VHT special 6 distortion mod

Here is a very simple modification one can apply to his/her VHT special 6 tube amplifier. The modern VHT amps are high quality overseas-made amps that use hand wired soldering and quality components. Replacing the factory tubes would be good though.
 This "Distortion channel" modification is based on a small circuit alteration.You will only need a pair of diodes, a switch and a capacitor. The purpose of the circuit is to add a second, distorted channel that will provide the amp with an Overdrive effect at low volumes. There are other mods for this special 6 amp but this is probably one of the easiest. Be very careful when working on tube amps though! Make sure the amp is unplugged for  at least  two hours before opening it and do NOT touch any capacitor leads. The high voltage capacitors store around 400V which is pretty dangerous.

 This "diode clipping" mod is based on schematics that Marshall, Mesa Boogie and many more tube amp makers use(see Marshall Jcm-900 for example). Using only one 12ax7 on its preamp circuit, the special 6 cannot produce loads of gain or distortion, which is why we are using clipping diodes for this modification.
 The diodes used could be almost anything but the most common diodes are 1n4001 type and LEDs. Different diodes produce different distortion sounds. All diodes decrease the overall output volume of the amp when the dist. effect in "On".
 Leds (red & green) sound smoother (overdrive style) and have more clean headroom. 1n4001 diodes sound brighter(distortion style) , have more clipping distortion and less clean headroom. Schottky type diodes sound more distorted("metal") and have the lowest headroom.
 Here are some interesting issues on diode clipping:


http://www.generalguitargadgets.com/richardo/distortion/index.html

http://www.diystompboxes.com/pedals/diodes.html

http://www.muzique.com/lab/warp.htm

http://www.muzique.com/lab/zenmos.htm

http://www.google.com/url?sa=t&source=web&cd=5&ved=0CDwQFjAE&url=http%3A%2F%2Fwww.beavisaudio.com%2Fbboard%2Fprojects%2Fbbp_DiodeMods.pdf&ei=cqBnTeGMIYiXOqqX5cML&usg=AFQjCNFlkxQgl7bFfxLFjL6fdWoQXFDvxw&sig2=2BFZpKV0TJo3L3OP6nqoGQ

http://www.diystompboxes.com/smfforum/index.php?topic=62203.0%3Bprev_next=next

The original schematic of the VHT special 6 can be found here and it's very useful:

http://www.vhtamp.com/pdf/VHT_Special_6_Schematic_5-17-10.pdf


The schematic for the distortion mod:

the diode clipping circuit is connected in parallel with the R18 resistor

Alternative mod:
for more volume and less distortion, the 2 diodes of the clipping circuit could be replaced with 4 leds or diodes as pictured below:
http://dreamfield.dyndns.org/_eric/_el/diode_clipping_01.gif


Also, to create a more tube-like, "asymmetrical clipping" distortion effect, one could replace only one diode of the circuit with a pair of diodes as pictured below:
http://www.generalguitargadgets.com/richardo/distortion/asym.gif 


Note: BEWARE, HIGH VOLTAGE!
There are very high voltages inside a tube amp(100-500V) and can KILL a person. You should not try this mod as your first project. It's too dangerous. Always let the amp discharge (unplugged for 2 days), check the voltages of the power capacitors with a multimeter, use plastic gloves if possible. To discharge the power capacitors use a voltage meter or do the following: Build a "discharge" wire with a set o "crocodile" clips, a 2.2kOhm / 5~10W resistor and some insulated wire. Then you can discharge the capacitors by touching the clips on their positive and negative leads.Don't touch the wire during discharge. Beware of the arc that may be produced( and wear plastic gloves).