Wiring Up Channel Switching Footswitch

[EETStudent]

I wanna wire up a channel switching footswitch that sends the signal through a set a gain stages(overdrive) or around them(clean). like a simple SPDT switch I suppose, but how do you incorporate this into a footswitch? Ive got a standard Marshall footswitch lying around, one button for Channel one for FX, how can I wire this up for this application?

[Enzo]

YOu would arrange a relay or relays in the amp, or a couple JFETs or LDRs. PLEASE don't try to send the signal out to a pedal and back. The pedal turns the relays off and on.

[EETStudent]

Well Enzo, I believe this is where i ask you how relays work. I did a search and saw a thread where you explained relays to another user, but it just didnt click with me. I suppose switches and relays and other micro stuff is something I havent been exposed to much. Anyway, could you explain relays?

[tubeswell]

My take on it is that a relay is a remote (or indirect) switch (like a solenoid) that is operated by power. The footswitch operates another 'powered' switch inside the amp that is configured either so that the relay is connecting one circuit or the other when you operate the footswitch. The footswitch either turns the relay 'on' or 'off', and when it is 'on' it connects one circuit, and when it is 'off' it connects another circuit. The footswitch doesn't carry any part of the amp's signal - it merely operates a remote switch which does. Given that transistors operate by turning 'on' or 'off' depending on the signal you feed into the middle bit, you could therefore make a relay out of some transistors. (I suspect you would need say 2 PNP and 1 NPN, or vice versa? or is that overcomplicating things?)

[Enzo]

Not regular old bipolar transistors, but JFETs - junction field effect transistors. They don't come in PNP and NPN, but they do come in N-channel and P-channel.

A relay is simple, it is nothing more than a switch. Except it is not flipped by your finger. it has a little electromagnet coil inside that moves the switch when the coil is energized. The magnet coil circuit is totally separate from the switching parts.

Look, imagine a toggle switch with a nice long toggle sticking out. It flips up and down. Now instead of using my finger to flip the switch, imagine I glued a little hunk of iron on the end of the toggle. Now if I hold a powerful magnet near the switch, it will pull the toggle towards it by attracting the iron hunk. I can then move the magnet to the other side to flip it the other way.

In a common relay, the switch is held one way by a little spring. Then when we turn the magnet on, it pulls the switch to the other position - tugging against the spring. If I turn the magnet off, the spring pulls the switch back to the starting position.

SOmetimes I might use the terms on and off with relays. I really try not to, but I do anyway. SInce they are switches inside, it might confuse. The switches inside have "normally closed" contacts, and "normally open" ones. The normally closed ones are the ones closed when the relay magnet is not energized. WHen the magnet is energized, it switches the contacts, so the normally closed contacts go open, and the normally open contacts go closed. Just like a toggle switch - when one set of contacts open, the other set closes.

The switches though don't really have an off or on position. Think of an AB switch, it has A and it has B, but it doesn't have off and on. We say contacts open and close. When I say turn a relay on, I refer to the magnet coil inside. Turn on the magnet to energize the relay. But in the interest of not confusing anyone, I try not to say on and off at all. Not for the coil, and not for the switch contacts.

SO the beauty of a relay is that it can sit right in the middle of the circuits, and the magnet can be operated remotely.

[EETStudent]

Well, how do you assign a relay to a certian function in a circuit? For example, switching a gain stage in or out. You attach the switch/relay at the input of the switch, where switching it allows the signal to go through or around. Would the relay be specially assigned for this location, or is just a matter of where you place the relay determines what it affects, just like a switch?

And, you mentioned they are remote controlloed.....What remote?

I guess Im seeing how relays work, but I do not see how they are activated.

[tubeswell]

The footswitch is the remote control. The relays need to be activated by their own power supply (the voltage for which depends on the particular relay). PV Classic 30 has relays for its channel switching and reverb, which is powered by a -36V supply.

[Enzo]

Really, it is not cosmic, there is no hidden meaning here. A relay is nothing more than a switch. The symbol in a schematic is the same as a switch, except the magnet coil part.

If you wanted to switch a cap in and out, like on a cathode bypass for gain boost, instead of wiring the cap to a switch contact, you wire it to a relay contact instead. the relays used in a lot of amps, like that Peavey Classic 30 or in their 5150, are about the size of a 5 watt cement power resistor. SO it fits on a circuit board easily.

SO wherever you want some switching function, you can mount a relay instead of a switch.

The magnet coil inside the relay requires some power source to energize it. The coil is of course right inside the relay. So we connect one side of the coil to a power supply (5v, 12v, 24v, whatever - they come in many voltage ratings.), and then we complete the circuit to ground from the other end of the coil through some sort of switch. It could be a footswitch, or a button on the front panel. Point is you don't have to touch th relay itself. You can turn on the coil from someplace else.

Try this. DO you understand how the LDRs work? There is a light bulb inside that shines on a photocell. The photocell varies its resistance depanding on how bright a light falls on it. All you need to do is get that inner bulb lit and the thing turns on. But the switch to do that doesn't have to be there at the LDR, it can be remotely located - that just means located somewhere else.

There is probably a light bulb up on the ceiling of your bedroom. The bulb is up there, but the switch is down on the wall. The switch remotely controls the bulb. "Remote" doesn't mean anything fancy, it just means someplace other than at the thing it controls. The switch would be real inconvenient of it was up on the ceiling next to the bulb, right? So imagine instead of a bulb on your ceiling, we had an electric magnet coil. You could turn it on and off from the wall switch - remotely - and if we stuck a set of contacts on it, it would then be a relay.

