Grand Meister Deluxe 40 Noise Issue

Hello All,

I have an H&K GM Deluxe 40 that is making popping sounds once the amp is turned on, and graduates to a variable hum/buzz as the amp heats up over about 30 minutes. The hum/buzz is present regardless of weather or not it has an input connected. The hum/buzz is present when connected to a speaker cabinet or through the speaker emulation output into a mixer. The hum/buzz stops when the stand by mode is turned on. I have recently replaced all the tubes in an effort to fix this issue, but that did not seem to be the problem. When a guitar is connected, the guitar sound is clear and unaffected, but the hum/buzz is still there. The amp is getting more hot that I think it should as well. I am using it on the 40W setting, but the issue is present on every Wattage setting. The tube bias indicators are all good. I suspect there is something going on in the power transformer, but have not had to work on amplifiers before. I have an electronics degree and background, so technical advice is most welcome. Thanks in advance.

Hi Brian.  You seem to have eliminated the first obvious area of concern, the valves themselves.  Popping sounds are a common valve trait when they start to go flaky.  That said, it could be a dodgy cap as well but let's hope it's simpler than that.  I would start by simply cleaning the switch wafers of the Fx Send and Return sockets.  In most amplifiers these are used to route the signal and they do corrode in time which leads to noise problems and even complete cutout of sound from the amp.  A spray with contact cleaner then a forceful repeated insertion and removal of a jack plug a number of times should clear it.

The GM40 schematics are not out in the wild.  However, the GM36's are and there is a shared family design to them, so that gives us a very good idea of how the GM40 is set up.  It has the Fx loop of course and this is a good way of separating the preamp and power amp areas.  Try simply putting a lead into the Fx Loop return.  This breaks the signal path between preamp and power amp.  If the noise is still there then it is power amp related.  If it stops it is preamp related.

Valves can usually be removed temporarily and the amp run without any bad effects.  You could try removing the preamp valves one at a time starting at the input V1 and progressing through them one by one putting the previous one back in each time.  If the problem is in the signal stages then it should stop when that valve is removed.  If it is in something like the power supply then it could be unaffected by that.

This doesn't really address your hum and heating issues but it would be a start at getting a picture of what is happening.  Are you sure you have a hot amp?  The GM series uses the case as a heatsink to draw heat out of the valve cavity.  It will run hot to the touch and I know from my own design job that the temperature you feel is not what your brain tells you it is.  50-60degC is very hot to the touch but is not an issue for electronic components.  Unless you have noticed a sudden increase in running temperature I would guess that might be a non-issue.

Thanks for your insight. I'll do all of this this weekend and see what I can come up with.

Well, we cleaned all the inputs, pulled all the tubes one by one, separated the Pre and Power tube sections with the FX jacks, tried different chords and a different power conditioner, and all for naught. I have a video of the issue, but the forum will not let me post a link to it yet.

Ok.  Let's take stock.  If you have swapped out the valves then that should rule out a dodgy one causing this.  Putting a jack into the Loop Send disconnects the power amp from the preamp but that didn't affect things so the problem is not isolated to the preamp stages.  Putting the amp into Standby stops the issue let's think of that.

When the amp is put into Standby H&K follow a different method to what is usual.  The old way to implement a Standby, (Marshall and others), is to cut off the HT to the output valves and sometimes even the PI valve.  This is a very poor way to do it!  Fender and a few others often follow the method of splitting the HT line after the rectifiers.  That method is also poor and sometimes even dangerous.

Allowing a valve to continue to emit electrons into the cloud, (i.e. stay heated), with no HT applied to sweep them away will cause the eventual death of the valve by cathode poisoning.  It slowly builds up a non-conductive layer between the cathode base and oxide coating which chokes off the cathode current.  If it is of interest, here is a good short explanation with a wealth of other info on valves:  Valve Wizard: Standby is a bad thing  Use the Home link to access all of his other info if you ever want to get to grips with valve work.  The Fender approach is also discussed there with its drawbacks.

The answer is - - - why use a Standby at all?  Despite "Common Knowledge", (neither common nor particularly knowledgeable), it is not saving the valves in any way.  It serves no real purpose.  Originally guitar amps were designed without them at all.  Then someone pout one in as a "feature" and suddenly every amp had to have one just to keep up with the Joneses.  It even led to the embarrassing Vox fiasco where their amps could explode, all caused by the poorly implemented standby circuitry!  You have a Volume control.  Look at your setting then just turn it fully down.  Your amp is now silent and ready to go by just turning the Volume back to your last setting.  That is often too simple for "today's guitarists"!

