It’s been a long time since I’ve hardwired an entire build without one breakaway connector, so I was very happy to fire it up and have every voltage test point measure spot on. A +4dBu sweep at unity gain was dead flat from 20Hz to 25kHz on both channels. The tube balance calibration measured less than 1mV on each channel (-60 and -62dBu) with the NOS GE 6SK7 steel tubes. I also bought extra bundles of NOS RCA (also steel), Sylvania JAN, and Hit-Ray (both glass), but haven't felt the urge to swap out the GE’s. The AC-DC power adapter is a TDK-Lambda DTM65PW360C that I bought a couple of new on eBay. The datasheet for it listed the model of the Kycon connector, so sourcing the correct chassis power inlet was easy. I used super bright clear lens LED's and adjusted the 4.7k resistor values to taste (R4, 7, 10, 13, 16, and 19), sending the 25.3-ish volt power for them out to a breadboard and working all that out beforehand. Each color change ended up with a different value for them to all match in brightness, as expected.
I’ve been wanting to design a front panel inspired by a vintage HH Scott integrated amp that I read about many years ago in a tube amp magazine, with a gold faceplate and slide switches, and this project felt like the perfect fit. I did a fair amount of CAD work to lay out the symmetry and figure out the side chain mounting panel (to both conceal the hardware behind the small knobs and buy myself some space for where I wanted to mount High-Low switches), GR meter PCB locations, power switch plate, and case layout. I wanted the GR LED’s to be on the same centerline as the slide and rotary switches, so that meant bending an offset on the ones for Channel 2. I drew the LED with the offset in CAD, then printed it and taped it to a wood block with a cavity for the LED to sit in and bent them. It worked like a charm when installing the meter PCB’s. I clamped the face of the front panel to a piece of 4mm aluminum I had laying around, placed all the LED’s in the PCB’s, mounted the boards to the standoffs, used a hex driver to reach in and push each down until they hit the 4mm panel, then soldered them. To clean up the look of the gap through the power switch hole and keep any light emitted from the backside of the meters from shining through it, I cut a scrap piece of tool box drawer liner, used a leather punch to make the holes for it to slide over the standoffs, cut a slit in it, then affixed it to the front panel with E6000. In case of future surgery or potential initial troubleshooting with everything hardwired, I wired it with everything coming up from the bottom to the controls so the front panel can be unbolted and rotated down flat on the table.
Thank you Heikki for the wonderful and unique project! I'm looking forward to building others that are on here!
With a desire for finer gain adjustments, but not the mastering version of Heikki's 23-step gain, I recently recalculated the 12-step coarse gain switch for 23 steps, and designed a PCB for an Elma 04 switch. It's the same overall range as the 12-step, just with an extra step between each of those. Position 13 is "0", so it would be in the same spot as position 7 of the course gain and match my front panel labeling. I also did some tweaking of the values along the way to get the targets even closer. It's great! I'll be posting my schematics and Gerbers for both types of switches in the Gerbers section in here, if anyone would like to use them.
