on the output transformer is +340V. As shipped this thing specifies a 5U4 as the rectifier. I don't actually have a 5U4 in hand - I have a couple other octal base rectifier tubes, but they are all old. Tube Depot sells a JJ version for about $15, doesn't sound like an excessive risk. I've read a few things hinting that the higher line voltages today compared to when this thing was designed may result in a too-high B+. Looks like there is a way to insert a high power zener diode into the B+ ground return in such a way that you can "lift" the voltages by virtue of a bias voltage, but I'm not super clear on how that works and how you might adapt a circuit such as the one above to using that, if it becomes necessary. Other thoughts, summarized as this is already TL;DR -- a) Use the power amp section intact. It has NFB "Factory Volume Adjustment" that might allow insertion of a presence control. Being as I'm lazy and the power amp section is already built (modulo electrolytic cap replacement) I am really leaning heavily in this direction. b) Use 1 left over 12AX7 for the input and maybe subsequent cathode follower to drive a standard passive TMB tone control section. c) I've looked at some things like the Phaez Daisycutter, which uses this tube complement, and as far as sound samples it's all well into the sizzling lead and hot crunchy rhythm zone. That's great but I also use clean tones a lot and I'm wondering what it takes to get something like this to work at least "OK" in the clean zone.