The circuit I decided on. Simple tone control (bass and treble) with feedback. Uses an initial cathode follower and a final gain stage to compensate for the losses in the tone control section. Because it's an Active tone control, it doesn't need audio taper pots. Linear ones work fine. I managed to find some with a centre detent as well!
I decided to make this on a PCB just because I hadn't done this before. So being a complete novice at PCB design, everything is done by hand. No auto routing or anything else. This was my initial sketch
Then I designed the board. Measuring the components I intended to use by hand with the micrometer. Yeah, when I said "First Principles" I meant it.
The board, ready for etching. The design was made according to the size of the board blanks I had to hand, to avoid having to do any cutting. The transfer process involves printing the design with a laser printer onto a plastic sheet, then using a clothes iron to transfer it on to the board blank. Usually this requires a bit of touch-up with a Sharpie-pen but in this case it was almost perfect.
Work in progress. Starting to build the board. I discovered to my dismay that I did not have all the resistors I need, so there are some unfilled holes in this board.
Also there's the power supply board which I made the same way, it's very simple and boring.
This whole project started because I bought the power transformer from the local auction site for $9. It has 213 - 0 - 213v secondaries, plus 6.3v. This gives a nice B+ of around 300VDC.
Also I had two 12AX7 tubes left over from a previous project - these are horrible cheap Chinese ones, but they work OK. Sufficient to test it, if I like it I might put some JJ or EH ones in.
More photos later when I get the case back from the CNC and laser etching...