Extreme volume loss on clean channel, lead and ultra unaffected, tube failure?

Hi guys, great forum, already learning lotsa stuff.
Just purchased a second hand GM40, have dreamed about a GM for quite a few years....ever since the 36 came out.

I have to say I have seen every single YT demo of the GM40, at least twice.... for the past 3 years before pulling the trigger... great sound, features, and it seemed quite fit and flexible for what I play, Floyd, U2, 80's metal (yeah... I'm 51....50 lol). I can equally say I have been really disappointed by the sound I'm hearing in person, harsh, fizzy, thin, boxy, cold and sterile... a high gain tone that sounds like a bad solid state distortion of 30 years ago...in fact the shock had me almost resell it immediately... but I took a breath.... I will give it a second chance.
Fortunately I have definitely found all I needed to know on the tube swapping thread, thanks to all of you.
I guess power JJs and a 5751-5751-83s combo might do it for me.

Now, I have an issue/failure that I believe tube-related, hopefully someone here has a clue.

After playing for a couple of hours just on the clean channel, there was a massive drop in volume, I mean easily 80%.... the crunch channel is affected , say by 50%, lead and ultra are as loud as ever.  I would imagine it might be a preamp tube assigned to the clean/crunch channels that just failed?  I took all tubes out and put them back in, they all seem to glow normal, no klonking sound when I tap them, so.... I'm hoping it's just the one tube that decided to go south.... I messaged the HK team on FB, they were very nice but didn't seem to have an answer.

Oh, and for the record I'm playing through a beauty of a Panama 1x12 cab with an aged V30, and despite the speaker influence, I can say I have played it with a 5w class A Baroni Lab head and it sounds sweet, not Fender-ish of course (also EL84s) but warm and clean, so I can't blame the cab ( and that doesn't explain the loss of the clean channel anyway).

*UPDATE*
I've run a few tests:
*Took a 12ax7 from my other head and swapped for each of the preamp tubes in the GM one at a time, and checking for some difference every time. Nothing.
*Swapped the outside EL84 pair to the inside. Nothing.
*Took an (the) EL84 from my other head and swapped for each of the power tubes in the GM one at a time, and checking for some difference every time. Nothing.

The clean channel is basically dead... I have to put the master AND the gain at 10 to even hear anything...
Meanwhile the crunch channel is now a pristine clean, and I have to put the gain way up and use a humbucker for it to start crunching...
From limited experience but common sense this would seem to me like a preamp gain problem, so tube failure, but replacing one at a time didn't do anything      I suppose the only way to be sure would be to change the whole set... but buying a complete set of tubes for an amp that may be faulty sucks the big one.  Damn, really not a good start.    
btw I also find the lead and ultra channels extremely loud on 1W setting.... but I suppose that is normal.... " yessss yesss..metalll... hehe  hehehe hehe"

Any clues much appreciated. Thx in advance.

Hi Firewind, good stuff, in the right place now.

Jonny was right, no nonsense there.  Don't worry about your valves, valve choice is the last thing in line in a chain of factors giving you that harshness.  Valves will make very little real difference to your sound, their affect is "subtle at best" as you will have heard from VoodooJeff concerning his Wathens if you read the whole of the GM36/Valve Swapping thread.  The most important by a long way is speaker choice just as Jonny advised.

All of this series of amps by H&K are very sensitive to what speakers they are paired with.  You probably heard the good sounds on YT coming from something like H&K's own speakers.  These are Celestion V30s but the thing people miss is that these are specially voiced by Celestion for H&K.  I had a lot of problems settling on a speaker for mine before I used my old Marshall 4x12 with new G12M Greenbacks in.  That gives me a deeper sound than most younger players nowadays, (only 50/51 and you think you are old? ).  "So it's gotta like cut through the mix dood".  Modern hypetalk which we all think we have to say at least once in any conversation with other musicians to be considered worthy!  No, for me it has to match the rest of the band to make the way we all play the number sound better.

