TM36 - clean channel is too quiet, crunch pot adjusts clean channel volume

Hi everyone,

I have an issue where I turn on my TubeMeister 36 amp with my TM212 cab hooked up to it, and the clean channel is much too quiet and also a bit thin sounding compared to how it should sound. When this happens, I can use the knob for the crunch channel to turn the volume of the clean channel up or down (but it's always much too quiet). Usually this resolves itself after a while and then the clean channel suddenly "pops" up to its normal volume with a nice full tubey sound. The other two channels always seem to work fine.

I use a self-built channel switcher but I have the problem if I don't use it, too. In fact, when this happens I usually disconnect everything except my guitar and it doesn't fix it.

Is this something that I can fix myself or that I can get fixed at my local guitar shop?

Thanks in advance for any tips,

Toon

Hi Spin81.  I'm really sorry I missed this for so long.  For some reason I'm not getting emails from the system to let me know when anything is posted.  Anyway, assuming you haven't fixed it in the meantime, on to your problem...

The background:

This smells like either valves or relays, and most things of this nature come down to valves.  The H&K TM/GM series basically works by first adding or swapping valve stages into the overall path to give different gain options in the circuit for the different channels.  You have 3x 12AX7s.  Each of those has two triodes inside it.  Usually, (I would say pretty much definitely here), H&K use them as 2x preamp valves and the third for the phase splitter in the power amp.  The 2x preamp valves give you 4x triodes to play with.  These are used as 3x gain stages and a DC Coupled Cathode Follower which offers no gain (x1 voltage) but allows for better driving of the following stages like the tonestack and adds a unique distortion mechanism into the mix.

The Clean channel is usually one triode from each of the first two 12AX7s with the second triode in the first valve ( ) unused.  The Crunch may be the same with extra gain added by altering component values or it may add in that unused third triode between them.  The third will definitely be used for the Lead channel.  The valve swaps are obviously dependent on the valves themselves working but also on the relays used to select which options are needed, (as are some of the other component changes).  The relays/switching elements are all high quality miniature sealed types or JFETs and don't usually give problems.  It is a no-brainer to go for the valves as your first area and see if that sorts things out.  If it's a valve it is an easy fix.  If it is a relay then you need to get it done professionally.

The test:

To test the valve theory, do you have access to a spare known good 12AX7?  If you do then swap it for each of the first two 12AX7s in turn.  If it is a valve problem you will get the amp up and running immediately.  If you don't have a spare 12AX7, (not even from a different amp), then just swap your existing 12AX7s around, (the one under the metal can and the one next to it).  You won't cure the problem but it should shift noticeably from one area to another.  If the nature of the problem changes markedly then changes back when you reverse the swap, you know you have a valve out.  Make sure you end up with the valves back in their original places, sometimes valves can be selected to be best suitable for a specific job.

The advice:

If you find a fault then just replace it and, for my money, for God's sake don't spend a month researching which valve manufacturer will make your amp sound like Eddie Van Halen's 5150s and swallowing any old bull that people hand you.  It just doesn't work that way.  There are only extremely subtle at best differences between valves despite what people, (either sellers of valves or their "white line followers") will tell you.  That much has been clearly and irrefutably demonstrated in the most precise and inarguable way now, and that was by someone who was trying to show that there were differences!  I can show anyone who is interested the proof, nothing to do with me but properly conducted tests with good results and a completely distorted and incorrect analysis of them afterwards. You can make your own mind up. A good old JJ is the valve of choice for most of us here.  It's cheap, is easily sourced, and does the job perfectly well.  (Isn't that what tone controls are for?) There is also a case for sticking with a Chinese made valve for the second one as the Cathode Follower has a very hard life and Chinese valves are shown to take that abuse much better.

There isn't much you can do to test the relay action and I don't have access to a schematic for the TM36 to advise you further I'm afraid.  There is a distinct family similarity between the way the TM36 and GM36 do things but they are definitely very different amps to work on.  I used to own a TM36 but moved up to the GM and sold it.   My advice is, if you haven't already, work on the valve swaps first then get back to us with the results.

Thank you so much for this reply, it may not be super quick but it's very much appreciated nonetheless. The level of detail and the advice here is very helpful. If the difference between tubes is indeed very subtle then I'm not going to be breaking my head searching for the perfect tubes - I pretty much only use my amp for punk rock, so subtleties are lost in the distortion and drums anyway.

What I have tried in the mean time is sticking my pick in the slot in the back, to see if the bias of the power amp tubes is the issue. One of them has one more "blink" than the others, but apart from that they seem fine and the problem exists on all wattages. I haven't tried if it happens with zero watts though but if you are right then I would expect it to happen with zero watts through the red box output as well.

