Soho QTR-1 footswitch build.

[Erik]

Hi All,
It's been a while since I was last here. I hope this finds someone that can lend a hand. I picked up a Soho QTR-1 preamp, but alas no footswitch. After doing a bit of research I have found a schematic for the Kittyhawk Quattro, supposedly identical item. Unfortunately, after opening up the Soho and poking around, they are not exactly identical but pretty close. I would like to build a footswitch for this, but can't wrap my head around how it actually works. The preamp has four channels, clean, crunch, lead, and hell. The hell channel is the crunch and lead channels combined. It looks like the footswitch has two switches and connects with a TRS patch cord. I can't figure out how the switching works, i.e. can you get to the hell channel from the clean channel and back. Is there voltage going to the footswitch to power LEDs and how much is there? Would it make any sense to use dual color LEDs on each channel switch for easier channel recognition, and coolness of course. I have a Fender dual switch pedal enclosure a couple of Fulltone 3PDT switches, jacks, some LEDs, and a 9V input jack(if needed). I've included the schematic I have for the Quattro, and it's easy to find online, and a Fender dual footswitch that I think would work.
While writing this I think I've got the channel switching issue worked out, but please feel free to expound if you wish.
Thanks in advance! Can't wait to get the old iron out and melt some lead.
Erik

[martin manning]

If the voltage shown on the schematic is correct, the channel switching relay system operates on 6V DC.
It looks like the chassis mounted switch can select any of the 4 modes, where the foot switch only controls Lead and Hell. I guess you choose Clean or Crunch with the chassis mounted switch, and then use the foot switch to select the other two. The foot switch is very simple, just two switches and no LEDs. If you add LEDs to the footswitch the added voltage drop is likely to prevent the relays from activating. Also, it looks like the filament connections are not drawn correctly since they are in series/parallel with only a 6V supply.

[Erik]

Hi Martin,
I'm pretty sure the footswitch controls all four channels. One switch controls clean/crunch and has a green LED. The other switch controls lead/hell with a green LED. The switching sequence would be:

No LEDs - clean
Green LED - Crunch
Red LED - Lead
Green and Red LEDs - Hell.

Another difference in the Soho nd the schematic is that the Soho has two LDR's not just the one by relay II, could these have something to do with the relays just running @ 6V? I don't know about the filament comment, I'll have to look inside again, could be another case of the two amps being slightly different. Thanks again for the fast reply, I really appreciate it!!

[pompeiisneaks]

The problem with any switching mechanism I've seen is that you need a wire per switch, they can all share common ground, so that's just one, but you're mentioning 3 separate 'switching' that's happening. That requires three separate lines. You can do some of this with a DPDT switch (two switches at once) but you still need a discreet line pulling each 'connection'. The schematic you've linked very clearly only switches relay II and III, on the footswitch.

A TRS jack has only three wires, therefore it has relay II, relay III and common ground.

the only other way to switch more than one is with some kind of multiplexing circuit that uses a chip in the footswitch and a chip on the board to handle the timing and multiplexing of the switches.

~Phil

[sluckey]

It's a binary thing. Two switches that can either be on or off... there are four possible combinations.

[martin manning]

Ok, yes I see how the two buttons can select all four modes:

[Erik]

Ok, great! So, what resistor values should I use, the Peavey is running 30V and this is only 6.3V. Or would an internal 9V be needed(easier)?

[pompeiisneaks]

Oh duh, that suddenly makes sense, it's like with the dumble, you can have clean, or enable PAB, or enable OD, or enable both PAB and OD. the bulb goes on.

~Phil

[martin manning]

So, what resistor values should I use, the Peavey is running 30V and this is only 6.3V. Or would an internal 9V be needed(easier)?

I think I'd be inclined to use a battery in the footswitch enclosure if you want LED's there, and select resistors that will run them at low current, say 10 mA to maximize the battery life. Adding two or three volts additional drop is probably not going to work out. If you had a fourth conductor in your cable you could bring a dedicated 6V lead into the box, but that would mean changing the TRS jack to some other type. That might actually be a good option since you are buying and building everything except that jack. A midi cable is a good solution.