Anyway, the H&K method utilises the MOSFETs in the cathodes of the output valves.  They leave the HT applied but cut down the cathode current to a very low level without turning it fully off.  That is why sometimes you can still hear a very low signal when in Standby.  It is perfectly normal given the method of standby.  This irritates some of "today's guitarists" who insist standby means fully silent.  Sorry, personally I just don't get that level of unnecessary impractical perfection.  This system shares the MOSFETs with the auto-bias system so if that is working, and you do say the indicators show it is good, this should not be the problem.

We are starting to look towards things like PSU caps or even what you feared, the output transformer.  I have not had a report of an output transformer problem for many many years here.  They just don't seem to happen, though they always could of course.  You mention a "Power Conditioner".  I'm for this sort of unit personally but have you tried the amp without that in place?  That could be the ideal area to introduce this sort of problem.  I would suggest, if you haven't already done it, just rule that out by removing it completely and retry the amp.

Just had a thought! This is obscure and very unlikely to be the cause of the problem but it is just possible. Are you able to meter out the power cable? I mean check with a meter that the live pin on the wall mains plug is actually going to the IEC live pin on the amp end. The IEC plug on the cable should have the live pin on the left as you look into the pin sockets. I have personal experience in my old job of cheaper Chinese import cables which look perfectly good being wired with their live and neutral reversed. That is not good and could even be dangerous. Worth checking as this was quite a common issue at one time and it is the sort of thing that can lead to increased hum and noise.

The power conditioner I mentioned is a Furman SS 6B Pro, we tried operating the amp with and without the Furman in several locations with the same results. I did not measure the power cable, I will do that tonight and report my results tomorrow. Thanks again for all your help!

I should have asked this before but, have you removed all other gear from the setup? No pedals or other units attached, just a clean guitar -> cable -> amp -> speakers and nothing else?

I've been digging around to see if there are any pictures of the GM40D internals out there.  There isn't much info at all.  There is a lot of shared "family" design between the GM40D and the preceding GM36 so I would expect the internal layout to be pretty much the same on the surface.  The HT and LT fuses are internal to the amp so access has to be available to change them and the PSU components are in the same area of the PCB as the fuses.

You may be able to see this area through the ventilation slots on the base of the amp enclosure but it looks as though the main caps are off to the side.  If not, the bottom is held on with 10 small screws and can be easily removed.  You don't need to remove the feet.  It may pay you to have a look at the PSU area for scorched resistors or diodes and "blown" smoothing capacitors.  THE WARNING:  INSIDE THERE ARE LETHAL VOLTAGES.  GIVE THE AMP A FEW MINUTES TO DRAIN THESE DOWN TO SAFE LEVELS BEFORE GOING INSIDE AND TRY NOT TO TOUCH ANYTHING UNNECESSARILY.  Apologies if I'm teaching my granny to suck eggs but I'm not sure how much experience you have with this sort of work.  You are looking for a cap which has a dimpled cross on its top which has puffed outwards.  I've attached a picture to show you what that means just in case.  This may not be necessary for you but others who are not experienced may find it useful.  The PSU caps are generally larger vertically mounted cylinders.  There are a couple with black plastic slips over their tops but you should be able to see the same puffed upwards appearance if they are faulty.

If we are in that area you will need experience to swap out these parts.  You are dealing with high voltages - and I do mean lethal!  It must be done correctly, neatly and safely.  Also consider that the GM40D is a modern design of amp with reliance on close packed PCBs with small surface mount components.  It is nothing like working on a "classic" valve amp in the Marshall/Fender tradition.  Unless you have experience enough and the right skillset to work on this I would pass it on to a tech with the workshop around him to make the job easy.  It's money well spent.  (Both GM40Ds and funerals are expensive businesses nowadays!  )

The issue happens when nothing is connected to the amp via the input jack. Amp plugged into power conditioner plugged into the wall. Speaker cab connected to amp. That's it. Issue happens when a single guitar cable and guitar are plugged into the input jack as well. I have already taken the amp apart and fished around for burning silicone smell, bulging caps, black fuses, scorch marks, stray wire clips, etc. I have a robotics BS and have designed and worked on automated electronic systems for a decade, so I'm good on all the voltage warnings and electronics knowledge, although if you (person searching the forums for a fix with the same issue in the future) are not trained, please heed those warnings! He is not exaggerating about voltages that will kill you. Unfortunately, Bordonbert I don't see anything obvious. This amp is absolutely smashed together and I certainly don't really want to take the whole thing apart to stare at it and wish I had a wiring diagram. I will post the video of the issue in a few days when the forum allows me to, but I'm thinking you are correct about getting it to an amp doctor.

