TU 18 Footswitch

This is the sort of complexity I am talking about.  Here is a quick sketch of how it could be done.  There is absolutely nothing special about any of the components whatsoever.

Q2 is not usually set up as I have it here.  The LED is usually in the collector branch along with the current limiting resistor.  With it in the emitter branch as I have done it you can use a single Red/Green LED as long as you make sure it is "Common Cathode" and that there is no current limit resistor built in. (If they have their own CLR built in they are usually described as "Indicators".)  Otherwise use any suitable red and green LEDs you can get your hands on.  I have assumed On voltage of Red 1.5V and Green 2.5V.  This is again not crucial in any way.  Suck it and see rules here!

Trim R4 to get equal brightness between red and green.  Lower values will brighten the green and higher values will dim it.

ANY NPN transistor type will do.  Yes, absolutely ANY!  This is simple low speed saturation switching, it is a totally none critical application.  There is too much BS about transistor and diode types, and especially opamp types, talked in internet forums.  In our very simple applications for guitar work most transistors will do any of our jobs perfectly as long as they can take the voltage and current levels required and meet sensible minimum gain levels.  Most modern transistors have fairly high hfe levels (also called Beta or described as current gain) and there is more than enough current allowed for in the transistor bases in this circuit for anything to do the job.

Built on a bit of veroboard or even a scrap of PCB this will mount on the switch terminals quite happily.  A small DC socket fitted into the footswitch and you are away.  The circuit does not interfere with the normal switch and LED action in any way.  Without external DC power the Green LED simply will not light up but the switching and the Red LED will work just as they always do.

It's there now for anyone to give it a try.  If you do then be prepared to play with the values a little or at least make some voltage measurements and post back here.  I'll give you tips on how to tweak things if they should somehow be a bit out of kilter.

Here is a fine technical point which may interest some people.  I don't want to cause panic in the ranks but it is worth being aware of this if you are adding LEDs into the footswitch control lines.  None of these calculations are critical in any way.  Bear in mind the relay coil resistance is specified as +-10% so accuracy on our part is impossible but the idea is sound.

Inside the TM18 the sensing circuitry for the footswitch lines are very simple.  The lines are fed from +22V, through their relay coil which is specified at about 1028R DC resistance, through a current limit resistor of 220R for the Channel and 470R for the Boost, through the amp's switch LEDs which are unspecified types, the channel line passes through a second red panel 3mm LED, and on to their socket terminals.  The LEDs will take up a few volts which we will have to guess at but, assuming about 2.0V for the red and 3.5V for the blue, and doing a couple of quick calculations we have this situation:

Channel line
+22V - 2.0V(red LED) - 3.5V(blue LED) = 16.5V across relay coil and resistor
Resistance coil + resistor = 1028R + 220R = 1248R
Coil current when activated = 16.5V/1248R = 13.22mA

Boost line
+22V - 2.0V(red LED) = 20V across relay coil and resistor
Resistance coil + resistor = 1028R + 470R = 1498R
Coil current when activated = 20V/1498R = 13.35mA

Looks like a good match to me so our assumptions seem sound.  The rated working current from the datasheet of those coils is 11.7mA.  The extra will cause no difficulty for the relay as it can handle over twice the rated voltage which means twice the current too, though I wouldn't want to design in anything like that.  H&K have built in a safety margin of operation of a few mA which they know we can lose before even coming down to the normal level.  If we add in a footswitch LED we rob the line of around another 2V for common Red and Green types.  The current levels will then drop down to about:

Channel line
16.5V - 2.0V(ftswtch red LED) = 14.5V across relay coil and resistor
Coil current when activated = 14.5V/1248R = 11.6mA

Boost line
20V - 2.0V(ftswtch green LED) = 18V across relay coil and resistor
Coil current when activated = 18V/1498R = 12mA

Now the whole point of this is that, even if we are adding in LEDs in the footswitch and we are stealing some of that voltage and therefore reducing the current through the relay coils, the level it drops down to is still within the absolutely reliable level for the relay they use.  That's reassuring.  Even adding a blue LED with a higher drop of 3.5V will only pull the current down to just over 10mA channel and 11mA boost.  The relay datasheet specifies the "must operate" voltage as 9V and this means about 9V/1028R = 8.75mA.