I started out trying it with a pair of V30 1x12s and couldn't live with it.  I got exactly the same results as you.  Harsh, ear slicing, fizzy top end and almost no bottom end to speak of.  I would strongly advise you to try the amp with a more mellow and not so "in your face" speaker.  Forget about valves, (other than fixing it), anything decent will get you out of the original Chinese pit and on the road.  There really is very little difference between manufacturers despite what everyone may say, and that can be demonstrated objectively on the bench.  No one yet has shown any evidence that I have seen that there IS a difference, and that should be very easy if they are correct!  Who says it does?  Well, not to be uncharitable but, who sells you the valves?  And once the unwary or undiscriminating have bought into it?  Then it has to be hailed as the best thing going or they have to admit they have been stupid enough to have believed pure hype.  The Emperor's New Clothes all over again!

Anyway, back to the amp.  In theory the valves used for your Clean channel are all shared by all of the channels.  The only difference between them is that the higher gain channels add a third stage into the signal chain.   So a problem in the Clean channel should show up in everything.  It may just be that the increased gain of the other channels is pushing the level back up to what you would expect after it spits out of the preamp, but I would have thought the quality of distortion would have changed noticeably, if not the level.  Any chance of that happening?  Do the Crunch/Lead channels in particular sound different if just as loud?

When you use the amp it is important to understand what is going on at the input.  This amp has a built in Protection/Buffer/Booster/Overdrive/Distortion unit before the valves.  It is in circuit all of the time and it responds only to the signal level put into it and the Boost switch.  At low input signal levels it is absolutely clean and buffers the guitar from the amp giving a gain of 4x to suit the valve circuitry following it.  You won't even know it is there.  At slightly higher levels it acts as an assymetric (one side only) clipping overdrive which adds a nice valvey type of overdrive independent to the valves' own.  At even higher signal levels it clips on both sides and becomes more like a traditional clipping distortion pedal.  With the Boost switch kicked in, it goes into a higher gain mode (7.5x) to make Tubescreamer type distortion more easily available with lower output guitars.  This circuit is a very clever addition and gives you a lot more sounds in your arsenal if you learn how to use it.  It is all done from the guitar volume control, don't be afraid to turn that down!  A high input signal with a low Volume setting is not the same as a low input signal with a high volume setting.  You can choose where your distortion comes from by balancing the two. If you hit the input with too high a signal and also try to force valve overdrive out of the amp you may well get a spikey "metal" sound.  The trick is to get everything set as low as you need and that takes some retraining.

That said, from what you have posted about your swapping of valves around, (good work there! I hope you put them all back in the same slots?), it sounds as though it isn't a valve problem. As all channels are affected the problem could be in any area of the amp. Other areas that spring to mind are the Fx loop and the power soak. Try the following:

Insert a spare guitar lead in the Fx Send and straight back into the Fx Return. If it stays the same this should rule out the switches of those sockets as a problem.

Take a signal out of the Fx Loop Send and put it into your other amp or something like a PA mixer, (with its volume low, it's a big signal in comparison to the usual inputs). Do you have the problem then?

Take a signal from the Redbox out and put it into another amp in the same way.

Doing both of those tests not just one is good as they are actually different! The Fx Send is before the main power amp and the Redbox is after it, from the output before the power soak and speakers.

Have a look into the bottom of the amp through the ventilation slots. You can see quite a bit if you get it in the right light and turn it until it is clear. You are looking for the large grey resistors you will immediately see there slung across slots in the edge of the circuit board, and a couple of others underneath them if it is possible to see. Here is a link to a thread which shows them clearly. Look for the pics! Do any of them look scorched or discoloured at all?

Let us know what you can find and we will go further from there, though I would think it will be unlikely to be a simple fixable issue if these come up blank.

I should also have said, have you tested the output valves with the TSC system? Are all of the LEDs behaving as they should? That's all out for 40W and the inner ones lit for 20W. Try the "pick in the slot and count the LED blinks" on the back. This should also give you an idea as to whether you have a questionable output valve.

