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.
Step 2: Parts
Oh, and dont forget some hookup wire for building the footswitch internals and relay connections. And, some 3-conductor cable for the footswitch---> amp cable (if you happen to have a stereo patch cable, that will work.) Some zip ties and something to mount the relay board will also be needed. I used some sticky backed fastners for use with zip ties. I'm not sure what to call them, but you can see them in a later pic.
The first pic shows a big toggle switch that matches the power/standby switches on the amp, but it won't fit in the hole left behind by the clean input jack. That's why I changed to the smaller switch shown in the 2nd pic.
I also had a 14-pin IC socket that I wasn't using that I decided to use for the relay. That way if the relay fails the replacement is a no-solder plug in.
Step 3: Building the Relay Board
I had some long header pins salvaged from some old circuit board or other that worked well to get the contacts out and away from the relay pins for easy soldering to the wires in the amp.
Step 4: Opening the Amp
You know that amps, especially tube amps, have hazardous voltages inside right? Look at TP13 on the schematic... 350 VDC! ouch! And those capactiors in there like C39 and C45 and all their neighbors will store that dangerous voltage until they can discharge through some load. Shorting them out will make fireworks and can damage the capacitors connections and could even hurt you. We can discharge them less violently by making a complete circuit through a resistor rather than a short. The standby switch allows us to do just that...
So, unplug your amp, switch the low/standby/high switch to high, and go have a cup of coffee. Come back when you're done
Now that we're caffinated, it's time to get AMPED!
Start by removing the 9 screws on the back of the amp. This will give access to the tubes which will have to come out before we go any further.
The 2 preamp tubes have protector sleeves on them which come off with an easy 1/4 turn counter-clockwise. Then gently pull the tubes straight out. The power tube is a little harder because it has a retainer spring that you'll have to push down slightly to get it to release the tube.
once the tubes are safely stored, continue by removing 2 of the handle screws (the 2 closer to the back of the amp) and the 2 screws on the sides. You'll notice the chassis getting loose in the cabinet at some point, be careful to keep it oriented so that it doesn't fall.
Once you have the chassis out, have a look around in there. Grab your multimeter (you DO have a meter, right?) and find the test points from the schematic with high dc voltages and measure them referenced to ground. No voltage? Good.
Step 5: Removing the Parts We Don't Need Anymore
You will have to unsolder the black and white wires that go to the clean preamp stage and move them out of the way. Then unsolder the ground wires from the jack. One is long and comes from under the amp's circuit board, and the other two are short and is just jumpers to ground the volume and ultra knobs. One more wire that has a resistor and is connected to the middle pin of the ultra knob is all that's left now. unsolder it and you should be able to remove the clean input jack by loostening the hex nut on the outside of the chassis.
When you get the jack out, you'll have to unsolder the 1M resistor that is still on it because you'll need it later.
Next unsolder the red and black wires on the footswitch jack. This leaves the jack held by 2 solder joints on a single solid ground wire. This part is a little tricky. You have to loosten the hex nut to free the jack, and then alternate heating the 2 solder joints with your iron quickly to get them both melted. Then the jack slides easily off the wire. DO NOT CUT THIS WIRE! We need it when we install the new footswitch jack. If you got over-ambitious and cut that wire you're still ok. You can use a piece of hookup wire to restore this ground connection.
Step 6: Installing New Parts (part 1 Toggle Switch and FS Jack)
Now that the footswitch jack in installed, it's time to resolder the boost switch wires. Do you remember, at the beginning, when i said that the schematic saved my butt? Well, this is when it happened. I didnt pay attention to where the red/black wires went on this jack. I soldered them back on backwards. I found the problem when i was doublechecking with my meter and I had lost ground to all the rear panel jacks! I figured out later that the ground was only missing when the boost knob was pulled, then a quick look at the schematic and i found my mistake. DOUBLE CHECK EVERYTHING! Better to find it now than later with power on... So, do what makes sense and solder the black one to ground.
Time to start wiring the toggle switch. Ground is going to go to whichever side of the switch is closed when it's in the position you want for ultra selected by this switch. Often ground is on the middle pin on these switches, but that's not the case here, so pay attention to the schematic. You can take the long ground wire removed from the input jack coming from the amp's circuit board and solder it here. Then solder the short ground jumpers that go to the volume knob and the ultra knob to the same place completing the ground connections to the toggle switch.
You can take this time to connect the opposite side of the toggle switch, not the middle pin yet, to the ring contact (middle) on the footswitch jack. This circuit will allow the new footswitch to operate channel switching when the toggle is in this position.
Time to get the relay board mounted. In the 2nd pic here, you can see the sticky back tie down points I used to mount the relay board. Just another way to keep from having to drill new holes in the chassis.
Step 7: Installing New Parts (part 2 Relay Board)
Note the sticky back tie points in the first pic. This is how i chose to mount the board, but you can improvise if you remember to keep the solder joints on the back side of the board from touching the chassis. The stickies hold the board up enough to achieve this, but I covered the joints with hot glue for extra safety here. If the relay board is in contact with the chassis you'll probably blow fuse F2 due to shorting the 6.2 VDC to ground and then nothing will work.