Adding in the little bi-colour LED circuit I have shown would increase the current draw through the line a tad when the footswitch is engaged so the red LED activated to feed the base of Q1.  However, it is less that 0.5mA so it is absolutely inconsequential.  With the footswitch disengaged, red LED off and green LED on, the amp line is totally disconnected anyway so there is no effect at all.

I originally decided to take a quick look into this as I wondered if adding the footswitch LED could alter the coil current enough to set the switching action close to the edge of unreliable.  Once again the H&K designers have their sums spot on and it is already compensated for.

You can't fudge the engineering and that means the Maths!

Bert, it’s been some time since I’ve been back here. Your help with the wiring of my FS-2 pedal to work with my Octa-switch got me rocking again.

I did however experience something weird at rehearsal yesterday. My boost led kind of flickered a little before finally checking out. Anytime I used a preset on the Octa-switch that required both boost and lead on, the boost led wouldn’t light up and the rig sounded weak. Like it was missing its “balls” so to speak. The LED’s on the Octa-switch showed both boost and lead to be on but it wouldn’t reflect on the Tubemeister 18. Only the red led would come on showing the lead boost was activated. I took the FVS-2 pedal apart and if I went around the led with a jumper wire and could get the channel switching to work on the Tubemeister just fine. Can an LED burn out and if it can, would it prevent the channel its associated with from engaging?

I ordered another pedal and was going to rewire everything the same and hopefully that will work. I believe the problem to lie in that one LED but I’m not positive. Also, if I wanted to change the LED colors, do you know what wattage and such I would need to make that work?

I’d love to send you our(Alien Armor’s)first three tracks fresh out of the studio and you can hear my rig in action. That is, if you’re interested. Just shoot me your email address and I’ll send you the links.

Thank you again Bert. Your expertise helped me immensely!

Cheers,
Rick

Hi Rumworks. Glad things have been working until now. It is possible your LED has given up the ghost, at some time all components will, but it isn't common for a LED to go pop if it is being used at sensible current levels which yours are. It is possible that the fault could be in the LED but it could also be in the stomp switch, the cables, the sockets, or even the Octaswitch's control relay.

From what you describe it doesn't seem as though the Channel side of things is faulty, it looks more like the Boost side. We had best make totally clear what is actually happening here. You say the Boost LED flickered then went out. Do you mean on the amp or on the footswitch or both? Any time you used a Lead + Boost preset the Boost LED didn't light up. What were the Channel and Boost LEDs on the TM18 showing? Was the Lead channel selected there and did the Boost stay off matching the footswitch LED? We must identify which of the two separate actions of Clean/Lead selection and Boost On/Off switching are working and which are faulty. Remember, you do not have three separate options here, there are two totally separate options giving four possible outcomes. The Boost switching has no connection to the Channel switching and vice versa.

There are a few tests you can easily carry out here which would help. Let's try them in turn. Firstly we will rule out the Octaswitch. Plug the footswitch into the amp as it should be and pull out its two plugs to the OS. For each plug, short out its tip to its sleeve with a short piece of wire just wrapped around it. Now the footswitch should work as a normal control unit. Test using just the footswitch whether the Channel switching works, then whether the Boost works for each channel. We need to know if the LEDs on both the footswitch and amp light up as they should do matching each other.

Try that first and if we still see a problem then we know the issue is in the footswitch or its connection to the amp and we can forget the OS. If the problem goes away then we know the OS is at the heart of it.

Bert,

Thank you for your response. We left for vacation right after I posted that. Sorry, it’s been a little while before getting back to you.

Ok, so…the LED on the first boost switch(on the FS-2pedal)is what went out. The LED for the second boost worked just fine on the Tubemeister 18 when I stomped on the footswitch for second boost.

As for the Octa-Switch, both stages would light up(or not)like they should whenever I picked a preset that had one, the other or both. The Octa-Switch worked as it should from the best I could tell.

