Tubemaister 18 Line& HX effects switching problem

Hello guys, I need some help from someone that understands electronics inside the TM18

I have the TM18 bought it as soon as it came out. It does have som quirks.

I use the amp in 4cm with a line 6 hx effects, and am using the helix hx to switch the amps. It works fine for a while, but when the amp gets hot (after an hour or so of playing) when switching to clean channel, i get pops and cracks in the sound, making the clean channel unusable, as if there was some kind of a voltage buildup. If i then change to the FS2 foots switch, the clean channel works just fine.

Is it possible, that the lack of led lights and resistors, that are in the foot switch, is causing this problem? Would the problem resolve, if i make a circuit that's identical to a FS2 without the foot switches, leading to a trs jack and than into the helix HX?

Thank you for the answers!

I do have another quirk... when I play on hot stages, and for a long time, clean channel starts acting crazy, as if it would bypass the master volume... does anyone have an idea what would be wrong? Maybe some cold solder joints?

Thank you all!

If I have this right you are getting popping and crackling in your Clean channel after an hour or so when it is under the control of the Helix. I'd better get this clear in my head as I have never come across this before. The Helix, as I understand it, is an Fx unit which sits in the Loop generating your Fx sounds for you and which can also be programmed to control MIDI devices at the same time. I wasn't aware it could control your amp channel selection via the Footswitch socket. Is that how you are controlling the TM18 which does not have any MIDI capabilities? Does the Helix have the ability to act as a plain footswitch to replace the standard FS2 type usually plugged into the TM18 Footswitch socket?

When you swap out the Helix and just use a simple FS2 switch you say the Clean channel goes back to quiet again. If that means you are removing it from the Fx Loop too then it does seem as though the problem could easily be in the Helix. LEDs and resistors in switch units will make no difference to noise at all. The control functions inside the amp are based on completely different circuitry to the signal side.

Hi @bordonbert, You're right... I am using the helix with a 4 cable method... meaning, i use some of the effects on Helix in front of the amp, and some in the amps FX loop... all of the helix line have midi control, and they also have the option of controlling your amps channel switching with dedicated control outputs... The EXP pedal outputs double as relay switches for your amps or fx... you can use a mono jack, or trs jacks.. and then you can assign a pedal on the helix to switch the channels on the amp... I use a trs (stereo) cable and am plugging it into the amps foot switch output, the other side than goes to the helix....

The switching works fine, no problems at all, just as well as a fs2 would work, but i can have it programmed per patch on helix... changing a preset on helix, switches my Tubemaister, and adds effects, or changes the routing, whatever I want...

And then after a while, the popping and crackling sounds appear... when switching from drive or boost channels to the clean channel, the sound is fine on drive channels. When I remove the trs cable from the amp's foot switch input (or change it with the FS2), the amp functions normally. That is the only thing I do, helix stays connected into the front of the amp and in the FX loop.

The problem I think is, that helix uses relays to do the switching, which is ok i guess, shouldn't be a problem, I think the problem is that there is residual current that builds up, because of the lack of LEDs and resistors that are in the FS2. I will try to solder a led and a 1k resistor to the trs cable I use, and see what happens, it can't hurt...

Do you by any chance have the answer to the second quirk? "I do have another quirk... when I play on hot stages, and for a long time, clean channel starts acting crazy, as if it would bypass the master volume... does anyone have an idea what would be wrong? Maybe some cold solder joints? " This problem appears only on really hot stages, and it does so with or without any effects plugged in.

Thanks!

That sounds like a valve ageing but there should be no way it can only occur when the Helix is controlling the amp Channel/Boost selection. If you have a noisy valve it should be the same whether the footswitch jack is inserted or not. Sounds like a bad valve is too simple here. Dirty loop jacks don't seem to apply here either though it never hurts to give them a clean occasionally.

When I got my first H&K amp it was a TM36. I started using that with a Thomann generic footswitch which had no LEDs or resistors inside it at all without any problems so there is absolutely no need for them to be present for it to work properly. I also know the circuitry around that part of the TM36 quite well too and can confirm it is all separate until the point where the changes you ask the amp to apply actually alter the circuit parameters through a couple of FETs.

