The On-On-On 3-way switch can be a bit confusing, but its quirky switching layout can enable some clever circuits. I’ve been tinkering with an old project guitar to add both a kill switch and a passive-tone-shaping switch to the same pickup. With the On-On-On, I can combine both features into a single 3-way switch so I don’t have to drill another hole in the top.
The switch I’ll be using is marked FTPA41. I couldn’t find a data sheet online, but it’s always a good idea confirm the switch behavior yourself anyway. The best approach to understanding any switch is to make a chart of which pins are connected in each of the switch positions. I use my multimeter in continuity mode where it beeps whenever a connection is made (or you can just watch for zero resistance). I touch each pair of pins in each of the switch positions and listen for the beeps, and make a drawing indicating all the connections. For the my switch, the chart looked like this:
Pay attention to which way is up on your switch- it’s not always obvious, so add a “top” marking if necessary. Also, note that there are two types of On-On-On switches – with the pin connections different in the center position, so always confirm with a multimeter.
Now, how can we do something useful with this?
It’s helpful to think about a two pole switch as simply two independent switch-circuits, controlled by the same actuator lever. So the left-half and right-half of this switch are totally independent, unless they’re deliberately connected together.
First, I’ll use the right half of the switch to implement the Kill switch. This is trivial- I connect the pickup signal wire to pin 4 and and connect pin 5 out to the rest of the guitar circuit. Now when the switch is up/center, pin 4 and 5 are connected and you can hear the pickup, but when the switch is down, pins 5 and 6 are connected which disconnects pins 4 and 5, and kills the signal.
I can use a jumper wire from pin 1 to 5 to connect the two halves of the switch, so that a second circuit is brought into play. When the switch is up, this jumper completes a circuit from pin 5 to pin 1 to pin 2, where I’ve connected a .0022uF capacitor to ground (which gives a gentle treble roll-off).
In the center/down switch position, pins 1 and 2 are disconnected (removing the cap from the circuit), while pins 4 and 5 stay connected providing the full pickup signal to the output.
So there we have it- a combined Kill/Treble-Cut switch. When the switch is down the pickup is killed. In the center position, the pickup is full-on and bright. And when the switch in the up position, the passive treble-cut circuit is added, which gives a warmer sound.