If you haven't seen the VHT Special 6 Ultra amp you're missing out on a very inexpensive tube guitar amp with a ton of features usually found on amps with a much bigger price tag. The manufacturer probably saves a lot by having them handwired in China, but hey, It's handwired! That makes the amp relatively easy to modify, and what ultimately made this instructable possible.
After I got mine (I got the head, but the combo has the same chassis) I noticed a few things that I thought would make it better. This list includes...
1 Channel switching. Out of the box you have to choose between the clean/ultra channels by plugging into the corrosponding input jack
2 FX loop/Line out level control. These output's levels are dependant upon the volume settings of the preamp stages causing distortion on some outboard equipment at high gain levels.
3 More positions on the 3-position texture switch. I'd like to fine tune the high end response a lettle better.
In the future I may build some outboard gear to handle #s 2 and 3; basically an attenuator/tonestack that can be plugged into the FX loop.
That just leaves #1... My goals during this project were simple. Achieve footswitchable channel switching with a reversable mod that required no drilling or other permanent change to the chassis. Achieve the same via a toggle switch positioned in the hole where the clean jack normally is. Maintain a completely silicon free signal path (no transistors!) And, maintain boost footswitch function with either the new 2-button switch or the stock 1 button switch. I am happy to report that all of these goals were met...
ok, still here? The description of the functions are like this: The toggle switch where the clean jack was is a 3 position on-off-on type. This allows building a circuit that can still use the stock footswitch. The up and middle positions are manual switching between ultra and clean and allow the stock footswitch to operate the boost function. The down position allows the new 2-button footswitch to operate both boost and channel switching! Slick!
(*note* with the toggle in the down position the stock 1-button footswitch will cause the ultra channel to be active all of the time... you can make a small change to have the 1-button switch operate the channel switching and the boost will be active all the time if you wish. There is a note about this on the modified schematic pic, but the yellow boxes are hard to see)
So, you want to tackle this mod? Read on...