The switching in the TM18 needs simpler requirements but is exactly the same in principle. It has its Channel and Boost control circuitry under the control of the front panel switches. The switches just ground or leave open two voltage lines which cause later circuitry to switch a number of FETs used as switches in the amp circuitry. That is the only point where the two systems touch. They in turn apply additional resistors in parallel with others in circuit permanently to alter their value. When you plug in your Helix jack all you are doing is swapping the footswitch switches for the panel switches, the control circuitry is exactly the same for both. So if it works properly without noise via the panel switches when the footswitch is unplugged, I really can't see a way that it could become noisy with it plugged in.

If it were me I would try to rule out the Helix as that could possibly be at fault too and introducing the noise through its Fx lines. I would set the Helix up to control the switching in your amp in exactly the same way but I would take out all four connections for the effects, both before the amp and in the loop. And I mean physically remove the cabling from the amp. Just put a guitar straight into the front end and don't use the loop at all. Test the amp for a good while like that using the Helix to switch things occasionally. If you don't get the problem showing up then you know it isn't being generated in the amp by the switching connections which are still there.

If you don't get the problem then leave your Helix controlling the switching and put in just the Fx loop cables. Run it for a while like that too and see if the problem occurs. If it does the Helix has a problem in that area. If it doesn't, then remove the fx loop cables and put in the cabling for Fx before the amp. That tests that section in the same way. Finally refit it fully and see if it still becomes noisy. You are first isolating everything except just the Helix switching to test whether it is the amp which generates the noise with the footswitch connection in place, then seeing if the Helix is introducing the noise in its Fx section, then if it is the "before amp" Fx section, then if it is some strange interaction between the two.

Can you get time to do that?

I know that is a long process but if you want to get to the root of the problem it will take a little bit of this type of work to be sure.

Huh...

First of all, thanks for your answer, troubleshooting the problem like you described never even crossed my mind.

After about 3 hours of troubleshooting, and trying out all the different options, the conclusion is that the problem is in the switching, not in the helix.... if I plug directly into the amp and use the helix just for switching the amps channels, there is no difference, nor is there a difference, if i use the helix in front of the amp or just in the fx loop..... different cables etc don't work as well.

I did notice a peculiaritie.... when the trs for the switching is plugged into the amp and helix the led on lead channel is on just slightly, the channel select button led is on just a bit too, that is however not the case when the FS2 is connected.

I've ordered a couple of 1k resistors and LEDs... will try copying the schematics of fs2 without the switches and then see what happens...

ah well, enough playing around, off to work now

Excellent work there Lukas, that's very useful info and makes something clear.  By testing the Helix in its control mode only with no Fx connections you have shown that the problem is definitely in the interaction of the TM18 and Helix.  The TM18 has no problem when it is used with a standard footswitch, it isn't faulty.  The Helix has no problem in introducing noise into the signal when it is simply controlling the amp because the problem still happens even though it is out of the signal path.  You have ruled out the Helix as having a problem where it was bleeding the noise into its own signal path and feeding that into the amp input or Fx loop.

However you have shown up something wrong, that Lead Channel LED should not be showing any signs of lighting up.  It is controlled by the state of the switch on the tip, either connected fully to ground or floating fully free.  It looks to me as though the Helix control circuitry is not able to control the TM18 directly.  Did you spot the following in the user manual?

Helix Manual wrote:IMPORTANT! Your ability to control external amp channel and/or reverb switching with the HX Effects device has been tested with many popular amps and heads. Unfortunately this does not guarantee compatibility with all products.  Note that, depending on the circuitry of the channel switching jack in the guitar amp used, the EXT Amp function may not operate as expected.

