12AU7 tube preamplifier has been slightly modified so that I could use parts I had on hand. Here is the schematic of the 12AU7 Tube Preamp I came up with (sorry about the over sized schematic). I used 12AU7 tubes and a B+ voltage of about 150V. Even at 150V I could not get the 12AX7 tubes to bias properly.

The power supply is also very simple. 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. For the filament supply I used a simple 7812 regulator circuit to reduce noise. 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. This plug works quite well and is fairly sturdy.
Construction - 12AU7 Tube Preamplifier

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. So the board is proto-board that was reused from a power supply I built, but was never used. I do personally like point-to-point connections, but for something this small a PCB may be a better choice.

12AU7 Tube Preamp on Protoboard
Photograph 1: 12AU7 Tube Preamp on Proto-board

12AU7 Tube Preamp Power Supply
Photograph 2: 12AU7 Tube Preamp Power Supply

I was pleased with how well it turned out, at 30 bucks it looks pretty professional with the solid gold plated jacks on the black case. A neat feature I added later on is the ability to adjust the brightness of the front LED. The yellow LED is to indicate that the tube filaments are active.

DIY 12AU7 Tube Preamplifier
Photograph 3: Finished 12AU7 Tube Preamplifier
Sound - 12AU7 Tube Preamplifier

I now listen to many different recording types mostly rock, and a bit of classical and blues in between, what ever is on my iPod. Oh, yes it sure does sounds good on my transistor amp - it really softens up the cymbols in the recordings and pulls out the true detail. When listening to Peter Gabriel (as I was when writing this) you can tell there is high detail and depth in some of his recordings. Personally I find the preamp makes the transistor amp sound more natural, with out over contrasting it like most solid state preamps do. The most evident thing I found was the added softness of the bass, much more warm sounding. Vocals in some recordings really stand out as though they were in the room with you, I'm not one to brag, but this cheap little preamp sounds about as good as some of my uncle's tube amps, it's just truly amazing for something so simple!

If you have any questions or comments about this project, please feel free to ask them in the Mini 12AU7 Tube Preamp Support thread on the forum.

12AU7 Tube Preamplifier with Power Supply
Photograph 4: Finished 12AU7 Tube Preamplifier with Power Supply