Now, when I took the jumper wire and went around the first boost LED(the one that wouldn’t work), I could get the amp to switch just fine. It switched and it functioned as normal. My “guess” is that it’s the LED by what I am seeing but my troubleshooting. I’m not sure what to meter to confirm. My new FS-2 arrived while we were gone. I’m just afraid of being wrong on my prognosis and ruining another pedal.

What course do you believe I should take to confirm?

P.S., you never shot me an email address so I could send you our first three studio tracks.

Rick

I forgot to answer one of your questions. If I stomped on both boost and lead, the LED for the lead worked as normal on both the amp and the FS-2 footswitch. The LED for the boost didn’t work on either the FS-2 or the Tubemeister head. The Octa-Switch always showed the proper LED’s according to whatever preset I chose.

Once I created a short going around the “suspected” bad LED on the boost side, the amp head LED’s worked as usual and the amp had its “balls” back.

Well done for doing those tests and getting the results you did. You've shown that the OS is not at fault, we can forget about that. You also have proof that the TM LED and switching action is not at fault. The pedal is the definite culprit.

So we need to address the pedal. The switch in there is fine once the LED is shorted so the LED is the only other cause of the problem. I would just replace that, I seem to remember you can solder so it isn't any really difficult job. When you are soldering LEDs the temptation is to shorten their leads and solder to the little stubs for security. This puts your soldering skills under more pressure. You have to be able to do it with just a second or so of the iron on the joint.
LEDs, like other active components, are very heat sensitive. Too much heat will damage them or at least weaken them so they do fail later in the day. That may be what has happened to yours.

If I am doing something like this I first ask myself if I can get away with leaving the legs full length or at least as long as possible, and in a pedal that is usually ok. That helps with less heat travelling up the leg and into the LED. It means you have a little more time to complete the solder action if your skills are not absolutely top notch. The extra mile is that you must sleeve at least one of those legs to prevent them shorting out in the future. I do think that just soldering on wires and bending them apart is not enough unless the actual leg of the LED can be soldered direct to a fixed point like the switch or socket terminals. Rough handling of the pedal can move those thin little legs if they only have a flexible wire attached and it isn't silly to plan and work to prevent this. I keep a mixed stock of heatshrink sleeving for jobs like this. It is cheap enough on ebay or the like and if you can't get that then consider stripping a little heavy duty mains wire and using the outer insulation you pull off. As long as it is a firmish fit it will stay in place. You could even use a dab of glue on the end to hold it in place.

The other thing you must make sure of is that the LED is the correct way around but you already know that. In your case you MUST have the anode towards the TM18 and the cathode towards the Octa. The anode is the slightly longer terminal leg in a standard LED package. You can look it up online if you are at all worried about it, there are plenty of sites where they will show it in pictures.

Take your time and be gentle with trying to get the old LED out. From your pics it looks as though it is mounted without a panel bezel of any sort but that may be wrong. These are usually a small plastic ring which the LED pokes through and this has legs on the inside which clip behind the LED holding it in place. Sometimes there is a locking ring pushed onto the back edge of that and that can be flicked off to release the LED. Sometimes the LED has a dab of glue on the back to hold it in place. That can usually be picked off with a sharp point then the LED comes out with some gentle teasing of the bezel leg clips. However yours is fitted, make sure you don't damage that mounting clip. You can get replacements but it's much better to just take a little care and keep the originals.

Anyway, you should be able to get on with that and then be sure to let us know how things turn out.

Bert, thank you for your time again. I decided to go ahead and use the new pedal and wire everything just like we did the last one. It gave me the opportunity to use a little longer cables for the two mono jacks that run to the Octa-Switch this time as well as another chance to do a cleaner job of the wiring. I’ll test it out at our rehearsal space tomorrow evening. Hopefully, all will go well.

Do you know what the ratings are for these LED’s by any chance? I wouldn’t mind changing the color from red to blue. Thanks again Bert! I’ll let you know how it goes.

Don't worry, there are no complicated ratings for LEDs for what we are doing with them.  You are going to use it at a relatively low current, around 10-20mA, which is fixed by the circuitry in the TM18.  You won't notice much difference in light level within that range even though it seems wide.  For my own gear which I build or mod I usually run them much lower than that for use on stage in lower lighting when they can seem like spotlights, it's a bit of a matter of taste.  Effectively the TM18 will "squirt" close to the same controlled amount to any LED you put there.

