DIY single 12AX7 1/4W tube amp

 Here's a new tube amp project I recently finished! It's a 0.25W tube amp that uses a single 12AX7 dual triode tube. The 1st half of the tube is the preamp section and the 2nd half is the power amp triode section. It can only produce about 94dB of sound pressure level(spl) when connected to a 100db/1W sensitive 12", 2X12" or 4X12" guitar cab. That is as loud as human voice in a room.
 This tiny amp is perfect for bedroom practice using a distortion pedal in front of it. I used a boss Mt-2 metal pedal in front of it and it really rocked! Clean headroom is almost zero...
 Be careful, this is a High Voltage project and uses up to 400Volts d.c.! It's not a beginner project. Always unplug the amp and wait for it to cool down before making any modifications to the circuit. Never touch the Powe Supply Capacitors , it's lethal! Always check the voltages using a multimeter. The wires used must be insulated.
 Here's the schematic circuit of the amp, enjoy!

(Click to enlarge)


12AX7 specs:
http://www.audiomatica.com/tubes/12ax7.htm


12AX7 Pins:

Here is something very important:
*** SAFETY WARNING ***
READ THIS FIRST!!!!!

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Working inside a tube amplifier can be dangerous if you don't know the basic safety practices for this kind of work. If you aren't prepared to take the time to learn and apply the right precautions to keep yourself safe, don't work on your own amp. You can seriously injure yourself or get yourself killed. This section is not intended to be a complete guide to safety in tube equipment, just to hit the high points as refresher for those of you who have some experience. The best way to learn the requirements and practices for safety in tube equipment is to find someone who will teach you one on one.

BASIC REQUIREMENTS

• UNPLUG IT FIRST Pretty self explanatory. Do not, ever, ever, leave the equipment plugged in and start work on it. Leaving it plugged in guarantees that you will have hazardous voltages inside the chassis where you are about to work. This is like setting a trap for yourself.
• LET IT DRAIN If the amp has been turned on recently, the caps will still have some high voltage left in them after the switch is turned off. Let it sit for five minutes after you turn it off.
• SUCK IT DRY When you open up an amp, you need to find a way to drain off any residual high voltage. A handy way to do this is to connect a shorting jumper between the plate of a preamp tube and ground. This jumper will drain any high voltage to ground through the 50k to 100K plate resistor on the tube. To do this successfully, you will need to know which pins are the plate pins. Look it up for the amp you're going to be working on. You'll need to know this for the work anyway. Leave the jumper in place while you do your work ( high voltage electrolytic caps can "regrow" voltage like a battery sometimes. Really. ) Remember to remove it when you finish your work.
• TEST IT Take your multimeter and ground the (-) lead. Probe the high voltage caps and be sure the voltage across them is down, preferably to less than 10V.
• BUTTON IT BACK UP FIRST Take the shorting jumper out. Put the chassis back in the cabinet, making sure all of your tools, stray bits of solder, wire, etc. are out of it. You don't have to actually put all the screws and so forth back in if you believe more work might be needed, but make sure that the chassis is sitting stably in the cabinet and won't fall out. At the end of a listening test, either continue buttoning up if you're done, or go back to UNPLUG IT FIRST.

            Source: http://www.geofex.com/tubeampfaq/tube_amp.htm 

UPDATE! Here are a few modifications I 've made to the original circuit for better performance. There are some new ideas and tips on the schematic as well!

New Update!


MK2 amp: