Diy onboard guitar preamp8/30/2023 So the board is proto-board that was reused from a power supply I built, but was never used. I really could have made a PCB, but the materials required cost money and take time, besides I wanted to see how "cheap" I could really be. This plug works quite well and is fairly sturdy. Because the preamp is very low power, I was able to use a 5 pin key board plug, not the PS2 type but the old bulky type. For the filament supply I used a simple 7812 regulator circuit to reduce noise. All you need is couple of 12 volt transformers (which you can put back to back to isolate 120V), a bridge rectifier, resistors and some caps. Even at 150V I could not get the 12AX7 tubes to bias properly. I used 12AU7 tubes and a B+ voltage of about 150V. Here is the schematic of the 12AU7 Tube Preamp I came up with (sorry about the over sized schematic). 12AU7 tube preamplifier has been slightly modified so that I could use parts I had on hand.
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