Davide Gironi wrote a post on his blog showing how he modified the Behringer v-tone GMX212 Hybrid tube amplifier:
For this project I’ve used 12AX7 vacuum tube. This tube is one of the most used for the guitar preamplification. So there are many resources available on web.
The first problem i faced it’s the power stage. Tubes needs high voltage in order to work properly. The solution I’ve found was to connect a 230V/9V power transformer with secondary and primary reversed. That way, connecting it to the 10.5V AC output of the main Behringer transformer i can get almost 160V DC.
Any tube also needs a low voltage current to power up the heater. I choose to use a 7805 voltage regulator, shifted by 1.3 volts using two switching diodes. This gives me the 6.3V DC I need.
More details at Davide Gironi’s blog.
Check out the video after the break.