Orange Micro Terror modification!

This is the Orange Micro terror amplifier head. Would you like it to turn it into a "Dark Terror"? Continue reading!
www.thomann.de/gb/orange_micro_terror.htm?partner_id=73786
The MT-20 a tiny hybrid (tube and solid-state) amp with a 12ax7 preamp tube and a 20W TDA-series IC power amp.  An external power supply rated at 15V/2A (30VA) is included as well. The amp is so tiny and there's only one channel with gain, tone and volume controls. That's it!... There's also the new Micro Dark Terror model out which packs more gain and distortion. I 've tested out the amp in its stock form and it sounded cute through a 2X12" Celestion cab. A large cabinet is what this little amp should be paired with. I know it sounds like an oxymoron but you won't believe your ears (and eyes) when you plug this little box to a speaker cab that's got at least one 12" guitar speaker in it. This amp can produce some serious low end which is unfortunately not controlled by the amp's tone pot (it's not a "contour" control as used is Marshalls, just a simple treble cut/boost pot).
The tone is really good right out of the box even with the stock cheap 12ax7 tube. This tube acts more like a pair of clipping diodes rather than an all-tube preamp. That's the case with most solid state/hybrid amps that use only one 12ax7 to "warm-up" the tone.
The tube certainly has a purpose here, it's not just a tube with an Led stuck under it with no ginal passing through it (that's what a few dodgy cheap HiFi manufacturers do by the way...check this article). So, replacing the stock tube will alter the tone of the amp and you can experiment with different tubes, even the 7025, the 12at7, 12au7 , 12ay7 ones or their ECC-series equivalents. It's like replacing clipping diodes with leds, mosfets and so on. Some will sound better/louded/clearer, others will not. It's a matter of taste really. I for instance am a big fan of higher gain distortion/overdrive so I kept the high gain Chinese tube on the micro terror.
The amp can produce lots of gain with a few mods, just like its darker cousin does. These amps are used by rock, metal and doom guitar players a lot so it would be nice to have a bit more gain that you can actually control.

Modification

The first thing you have to do is make sure the power supply is unplugged and undo the 8 screws that hold the white metal grill in place. Don't remove the metal handle. Then, you gain access to the 12ax7 tube, Remove the tube by gently pulling it up. Wiggle it a tiny bit if needed.




This is where things get really tricky... There is a sheet of metal on top of the amp's electronics that needs to be removed and then you will see the PCB. The PCB is really hard to remove from the chassis. There are a few screws and cable connectors holding the PCB down as well as the control pots, jacks and their washers. Even if you undo all of these, you still have to bend the chassis a little to free the control pots. It's tricky and you have to be extra careful not to damage the controls, their shafts, or the PCB itself. The Orange LED can also get in the way...



I finally removed the chassis-mount LED as I wanted to replace it with a bright blue one. The LED metal casing is held in place with a washer. The actual LED bulb of this casing is glued in so I had to drill it out and destroy the led before placing the blue replacement one in the metal casing. I could simply buy a chassis/panel-mount LED(see pic below) but, where's the fun in that?...

Once I removed the PCB, I experimented with components and values a little bit before deciding what to do. I examined only the preamp section of the amp. At first I altered a few components related to the 12ax7 preamp stages but the sound was getting worse and the volume was dropping. Therefore, I decided to modify only the opamp sections of the preamp.

The tube is properly biased for this application and you don't really want to mess with this section unless you know what you're doing and what you want to achieve. The signal that the 12ax7 receives from the opamps can be easily modified though.

MODS!
Here's the list of mods you can do. You can do one at a time, step by step and hear the modified result. If you don't like it, just take a step back.

-At first, I replaced the R1-marked resistor with a short-circuit (a simple jumper). This mod increased the gain and treble a little bit.
-Then I replaced the R14 resistor with a 47kOhms (0.25Watt = 1/4 W) one. This also increased gain a bit more.

The next mods increased gain even more and gave the amp a marshall-like tone:
-The C10 capacitor was replaced with a 47nF (nanoFarads) one. You can use a film capacitor or even a cheap ceramic one. The film type sounds a bit better. This mod removed some of the low end muddiness/unclarity. If you prefer the doom-metal sound of a highly-overdriven orange, you can leave this capacitor stock. However, I wanted a marshall-like and a bit clearer tone.

Then, I increased the gain more and more by performing the following mods:
-Replaced the R4 with a jumper (short circuit)
-Replaced the R7 with a 1kOhm resistor (1/4W)
-Replaced the R12 with a 2.2kOhm resistor (1/4W)

Each resistor replacement increases the gain and distortion of the amp. It sounds like a beast now! Perfect for high gain riffs and solos! No need to purchase the "dark" version of it!...



However, if you want to maintain most of the clean tone and headroom of this little amp, DON'T DO ALL of the resistor mods. Be sure to do these mods step by step, up to the point you feel there is enough gain and distortion for your taste. You can also skip a step if you want to experiment more. Just make sure you note down each step and keep the stock component, in case you want to undo a modification step. Always use a quality soldering iron with a fine tip, high quality solder and apply soldering flux to the points you want to solder/desolder. A solder pump and some solder wick will make desoldering way easier.

Finally, I placed a blue led under the tube itself for an optical effect. This led runs in parallel to the blue led I placed at the front of the amp.



UPDATE! Hey! Here's a quick video demo of the modified MICRO TERROR amp! It shows that the amp has much more gain (suitable for dropped tunings and soloing). The clean headroom is way less though, which is normal due to the single channel topology of the amp. Check it out: https://youtu.be/5nZR-Njt7qo

Cheers,
Thanos

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