Glad you understood about the importance of warnings on an open chat site. This info sits here long into the foreseeable future and who knows who will come across it. The next guy might just be the numpty with the pacemaker who sticks his fingers onto the HT! We have to try to protect such extremes of the gene pool from their own curiosity.

I'm really glad you also weren't insulted by the thought of getting it to someone with specialist knowledge. I was a bit worried you may get precious about it. It's good to hear you take the pragmatic approach and admit there will be a point beyond which you won't go yourself. I am the same. As a design engineer in a different field, (admittedly an opinionated one in some areas), many people assume I have a poor opinion of guitar techs from the way I constantly push factual info over common knowledge and guitarish scuttlebutt, often at their expense. I only have a poor opinion of the guitar techs who talk a hugely informed game but actually have very little understanding of what they spout off about when you look at the engineering evidence. Some are prone to want to go way outside their area of expertise with their claims as gurus. Arguing with a man who believes some of the tripe about differences between valve makes, (note I said "some" there ), when there is solid evidence to refute his claims is a frustrating process. When we should be pointing each other to bench measurements and plots which support our points of view we end up with "everyone knows", "it's common knowledge", "my old mentor who taught me electrickery at his knee always said that is wrong" and any real proven validity is long lost.

I could not do what a tech does. I simply do not have the huge spread of info about and experience with so many models of amplifier, and the knowledge of their quirks and most common faults at my fingertips. I can read deeper into a schematic and chase through circuitry quicker and improve it more fully and correctly than most which the average tech doesn't really need to do for their role. However I am very aware that the tech can often go to the correct solution without even consulting any schematics because he simply knows the unit at fault from many previous fixes. Hopefully you know a good one who can work on something as modern as the GM40D. My own GM36 has a number of small mods in it to improve reliability so I know the problems an "old school" tech will face if he can only work on rat's nest type wiring of lego brick sized components.

I'm sure in your own situation a good tech with the unit in front of him on a bench would get to the roots of this pretty much without breaking into a sweat. Those symptoms sound very valveish but you have proven they aren't. They can also be symptomatic of a cap going out which is now my best guess without having seen the unit. With a scope it should be easy to trace the noise back to where it originates and we have to hope the fix won't be too dramatic. You never know, you might even end up with an insight into the GM40 circuitry with documentation. Do post that vid of the fault, or maybe put it on YT and just post the link? It would put a bit of flesh on the bones of the issue.

Here is a video of the issue. I took the amp to a local H&K certified repair shop. I told the tech all that I had already done to try and isolate the issue. He said that's how he would have started as well haha. Hopefully he will have repaired the amp in the next few days and will enlighten me on what was actually happening. Thanks again.


https://www.youtube.com/watch?v=pyC5kY4SYhA

That is definitely the digital signals controlling the amp bleeding through to the audio side. The "spitting" is too regular to be a random thing. It is definitely a regular signal pulse stream breaking through. You can just about hear this in normal use with the GM series but it is very soft and low. Yours is way beyond what should be audible of course.

It could be a simple grounding/earthing issue or maybe a power supply problem. With test gear and a chopstick he should be able to chase it back and get to grips with it. Do please keep us informed, this is an interesting one which I've never seen, (from this side of the issue of course, not so much from yours ).

Hi,

i am having almost the same issue with my GM40 which i bought 2 Months ago new.
Did yfind any solution to the problem?

I had to send it to Yorkville Audio in New York for repair; my local dealer could not figure it out. It was initially repaired and sent back, but had developed a phasing issue which also made it unplayable. It was sent back and repaired and I haven't encountered any issue with it since. They did not describe what the core problem was when I asked, their customer service representative only told me it was a "digital bleed issue". It was repaired under warranty. All in all it took about 3 months to get corrected.

Thanks for message. I am experiencing the same digital bleeding and varying noise in the background. Will send it in for repair since it is still under warranty. Luckily I am living close to the H&K factory in Germany so I hope it will go a little faster. But still annoying that it happens with a almost new amp.