Hello Again!
I'm back with a new Instructable! Wheeyy!
In this 'able, I'll show you how to make a Low-Voltage (Around 60 Volts) Tube Pre Amplifier for your Guitar!

Youll need, lets say Basic Electronics Knowledge for this.

Step 1: Parts Youll Need

I've listed all parts youll need here.
One thing I have to say before I write further.
I cant' tell you the exact sizes of the holes etc you need to drill because it depends on which parts you use and what kind of housing you choose.

So, Youll need :
A Case.
1 12AX7 or ECC83 Tube
1 9 Pin Tube Socket + Screws
1 200V 470uF Electrolytic Capacitor
1 1M Ohm Logaritmic Pot
1 100k Ohm Logaritmic Pot
2 Switches, 1 DPDT and 1 SPDT
2 35 V 2.2uF Electrolytic Capacitors
2 6.3mm Jacks
1 400V 0.68uF Polyester Capacitor
1 100V 1uF Poly Capacitor
Lots of wire.

Resistors :
2 4.7k Ohm 1/4 Watt or Higher
1 68k Ohm 1/8 Watt or Higher
2 100k Ohm 1/2 Watt or Higher
3 10k Ohm 1/4 Watt or Higher
1 1.5k Ohm 1/4 Watt or Higher
1 100k Ohm 1/4 Watt or Higher

Tools :
Soldering Iron
Solder
Wirecutters + Strippers
Needle Nose Pliers could be helpfull
Drill to drill holes in your Case

Step 2: Prepare Case

At this step, I couldnt help you much.
You need holes for :
Tube Socket
2 Pots
2 6.3mm Jacks
2 Switches
and for some Wiring that goes outside the case.

Step 3: Insert Sockets Etc in the Case

I think its easier if you put in the Parts first and then solder everything up.

Step 4: Prepare to Solder!

Bend the pins of the Socket a bit apart. That makes soldering a bit easier.
Then, put solder on every pin.

Step 5: Schematics

I'm sorry for the Hand-drawn Schematics, but I couldnt find any kind of software which could draw Tubes. If you dont' understand something, post a Comment. I'll try to Answer ASAP.

Oh. One thing. I forgot to add the Bypass switch ( the DPDT type).
I will explain that in the next step.

UPDATE:
Check out the last Page for the new, better Schematics.

Step 6: Beginn the Work

Grab the following parts :
2 100k Ohm 1 Watt resistors
1 68k Ohm Resistor

And Join together like shown on the Pictures.

Step 7: Adding Next Parts

Grab the following parts (According to the new schematics):
Jumper Wires
400v 1uF Cap
220k Resistor

Step 8: Cathode Resistors and Bypass Caps

... and the Coupling Capacitor between the two halfs of the 12AX7. And other connections...?
Get these Parts:
2 4.7k Ohm Resistors
1 35v 2.2uF Cap
1 63v 0.47µ Cap

400v 10nF Cap
100k Resistor

Step 9: Ground Those Things

Get some Jumpers out. And some Wire.

Step 10: Power Anybody?

You've got it nearly finished! Time to add... High.. er LOW Voltage!
Youll need a 60 Volt PSU for it.
And something around 6.3 Volts for the Filaments.

Step 11: Add the Tube and Some Knobs.

Well. Because this whole circuit wont Work very well without the Tube, Plug it in! Dont force it. it MIGHT take a while until you get it in.

Then, trimm the Pots and put some Fancy Knobs on them.

Step 12: Test It Out!

Now, youll have to apply power to it. Ive done that by Building some regulated switchmode Power supplies.
One Step-up Converter for 60 Volts and one Step-Down Converter fpr 6.3 Volts. Ive used the MC34063 For that. Not the perfect choice but it works.
I have applied 80 Volts to it, but that doesnt sound quite good. A little too thrashy. At 60 volts, the Gain is much better controllable and the distortion is a bit more harmonic.

Hope You Liked' it and Please Comment!

Step 13: Step Up/Down Converter

If you are interrested in the SMPS,I got the Schematics for you.
The Step Up converter is a bit more complex, but it aint as mind-breaking as it might look.

I uploaded the eagle-files for those who have got eagle, but youll also need the MC34063 Library file, which you can get at the Homepage.

http://www.cadsoft.de/cgi-bin/download.pl?page=/home/cadsoft/html_public/download.htm.de&dir=eagle/userfiles/libraries&sort=name
Scroll down to M, it should be there.

SMPS.rar
Download

Step 14: UPDATE 24.10.'11

Just made some tweaks at this Preamp and updated the Schematics.
Now you got beautiful Computer-drawn schematics (;

Notice:
C2 is only if you want a brighter tone.
R8 should be choosen according to your Amp.
If it has a high gain, use a higher value to prevent unwanted Clipping.