LEDs are a form of diode which all have a strange characteristic as you increase the voltage you put on them.  Connecting them one way around they refuse to pass any current at all, they are "reverse biased".  The other way around, "forward biased", they pass almost no current at all when the voltage is below a certain level, the "knee voltage".  The diode circuit symbol shows you that.  From one direction it is an arrow to show current can travel that way, from the other it is a bar showing current is blocked.  When the voltage reaches the knee voltage level they begin to increase the current they pass very sharply and the current increases enormously as you increase the voltage just a little beyond that point.  For a red LED the turn on point is usually at about 1.2V-1.5V depending on type and, if you try to force a larger voltage onto them, at around 2V-2.3V you will have destroyed them.  Here is a link to a useful diagram showing this:  LED Voltages

There are exceptions which are made for heavier use but for the class you are using in your pedal they will turn on around 1.2V and the TM18 will dictate how much current they then draw.  The TM takes care of the safety of the LED by adding a resistance in the feed to them which "soaks up" any excess voltage beyond their turn on point without allowing a huge increase in the current.  Imagine a garden hose with an On/Off valve or tap at one end and an adjustable nozzle at the other.  Connecting a voltage to the LED is like turning the tap from off to full on.  Without the nozzle there any amount of water can flow and you, your garden and your cat get soaked in the backlash.  However, fitting the nozzle acts like the resistor and restricts the flow to the level you want.  Choosing the value of your resistor is just like adjusting the nozzle.

Just get any general purpose red LED which fits the hole you are filling.  Make sure NOT to get a type usually described as an "indicator" which has a resistor built into it as this will interfere with the TM's control of the current and limit it too much.  Yours will be "3mm" types.  There are 5mm and 10mm available which will work but they won't fit.  You can find plenty of simple info on this online.  For the record Green LEDs have a higher turn on voltage level than red, and Blue are higher still.  You can see that in the graphs in the link that I posted earlier, there is a plot for each colour type (and for IR and UV types too).

If you change to a Blue LED from the pedal's original Red, the Blue's higher forward voltage means the TM18 will push in less current.  The other LEDs, the switch and front panel LEDs, all share that same current.  This means that all of the LEDs in the chain may not be as bright.  Light levels also vary even for LEDs of the same colour so a lot of that will be "suck it and see".  My guess is that it won't be too much of a problem if you just get a general purpose Blue type but it will be a little different and you may not like that.  LEDs are usually not expensive so you could easily get a couple of each colour and try them out.

It's hard for most people to get the idea that the LED responds much much better to the current you push through it than to the voltage you try to force across it.  In the first instance we all think of voltages when we think of electrical stuff. You can't think that way with LEDs, they are NOT at all the same thing as bulbs!  See if you can look at the graphs and imagine what happens to the voltage on the LED as you change the current through it.  Slide your finger up the vertical Forward Current axis and then horizontally across to the Red plot line and down to the horizontal Forward Voltage axis.  Try that for a low current and a high current.  You can see that if you control the current you can vary that a lot and get only a small change in Voltage.  Now do it the other way around starting on the Voltage axis and moving up and across to the Current for a low voltage around the knee voltage and then for an only slightly higher voltage.  Look at how much massive variation there is then in current if you try to control the voltage within a small range.  Remember, nothing in electronics is ever exact, components always have tolerances and some are quite large.  Voltages will always vary by a significant amount and good design removes most of the effects when they do change.  You control LEDs by setting their current and letting the voltage take care of itself.

Hope this all helps!

That’s some great information regarding LED’s. I ordered a handful of different colors just last night.

I did “rewire” the new pedal just like the first one we did and took it to our rehearsal place today to test it out. I don’t know what’s going on at this point. The LED’s came on but I could never get them to shut down. When I hit particular presets on the MK3, depending on which had boost or boost and lead together, the LED’s on the TM18 would get brighter or dimmer. Everything should be working but it’s not. I haven’t changed anything regarding the FS-2 since the first time I put it together via your instructions. It’s worked like a charm and literally changed how I do things where playing guitar is concerned.