Today my amp's clean channel started out great, but then when I switched to crunch and back the problem appeared again. So I'm thinking it might be a relay but I'm keeping my fingers crossed that it's a tube issue because at least that I can fix myself. I don't know when I can make time to look at this but it can be a few weeks before I can get round to it. I don't have a spare 12AX7 so I'll have to have a go at the ol' switcheroo and let you know how I fared.

The problem has gradually gotten worse with time and I've had this amp for almost four years now and have never replaced a tube in it until now. So it certainly could be a tube. Also I have a hunch that it happens more often if the amp is in a slightly colder environment. That kind of makes me feel it could be a tube thing (knock on wood...).

Would it be better to get a matching pair and replace both tubes with no can, if the problem turns out to be one of those?

Spin81 wrote:so subtleties are lost in the distortion and drums anyway.

Those are the wisest words spoken here for many a day! Keep that pragmatic attitude and you will go far.

The two valves you have to think about about are the one under the can and the one next to it. The third one on the left as you look at the front of the amp sitting behind the Lead channel switch is not part of your problem, it's the Phase Splitter valve in the power amp and it will be in place for all channels, so if it works for any of them it works for all of them. My guess is for the problem to be in the first valve as that is the one with a triode which is switched in and out for your clean/crunch channel.

That said, (after a little bit more thinking about it), it is not in line with our current picture of the problem to find that the amp works on the Crunch channel and not on the Clean channel. We are adding in the extra triode for the Crunch channel. If that particular triode has the problem you would think it would drop out when it is in circuit for Crunch not when we take it out for Clean. The other triodes are in circuit all of the time. It's leaning a little more towards having perhaps a problem in the switching such as one of the relays.

I would still go ahead and swap the first two valves over, (remember, that's the one under the can and the one next to it in the middle behind the crunch channel switch). If that doesn't give us a better picture of things then we have to consider it will be something like a relay. These relays are not expensive, about £3 in the UK but the cost is in the tech's time to diagnose the problem and fit one. They do have a specific footprint on the PCB so you must make sure to get the correct type to fit. Your tech would see to that if it comes to it.

Anyway, just swap those valves and let's see if it changes things.

Hi bordonbert,

I've just gotten round to switching the tubes around, the problem is in the same channels as before, as we both suspected. Off to the local repairman it goes! After all, this isn't an issue I can fix myself and I am not about to open it up myself.

Thanks again for your help, I'll be sure to post another reply if I learn more about the issue.

Toon

Hi @bordonbert,

I just picked up my amp from the store, they sent it off to their repairperson and he confirmed our suspicion that it was a bad relay. He apparently gave it a good once-over while he had it open and it looked good to him, and afterwards he gave it an endurance test, which apparently means loading it to the max and seeing what it does and it was 100% good according to the repairperson.

They added a year's worth of warranty, but said that they've been working with him for years and have never had a repaired amp come back. I can't test it out right now but I'll do so later this week. I don't expect any issues though.

Repair cost was about 130 pounds sterling, which assuming he spent a few hours on it is not bad a price IMO. Also I have a sneaking suspicion that I'm now good to go for another few years so all in all I'm a very happy camper.

Thanks for helping me debug the issue!

Toon

That's good news that it's fixed but it was a bit of a surprise at first to find out it had cost that much just to change out a relay. Then when I thought about it a bit more and took into account the time spent breaking the unit down to get at the relay and the time spent building it all back up again I realised it is pretty much on the ball. You have also had a general maintenance service performed and your amp given a clean bill of health which is worth a few pounds too.

We do have to take into account the complexity of these amps when we think of the likely problems which will occur. That is a factor we must expect to deal with when we take on an advanced design like the H&Ks. The "classic amp" guys with their fetish for turret board hand wired setups do have things easier there. (I have a late '60s Marshall SL100 myself too and I love them but not for this reason.) What they don't have is anything like the level of features and control that the modern approach can offer. It's horses for courses for me but of course to the majority of the ones I have spoken to over the years it proves that anything above a rat's nest of bare wire and well out of date components just can't offer anything like a good playing experience.

So it's time to get back to what your amp was made for and play through the thing. There's nothing like the feeling of getting your rig up and running after an unwanted break. Thanks for getting back to us with the results. It's comforting to know we were on the money with the diagnosis. Thinking it through logically is much better than just following random bits of advice thrown at you from all and sundry because "I heard someone once had this problem". That gets even more expensive very quickly.