[EETStudent]

ok i gotcha. You mention that relays need their power source. Where does that come from? Within the relay circuit? or somewhere within the amplifier?

Do relays already have predetermined footswitches? If I were to buy one from Weber would i have to get a 'matching' footswitch?

[Enzo]

The relay is just the switch and magnet coil in one package. Power is external. Yes, yo find suitable voltage in the amp. SInce relays come in so many voltage ratings, you can pick one that is convenient. Usually we make a separate supply for the relays and LEDs. For example, some rectify the heater voltage to make relay voltage. You could take the bias winding perhaps and add another rectifier for a relay supply. You could even use a battery, though that would seem silly to me.


A footswitch is just a switch in a box. It has no idea what it controls. it completes a circuit or not when you click it on and off. The only thing that makes a FS special is the labelling, and adding LEDs. Or of course when the thing is a multifunction behemoth. If you have two basically identical FSs and one says REVERB and TREMOLO, and the other one says REVERB and BOOST, the only difference between them is the writing on them.


Ever take electricity in school? Or in physics or science class? They set out a battery, a switch, and a light bulb, all wired in a loop: wire from battery to bulb, then bulb to switch , then switch to battery. CLose the switch and the light bulb comes on. The switch completed the circuit to allow current to flow. SOmetimes them wired in a large col of wire instead of a light bulb, and they would place a direction compass, like Boyscouts use, next to the coil. Now when the switch is closed, the compass deflects and the needle points to the wire coil. This demonstrates current through a coil of wire makes a magnetic field.

That is similar to our relay and FS. In the lightbulb example, we could replace the switch with a footswitch on a long cord. it doesn't matter if the switch is right next to the battery or 10 feet away with longer wires running to it. Step on the FS, and the bulb goes on and off.

ANd same with the coil of wire - move the switch 10 feet away and set it on the floor, and it still will deflect that compass needle when you turn the switch on.

SO instead of a bulb, or that large coil of wire, we now have the magnet coil inside the relay. INstead of a battery, we have a power source inside the amp. And instead of a switch on the panel, we have it ten feet away on the floor. CLick the switch on, and the magnet coil energizes, and that switches the relay contacts.

[EETStudent]

Ok Enzo, I think you did it. Seems fairly simple now. One of the main questions I had was whether or not I could use the footswitch I already have, and I guess not without the relay assembly considering what Im looking to do. I appreciate it.

[Enzo]

Oh, you COULD wire the signal to a cable all the way out to the FS, through the switch in the FS and back through a cable to the amp. But now there is 20-30 feet of excess cable in the midle of you r signal path. Not only will it likely pick up noise along the way, all that cable has capacitance which can affect your high end response.

Try this sometime. Play your guitar through a short cord, then connect a couple long ones together with an adaptor and play through them - like 40 feet long. Hopefully the short cord and the long cords are similar wire. You will probably notive through the long cord that it is not as clear and sparkly.

YOu COULD wire it direct, but don't.

The whole point of a relay is to control it here for something to happen somewhere else.

And another advantage: your FS has a single switch in it, so it can switch one wire from A to B. You can wire several relays to the same circuit if you like. Just like you can wire four speakers to an amp instead of just one. With multiple relays you can control a lot more stuff with that single FS.

For example, lets say we were channel switching. One relay could switch from one post volume control to the other. Another relay could select which channel the input feeds. ANother could switch some tone shaping in and out, and another could mute the unused channel. One could work some LEDs on the panel to indicate channel in use. WHatever you had them doing, ONE FS could turn them ALL off and on together. You may not need that now, but it allows more complex function for your FS potentially.

[geoffrod]

explanation please

Enzo, i wonder if you could be so kind as to explain what you meant by muting the unused channel, and why this would need to be done, and how would it be achieved. I have been pondering for a little while the use of relays myself, and wondering can i just stick this "switch" in the circuit where i want and what happens to the triod that is being switched out, or triods for that matter, how are they meant to be conected in the circuit whilst not in use, can they just sit there not recieving a signal and having nowhere to send one when they are not in use, does this have any effect on there part of the circuit or there operation????, or do they need a path in the circuit whilst not in use????
I hope what i am asking makes sense.
look forward to an answer.
cheers
geoff

[Enzo]

A relay is really just a switch - contacts that open and close - so it no more needs special attention than a plain old toggle switch, nor would the unused side have any more effect on the circuitry.

WHy would channels be muted when not used? Amps do it all the time. Mainly for crosstalk. Imagine an amp with a screaming high gain channel and a nice clean and clear channel. Like most amps, the signal goes through an input stage then branches to the two channels. On the overdrive channel, it can have all the screaming gain it wants. But when you select clean channel, there will still be a very hot dirt channel signal bouncing around in the dirt channel circuits. It would not be gard at all for the clean channel circuits to pick up some of this distorted signal through crosstalk. By grounding off the signal of the high gain channel, it can no longer bleed into the clean channel. We don't usually need to go the other way, because any little bit of clean bleeding into the dirt channel would not be noticed.

It is not necessary in all amps, but that is the sort of reason why it might be done.

[geoffrod]

thanks

Hi,thanks for the explanation,
so if i understand it correctly when my dirty channel is not switched in i send it to ground.
i have attached a very very crude drawing of what i understand, if you would please excuse the crudness of it, could you see if i have the correct understanding.
thanks again
cheers
geoff