I came home and put everything back in the old pedal and changed out both LED’s with the two from the new pedal. Without my amp head here, it’s difficult to know whether this one will work. It makes me wonder if they did anything different on this new FS-2 pedal. The switches were the same design but looked a little different. I’m going to give it another go Saturday and hopefully it will work this time. It’s really starting to drive me crazy.

Anyway, thanks for your help so far Bert. I’ll let you know how it goes. I’m considering buying a new FS-2 pedal and just using it like it was designed although, it kills the whole idea behind what the MK3 is for. Another week of this and I’ll be certifiably nuts.

That definitely sounds like you have a simple fault in your wiring. It sounds like you possibly could have a short to ground in the cables or in the FS2. Recheck your wiring and see that all is as it should be. Blobs of excess solder can be a bugger if you aren't careful!

Have you definitely put the LEDs in series with the switch between it and the cable to the OD? There should only be one connection between the LED and the switch and one connection between the LED and either the cable to the TM18 or the OS. TM18->Pedal Switch->LED->OS and TM18->LED->Pedal Switch->OS will both work but the LED terminal you connect to the switch will be different for each of those cases.

It is also possible that you have the LEDs the wrong way around. LEDs are supposed to act like normal diodes and block current in one direction. If the voltage is too large however they can "break down" pretty much like a valve that has been pushed with too much water pressure and suddenly lets go with a flood of water through it. If the current is limited to a safe level, which it should be by the TM18s circuitry, the LED can be undamaged by this and still work as it should. One side effect is that this has a different emission process which means it lights up by a different method within its crystal structure for which it is not designed.

None of that explanation is your concern of course but the "reverse voltage" across the LED that you have exceeded to get it into that state is too high for the switching action of the amp to act on. It needs to see a high (it starts at its own +22V) or a very low (below just a couple of volts as the pedal pulls this down to ground) voltage in order to decide to switch. If it is stuck to dropping only to 5V or more, which is the LEDs reverse breakdown voltage, it will not respond by switching. With a red LED in the correct direction the forward voltage is only about 1.5V or so and in the reverse direction it will be at least 5V, probably a few volts more.

Is it possible you have the LED the wrong way around? Is it also possible you have a LED which has a built in resistor, an "indicator" type and not just a plain bare LED?

Well, I wired the new pedal just like the first one. The LED’s were attached on both switches just like the old one. Maybe H&K changed something? I don’t know why they would reverse the LED’s but maybe they did. I’ll be able to test the pedal again this Saturday. Hopefully, it’ll work this time.

As for soldering, it was minimal. There weren’t any globs or anything like that. I think I will order another pedal as much as I hate to spend more money on a third pedal. If for some reason, I can’t track the cause down, I can at least have a sure fire way of getting the boost or lead necessary.

Man, I really need this to work. I’ve been spoiled with this setup for awhile now and have gotten very used to it.

I promise you, nothing has changed in the way the circuit works, H&K will certainly not have reversed the LEDs.  It must be down to a simple error somewhere.  Leave it for a day or so then go back to it and check it out.  There must be something simple which you can track down.  Perhaps you can post a couple of closeup pics of the interior of the footswitch and I'll see if there is anything I can spot.  If you can then make sure to get one from each side at an angle so we can see under things.  I should be able to trace the wiring accurately then.

Don't worry, it's something simple.

Good idea Bert. I’ll step back for a spell. Although, it hasn’t stopped circling in my head since the trouble began. I keep going over it again and again and can’t see it. There has been a couple times in the last 28 years where I’d get blinded so to speak by something so simple and then you take a break for the night and before you get to bed, it comes to you! You go in the next day and you’re back on track.