This is what is inside the TM18 which you are accessing with a FS2 or your Helix.  (The coil resistance is 1028ohms +-10% according to the spec sheet.) Your switching isn't pulling the connection down to ground or letting it rise fully as it should.  When using a FS2 type footswitch it is either fully open or fully shorted, period!  The Helix uses solid state circuitry to perform its switching on those sockets and that always has residual resistance.  It has to to be able to be set up for use as an expression pedal too.  It looks as though there is still enough resistance left in the Helix circuit to interfere with how fully the amp switching circuitry flips leaving it in an indeterminate state.  That is why you are seeing the LED stay slightly illuminated.  If that is the case then you will be finding the switching circuitry is leaving the final FET switches in the amp signal stages in a state they should not be in, partially on and interfering with the circuitry they are connected to.  This is giving you a contradiction with some areas set up in Clean mode, some in Lead mode and some in an illegal "neither one nor the other" mode.

The channel switching circuitry is designed so that a short to ground on the tip switches the action fully one way with no possibility of being caught in the middle, and an open circuit on the tip switches it fully the other way.  The circuit senses current not voltage, so the resistance of the Helix circuitry controlling the tip is crucial to the amp's action as that will alter the current drawn out of the amp.  I know there is considerable leeway built into this as I have played with that area myself in the past and know that up to a certain resistance will not prevent it working but outside of that the action stops working correctly.   All of the following control circuitry in the amp then falls into line to switch as it should instructed by that first current.  If the tip has a high enough resistance to ground in its shorted state where it should see a dead short, the current will be lower than it should so it is in the dead zone between open and shorted.  Then the rest of the circuitry can reach a state where it will also not be fully one way or the other.  Remember there is only one relay after the selector switch, the majority of the switching is done with transistors and JFETs used at their extreme one way (on) or the other (off).  The circuit is not designed for them to be sitting in the middle of their active regions neither off nor fully on.

As the problem is in the Clean state and that is selected when the tip is meant to be open circuit and no current flows, it looks as though the Helix has too small a resistance to ground even though it is supposed to act as an open switch. You may be able to help by adding resistors but it will be impossible to predict what value you would need.  It will have to be done by trial and error.  Adding resistance in series in the line to the tip decreases the current and will help.  The way to get LED action into the switching of the Helix is as always recommended by H&K, just put an LED in series with the tip line with its anode towards the amp.  No resistance is normally needed, just an LED!  Did others reading this for other reasons catch that?  JUST ADD AN LED IN THE LINE TO THE TIP.  It will then use the resistor in the amp to limit its current which is already the case for the LEDs on the amp.  This will help with the problem but it may also be helped by adding some resistance too though that could decrease the brightness of the internal LEDs.  I would guess a low resistance under 500ohms should do the trick but be prepared to get it right by trial and error.  You may even get it to work with just the LED.  As I said, anything which adds any resistance into the line outside of the amp will do that.

I can ask Line6 about the impedance of their socket which is what we need to know if you like but I am sure they will confirm that, for us, it is out of the range to reliably use with the H&K setup without tweaking.

@bordonbert, thank you very much for the answer and explanation and all the help so far. I'll do as you suggested, since the problem is in the clean channel, I'll try adding resistance, fist the LED and see what happens.

I like my setup, I love the amp, and would love to finally have it work without any problems, because it's inconvenient to say the least, when the amp starts to act strange in the middle of the gig...I gig 2-3 times a week, so you can imagine how reliable my rig has to be.

The resistors and LEDs I ordered should come by post in a couple of days, so I'll keep you posted about solving or not solving the problem

To day is a day to celebrate, well, actually yesterday was that day

It seems I fixed the problem. I had to test it by trial and error, but i got it to work. First I tried, to up the resistance, with just one LED, and it wasn’t enough, two 1k caps and a LED fixed the slightly on chanel led, the channel lights were changing, but the amp stayed on clean all the time, even though the channel select and boost LEDs were switched on. Lastly I tried one 1k cap and a LED, effectively copying what’s in the FS2.
It works like a charm, switching is ok, and the LEDs are either on or off, depending on the channel selected. I’ll try to test the switching thoroughly today, I’ll try to leave the amp on for a couple of hours erc.

Thanks again for the help!

Good news. Glad it turned out to be not such a drama.