Heya bordonbert,
Thanks so much for taking the time, very kind of you.
As I said, on my old twin the sovteks made a very noticeable difference in power, clarity and dynamics (then again the old tubes I had were from a 70's stock of used american tubes that I had found in a closet working at a tv studio in NY, can't remember the brand, the box was half red half yellow, but I digress...), but I suspect all the high end tube mystique (and prices) to be akin to the tonewood religion (which I won't discuss here so as not to hijack another thread, plus I have no desire to be burned at the stake lol).

Just to review the symptoms, I was just now playing a bit louder, neighbours being out...
(Mind you, I'm on 1W setting and this is LOUD, with master at noon this is already way too wild for appartment environment.... vs. my 5W head... but that's also one reason i got the Panama cab, it has an attenuator).

Anyway, these are the symptoms:
Ultra, Lead and Crunch are equally LOUD. Crunch seemed weaker to my ear before, but now I find it is on the same volume level as lead and ultra, plus, with master and gain at noon the crunch channel is now a beautifully warm crystal clean channel ???
Clean channel is essentially dead, as I said, I need to put master and gain on 10 to even begin to hear it. Even on 40W the volume is very slight on clean.

Yes I put the tubes back in the original slots they were in... BUT, I am now getting a slightly different TSC reading
BEFORE my initial tests, if I remember correctly all leds flashed 13 times, then the 2 on the right flashed once more and then the last one on the right flashed once again.
NOW, all leds flash 14 times, then leds 1,3, and 4 flash once more. I just repeated the test after another 30 min of the amp on, and now I get an additional 16th flash of the extreme left and right leds (1 and 4).
In any case the reading seems ok.

I have checked the grey resistors, it all looks clean, no scorchy smell of any kind after an hour of having the amp on.

Have connected the send of the FX loop to a combo input... at first, nothing..... then I pushed the button to activate the FX loop on the GM it results in the same problem, no clean channel.

Have inserted the spare cable to loop the FX loop, no change as expected after test 1.

Don't have the equipment to test the Red Box out, but based on the above, the problem being before the FX loop, this is a preamp problem, correct?

Now (thinking out loud), if all channels share all tubes, how the @#%& is only the clean channel dead? as a reminder, I was playing the clean channel when it just went bye bye back to Kansas....

Still I don't like lead much, and ultra is kind of atrocious, but this is taste, and potentially caused by the cab, which opens up a whole other can of worms, and additional costs.
Of course I could use crunch as clean, and build a pedalboard, but that would kind of defeat the whole purpose of a 1,300€ all-in-one MIDI amp...

Good post my man! That gives us some useful info.

Firstly, don't worry about the TSC tests changing, it isn't unusual to find devices like valves alter a little as they are heated and even just subjected to vibration in use. It may not be audible but it does show up with test equipment and the TSC circuitry is acting in that capacity here. As you said yourself, they are all within the recommended spec so aren't a problem. Valves are electrical in nature but also uniquely depend on their mechanical construction just as much. That gives them a wide range of latitude in how they behave. One tap and things can change a tiny bit as parts settle into a minutely different orientation. Ideally it wouldn't happen but the tolerances are so fine it will.

The Fx Send signal contains the same problem, ergo , the problem is before the Power Amp and Power Soak which means the Redbox test is not needed as that is between them. We now know it is way before that.

The Crunch channel is a "beautifully warm crystal Clean channel". That's interesting as it means that something is just reducing the gain cleanly and not introducing any distortions. You would be surprised how many small parts there are in that circuitry which trim the gain of individual parts of the overall amp with different channel selections. It's a maze of settings changes in there. One thing we can say is that the problem isn't in the second stage which is the one which is only switched into circuit on the Crunch/Lead/Ultra selections. The Clean is down too so it cannot be the cause.

Now one other area springs to mind knowing this. The digital control settings may be corrupted! If there is such a loss of volume without added distortion the gain is simply way down. That could possibly be a problem with the digital programmable pots that the GM uses. Do you have any specific patches set up yet? If so are you able to save them off the amp via the app? If you don't have any, then perform a factory reset on the amp. I would hope you have a manual for it but if not, here is a link to it on the H&K site: GM Deluxe 40 Manual The reset procedure is section 8.4 on page 12 but basically:

H&K Manual wrote:Press “Store” and “FX Access” simultaneously while powering the amp up by using the Power/On switch. Both buttons will flash to indicate the successful factory reset.