I will try the pedal again Sunday morning which is the soonest I can get to my rig. If it works, great! If not,  will take it apart again and send some photos. The thing is Bert, the ONLY thing I did before the problem started(which was at the very beginning of last rehearsal)was the day before, I brought my pedalboard home and reran my power cables for pedals. That shouldn’t have effected anything regarding the FS-2 at all. I’m going to go over my pedalboard again with a fine tooth comb but I just can’t see the problem arising from what I did then. However…the trouble started right after what I did that day. Go figure. lol

Howdy Bert…

I’m sorry for not getting back sooner but yesterday was the first rehearsal we’ve had in just over two weeks. During that time, I went ahead and rewired the 1st pedal that went bad and the 2nd one that didn’t work properly after wiring(or thought I was wiring) and I’ll be, THEY BOTH WORKED! I also replaced the original red LED’s with one blue and one green one which look nice. For some reason, the blue is a little brighter than the green but I can live with that.

So, I don’t know what I was missing while wiring them earlier. I do believe it was something simple like you said and I found it without realizing it. Thank you for your time again Bert. I sure do appreciate it.

Off topic but, what is the best way to confirm the tubes might be going or have gone bad in this Tubemeister 18? I changed out the original tubes a couple of years ago with some JJ matched tubes and have really liked the sound a lot. On occasion, my signal will get weak. Basically like it loses its power. I shut down the amp for few moments and then turn it back on and my beefy signal returns. Also, is there a better set of tubes that you could recommend? I really do like what the JJs brought but I’d be willing to try another brand if need be.

Haha!  I knew it would be something simple.  Glad you got it up and running.  LEDs are all very different in aspects like their intensity at the same currents so it isn't at all unusual to find your blue is brighter than the green.  The only way to change the brightness directly would be to add a resistor in series with the LED inside the footswitch to reduce its current.  Unfortunately that also reduces the current through the relay which is being switched inside the amp and that would make the switching action less reliable after a certain point.  You may get away with a small current decrease but reduce it too much and the switch just won't change the amp setting every time.  It is worse for a Green LED and is even worse for a Blue LED as these have higher Von voltages than a Red one.  That means more voltage taken up across the LED and less available for across the relay coil and the resistor you are going to add.

The relay spec shows it has a "Must Operate" voltage of 9V.  That means you have up to about 3V to play with which sounds a lot but isn't really in light terms.  Adding resistance might give you the right amount of light reduction but I have had this problem myself and it takes a fair bit of lowering of the current to get a noticeable reduction in intensity.  The amount of resistance you would need to add would have to be found by trial and error.

I'm assuming you are using the Blue for the Channel and the Green for the Boost and let's assume you may need to reduce the intensity of both LEDs.  For the Channel try adding anything up to around 330R.  Don't be surprised if the switching action becomes occasionally a bit dicey though I wouldn't think it would be too bad for values up to around 220R, we are exploring untrod ground here.  For the Boost with a Green LED try using anything up to 470R with around 330R being optimum.  The reliability will get better as you add in less resistance so don't add more than you really need, and all of this assumes you can reduce the light level enough by this method. Bear in mind you will also reduce the light level of the amp's own LEDs too, they are in the same line as the footswitch and relay.

There may be a simpler solution.  Is the Blue LED a clear type?  By that I mean, is the head of the LED a crystal clear plastic which only gets coloured when the LED lights up?  Blue are frequently like that whereas Red and Green more often have coloured cases which tend to make their lit state less dazzling.  If it is a clear one then open up your footswitch, take the LED out of its seating and try gently roughening or matting the round top surface with a fine sand paper to disperse the light coming from it and make it less like a pinprick of beamed bright light.  A very fine nail emery strip used very gently is great for that.  That alone might just be enough to reduce the effect.  If that doesn't do enough then get a blue paint or even nail varnish, dilute it, and put the thinnest dab on the head.  Get the amount of that right, it really will only take the thinnest coating, and you will tame down the brightness.  Let it dry fully before putting it back in place in the footswitch.

It is unfortunate that the TM18 and the other TM and GM models aren't able to carry lower currents.  It is a consequence of the way the relay inside the amp is switched with the current through the footswitch and LED being the same as that through the relay's coil.  You have only a little leeway to play with unlike many other amps.  They often have transistor circuits to sense the switch changes and then to amplify that to a constant relay level.  You can often do pretty much what you like in the footswitch.  H&K's solution is simple and reliable even though it makes this adjustment a little less easy.