Friday, April 1, 2016

Behringer v-tone GMX212 Hybrid tube amplifier mod


Some time ago I've repaired a Behringer v-tone GMX212 http://davidegironi.blogspot.it/2012/01/behringer-v-tone-gmx212-repair-bypass.html
That was the first post of this blog.

Now I would like to mod this amplifier, switching is original preamplifier board with a custom one.
This mods dives me in the vacuum tube world.

WARNING! - The project described in these pages utilizes POTENTIALLY FATAL HIGH VOLTAGES. Do not attempt to build circuits presented on this site if you do not have the required experience and skills to work with such voltages. I assume no responsibility whatsoever for any damage caused by the usage of my circuits.


To improve my knowledge on tube amplifiers, I've read articles from two really well written website: http://www.valvewizard.co.uk/ and from http://www.ax84.com/.

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.


The preamplifier circuit it's next to the AX84 P1 Guitar Amplifier, the tube load and ground resistor where changed to fit the load line of my tube for my power voltage. You have to calculate your own values it you use other voltage.
As a tone circuit, I've selected something similar to the Fender Tweed Princeton one.


The Behringer v-tone GMX212 has many output and input on the back that i do not want to lose.
To keep the back ports and to reach the amplifier board with correct voltage, I've built a connection boards that levels the preamplifier input like the original Beheringer preamplifier board does.
I've just measured levels coming out from the original board, and reproduced my version of that circuit using two TL072 opamps to adapt my preamplifier levels.


Eventually I would like to add some sort of reverb effect to this amplifier. So I've try the PT2399. What I've built it isn't really a reverb, it's more an Echo/Delay. But it's an interesting ambient effect i would like to have on this amplifier.
The reverb board use on TL072 to mix the wet and dry signal without any lose.


The amplifier stage is the original Behringer v-tone GMX212 one.

The amplifier plate log was replaced by a new one, engraved with my homemade laser engraving machine: http://davidegironi.blogspot.it/2015/12/a-diy-a4-laser-engraver-made-from.html


That's all for this amplifier... until the next modification.

12AX7 guitar preamplifier and power supply board:


Schematics of the PT2399 delay/reverb board:


Schematics of the mix level board:



Notes
  • read risk disclaimer
  • excuse my bad english