I would say that you could be running a pretty thin line with this mod to the footswitch. The TM18's switching circuitry is fairly simple as that is usually all that is needed for reliability. It has no positive switching action built in, it simply slides across one way and back as the current pulled out of the switch cable increases or decreases. More robust circuitry would reach a point then snap across fully with a little hysteresis designed in their too. This can only gradually transition to each state over a small range of current drawn. By running it from your Helix you are adding a degree of "running close to the line" just as Line 6 warned in the manual.

It depends on switching components inside the amp having enough gain to effectively still make reliable changes while they sense less change in the footswitch current because of the increased resistance of your resistor and LED added onto what is designed in the amp. All you can do is to keep an eye on it and trim that resistor if it needs it. It's a good idea to run it up for a good length of time as you suggested too. Heat build up is one of the ways component gain can change so you need to see it stays working at all temperatures you may play under.

That's one for the success file, good work!

EDIT!:

TURNS OUT THAT THE MOD DID NOT FIX THE PROBLEM

I guess I was too sloppy when I was testing the mod, the channel select LED partly on problem was fixed, I did plug the guitar in and test it, but I was short on time, so I did not notice the problem....

The amp was switching.. LEDs working normally, I just wasn't focused on the tone..... Turns out, the problem was not fixed, even though only the channel select switch was on, the amp sounded like it switched to lead boost directly... clean sound was perfect.... I noticed that on sound check... I had the FS2 on hand just in case, and did the gig with FS2

So today I decided to get to the bottom of this problem... so I wondered... if there should be a resistor and a LED on the ring wire too... did that.. problem was still there... then I decided to remove resistors on both wires, and only use a LED on each of the wires (tip and ring)... that effectively solved the problem.  The LEDs on the amp function normally, and so does the switching!!! Happy as a pig in shyte, as a friend of mine from Ireland would say

I have no knowledge in how electronics work, I guess I know hot to solder and stuff, I build my own cables and fix guitar wirings if needed, but thats it... I am stubborn enough to try things

The amp is on for a couple of hours now, and the switching works normally, I'll leave it on for a bit and see if it holds up, but so far, so good.

So Line6 HX effects (probably all the Helix line) feat. Hughes&Kettner TubeMaister18 switching problem mod for those who might need it:

                             WIRE A LED ON EACH OF THE WIRES (one on the tip line, and one on the ring line)



Thank you again @bordonbert for your help!

Glad you got it working.  I would still caution, as a fix it is kind of a "temporary probable" solution which you should think will work in most circumstances.  You could make changes inside the amp which would make it more robust but that is much more involved work and beyond what you need.  It certainly won't have any bad effect, the circuitry inside is just as safe with your fix in place so go ahead and run it like that.  It might just be that under some conditions, after running for a long time in a hotter environment for example, it may verge on the slightly unreliable again.  I would play that as you find it and carry a FS2 around with you so you can swap at short notice if it gives problems.  You may not ever see a problem again but, as we don't really know what the Line 6 circuitry comprises of, it is always possible that you just might.

There are a few areas in the circuitry which are switched together when the channel selection is made. What you have seen is, I suspect, one or two of these changing and the others either not responding or maybe not responding fully so the amp is put into an indeterminate mixed up state. That would not be unusual with the way we have changed the switching conditions both outside and, because of that, inside the amp.



For the record and for the benefit of others who may come across this, this is NOT a fault or weakness in the H&K switching circuitry, nor really the Line 6 Helix either.  They are each set up to respond to a particular configuration of the other device and unfortunately there is a degree of incompatibility between them as we have seen.  Note that Line 6 are aware of it and warn against it in their manual.  It is caused by the fact that the H&K control circuitry is set up absolutely correctly for a standard "Short/Open" mechanical footswitch.  Resistances are calculated to work correctly controlling their transistors with either a dead short or an infinite resistance, not fully fo what they may find in between.  The Helix unfortunately has a control output which is based on active semiconductor devices, (transistors of some description), which simulates that.  This means that when the device is on and simulating a short circuit to ground there is always a residual resistance in the device which interferes with the H&K circuitry, unlike the mechanical switch which is an absolute 0ohms (as near as damn it) when closed.  Likewise, when the device is off simulating an open circuit there can still be a residual resistance to ground rather than being infinite which can also interfere in the actions of the H&K circuitry.