Hello Dave, thanks for a lot of information and ideas!!! I was also able to (almost) complete my pair of 627 EQs. A bit about my setup: I used Igor's UTM Transformers (fabulous!) for the build. The frontpanel is from Franks frontpanels.de with UV printing on anodised aluminum - with a touch of "crackle coating" imitation. The tubes (most are matched) and the power supply are from BTB Electronic Germany, the power supply is unregulated - heavily based on your power supply schematic - thx man. I used 600 ohm T-Pad pads for input and output, the triple turn/gang frequency pots are 24 step switches - I have decided this is the easiest way for me to clone the originals this way. But it is certainly not "fully variable". As for the audio path I took Colins and also the original schematic to build the circuit. Unusually I built it on circuit boards to hold the tube sockets. These prevented me from making mistakes when building stage by stage without losing control of this really complex build. And yes, it worked pretty well for me. I grabbed two Soviet 6E5s Magic Eye Tubes. But the seller cheered me on an old worn tube, light power is nearly gone, the other works as new
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Everything works great, I had no real issues with the setup. One little thing was with the original C27B cap which caused a 13Hz hum. I replaced with a higher one, thats it. No hum at all, very low noise. As Dave described the headroom is limited. My frequency response which I checked "quickly" with the audio card looks good linear in a range of 2.5 dB range from 20-20K, all filter curves are looking good and working as expected. I use two Hammond 156c on the Output TX to get a nice Lowend. With the extra 130 ohms across R18A and R31A to ground I can get a maximum gain of around 13-16dB depending on the frequency range of the Hi and Low Band. Booth units are very matched with all curves and the frequency response are also very close.
It sounds absolutely fantastic, super class. Really different than a passive Pultec. Not as sweet. It rocks, broad, 3D, sexy, it has character.
Btw I would suggest this build to real masochists only. Not a lot money to save compared to Colins, really lots of time consumption, lots of time for studying the schematics, searching for components, building, improving, setup..... I think Colin has really a good Kit on the start. My opinion.