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...
Step 1: The Schematics
Obviously, the clean input jack goes away. We won't be needing it anymore and it makes a perfect empty hole in the chassis to put our toggle switch.
The footswitch jack needs to be replaced with a 3-conductor stereo type jack otherwise known as a TRS jack. We need the extra conductor to allow the extra switch circuit to be added. Remember that whatever switch is wired to the ring of this connector will be active all the time if using a mono- 2-conductor plug like the one on the stock footswitch.
We also need to add a relay to do the switching for us. A total of 4 contacts must change to make the switch, and some are signal paths. Using a DPDT footswitch could do the job, but I didn't think bringing the guitar signal out of the amp to get switched and then back in was a good idea, so the internal relay can do it and all we need outside the amp is one contact for relay control.
When switching from ultra to clean, this is what happens: The relay removes the guitar signal from the ultra preamp input and applies ground in it's place. At the same time, it removes the ultra preamp output from the clean preamp input and applies the guitar signal in it's place. And, of course, switching back is just the opposite. if you notice where the relay coil voltage is comming from, you'll see that we need a 6 VDC DPDT relay... the 33 ohm resistor is there to reduce the relay coil voltage to 5.5 VDC because I couldn't feel complete knowing I was putting 6.2 volts on a 6 volt coil every time I switched to ULTRA!
If you can't see on the schematic how this works, then stop, and find an amp tech to help you! These schematics saved my butt while doing this mod... and I designed it! You'll have to be able to read them.
ALSO! This might be very important to you... During my installation I was looking for a good place to connect the positive side of the relay coil and found that R52 on the schematic is actually R54 in my amp! So, be careful. I don't know if it's just an error, or if there are different versions of the amp out there so I left the "error" unchanged on these prints.