Start by soldering the connections to the relay coil through the 33 ohm resistor on the relay board. The negative side of the relay coil connects to the middle pin on the new toggle switch we hooked up in the previous step. You can see that I installed a piece of heat shrink tubing on each of the relay board connections to reduce the possibility of shorts. The positive side of the relay coil, through the resistor, connects to a 6.2 VDC point on the amps circuit board to get power for the relay. I spent some time choosing a place to tie into the existing circuitry to get this 6.2 volts, and this is when I found the "error" on the manufacturer's schematic. I chose the positive side of R52 (the LED resistor for the power lamp) to make the connection. It was labled R54 on my amp's circuit board though. Just look for the twisted red/white pair of wires from the LED. The red one connects to the resistor we're looking for and the opposite side of that resistor is the point where we'll get the 6.2 volts from. Solder a piece of hookup wire from that point to the resistor on your relay board to power the relay coil.
Now we just need the relay contacts to do the channel switching! Almost done!
The schematic will be very helpful here. Your relay should have 2 sets of contacts. A (normally open)(common)(normally closed) and a seperate (normally open)(common)(normally closed.) I started with one side and connected the 3 points using the schematic and then moved to the other set to keep from getting confused.
Remember to check your relay's datasheet for connection information.
First, I connected the 5.6k resistor connected to the middle pin of the ultra knob to one of the normally open contacts on the relay.
Next, get that 1M resistor left over form removing the clean input jack and connect it from the common on the same set of relay contacts to ground. I went to the ground point on the ultra input jack. Then, find the black/white pair of wires that were on the clean input and connect the black one to ground and the white one to the 1M resistor on the relay side. This reconnects the clean preamp input.
The remaining normally closed contact on this side of the relay will jumper over to the normally open contact on the opposite side of the relay, and then from that point to the tip terminal on the ultra input jack. Here you'll have to remove the white wire and the other 1M resistor. They will no longer connect here, but we'll need them somewhere else shortly. Leave the black wire (the one paired with the white one) where it is. It can stay there.
Now connect the normally closed contact on this side of the relay to ground. By now, you should be very familliar with where the ground connections are, so choose a convienent point nearby.
One relay connection left!
The common on the 2nd set of contacts. A little while ago, we had to remove the white wire and the 1M resistor from the ultra input jack to connect a relay contact there. Now, connect this common contact on the relay to the 1M resistor and then the resistor to ground (it should still be on the ground/sleeve connection on the ultra input jack.) Then Connect the white wire, which should be the only connection not made at this point, to the side of the 1M resistor that goes to the relay.
Wiring inside the amp is complete, so it's time to put it all back together!
Step 8: Putting It Back Together
once the chassis is back in the cabinet, but the back panel is still off, it's time to reinstall the tubes. The tubes can only be inserted one way, so pay attention to line up the pins correctly and not bend any of them.
Put the back panel back and set the amp aside so we can work on the footswitch.
this might be a good time for another coffee break. Or, you could fire up the amp and test that the new toggle switch switches channels correctly. Even without the footswitch, it's really nice to switch from clean to ultra by just flipping a switch!
I added some new pics of the amp internals after the mod was completed because my original pics were poor quality. These are a little better. Focusing was still difficult, but at least some of the connections that were previously unclear are able to be seen.
Step 9: Making the Footswitch
After the amp internals, this will be really easy.
First, you'll need a box to put the switches and jack into. Pretty much any project box from radio shack or any other sturdy box you may have will work as long as it's big enough so the backs of the switches and the jack won't touch each other.
Drill the holes where you want the switches to be, and drill the hole for the jack. You can see in the pics how I did mine, but you can do whatever you want here.
The wiring here is simple. The contacts from one switch go to the sleeve and ring terminals on the jack, and the contacts on the other switch go to the sleeve and tip terminals on the jack. That's it!
If you're making the cable to go between the footswitch and the amp you'll need some 3-conductor cable. Solder the plugs on each end the same so that tip is to tip, ring is to ring, and sleeve is to sleeve.
Now go plug everything up and make sure it all works!
Step 10: Conclusions
I also thought that the amp might get more noisy at higher gains. It hums some unmodded, so I was afraid that it would get even louder! This also didn't happen with mine.
You have to set your volumes carefully because the 2 channels are not independant of one another. But, you knew that right? This is more noticable now that I dont have to stop playing and unplug the guitar to switch channels. With a little tweaking I've found that a nice balance can be found between a nice clean sound and a dirty crunch with little difference in volume. With the boost switching still active, it's possible to dial in 4 different gain levels that can be easily switched!
I've found that the boost function is a little too dramatic for my tastes. If you find this to be true for you also, you can make a simple change to fine tune how much boost you want. R12 on the schematic is a 68k resistor that facilitates the boost function. If you reduce it's value to perhaps 47k? the change the boost makes will be less dramatic. That will reduce the overall gain a little though. Another option would be to add a resistor in series with R12 AND reduce the value of R12. Something like R12=47k and add a series resistor=22k. I haven't thought much about doing this yet, so maybe another instructable in the future...
I hope you enjoyed this instructable. If you choose to perform this mod I hope you'll be carefull around hazardous voltages and don't try to do anything you're not sure about. If you have questions I can try to help, or any amp technician worth his salt should be able to understand the changes I made to the schematic.