That may bring the volume back up. Can you try that first if you don't have a bevvy of patches to save and get back to us with the results?

Well, that was an easy test.  I had no presets to save or worry about.
I've only had this amp for a week. I don't have a controller yet, still deciding on which (either One Control Crocodile Tail or Musicom Lab MKV), neither the wifi gizmo to use the app...
I figured I would play this amp "manually" for a few days/weeks to see if I really liked it before investing another 400-800€...
I do have the manual, the previous owner (from the UK through evilBay) is actually a nice guy and obviously takes good care of his equipment (he told me he has a GM36 and a Triamp).

Heart pounding I proceeded.  

Good thing I was sitting down.  

Click, pop, flash...

Not only is the clean channel back and all channels level, but this is no "ampus horribilis" anymore.
I was beginning to fear this was made for extreme drop tuned metal and fanatic djentlemen exclusively....
But in a matter of minutes of febrile instinctive tweaking I brushed by some amazing tones I did not think this blue beast capable of.
Beautiful ambient cleans, excellent crunch, plexi-esque lead, 5150-esque (again ^^) ultra...  Instant switch from wanting to chuck this thing out the window to not wanting to switch it off and wishing my closest neighbour would be 7 km away

I can sense that the right blend and interaction of volume, gain, and boost opens up immense tonal possibilities, clean with gain up full, boost on, and volume slightly rolled back is actually a very nice Andy Timmons type light crunch.... crunch with gain full up and volume rolled back is pure southern rock... lead and modulation conjured tones from Sad Wings of Destiny to Screaming for Vengeance... and so on... yeah, a controller and the app are sine qua non to see what you're doing.
And you are absolutely correct, this amp is incredibly responsive to the guitar volume knob (I finally have a use for it ahahah).

I don't even have to worry about changing the speaker or the cab, it's purrfect.

I have no clue what the problem was, but I learned something additional today.  When I finally have the rest of the gear and a bunch of favorite presets, I'll do a backup first thing, in case this issue reappears.

In just a few days I learned a crapload of stuff, about tubes on the other thread (thx to everyone back there), and about troubleshooting on this amp.        

bordonbert I cannot thank you enough for kindly taking the time, and for your sharp methodic thinking, you have given me the key to this new amp, alleviated me of post-purchase remorse, saved me a lot of worry, and fo' sho' a whole lotta kopecks as well.
In doing so, beyond my most sincere gratitude, you have earned a monthly allowance of 25 karma points for life, and shall henceforth respectfully be referred to as Meister Bordonbert.

Cheers man... uuuuh... I mean Meister Bordonbert.

Wow what an amazing result. Meister Bordonbert you are the man!

Super happy that you can now enjoy your GM40 in all its glory

Naaahhhh!  I'm not that good.  If I was I would have told you to do that first!!!!  

It's the same with any highly digital gear of this type nowadays.  There is always the possibility of corruption creeping into the digital side and it causes this sort of issue.  In the H&K amps it is a rare phenomenon but we have had a couple of other instances over the years.  Glad we got to the root of it, I can once again sleep at nights.

When you decide on your controller setup I would make sure you look at the H&K stuff but be careful to look properly at other makes too.  H&K is always good gear but we all have to admit it is expensive.  You can justify this in most cases but, for my money, their wireless gizmo is very costly for what it is and there are others out there which will do the same job which are worth looking at.  MIDI is not demanding data at all and the idea of a secure connection is in the realms of computer network stuff and the prices of that gear are so cheap it is silly.  I have watched the prices of the MIDI specific devices rise astronomically as people started to pick them up!  It's a disgrace that gear which costs £30-£40 when it comes out, (that's retail and priced to make a profit), suddenly gets "upgraded" and rises to £120 because people start to see it as worth having.  We really do work and play in the most ridiculous field imaginable for letting ourselves be bent over the kitchen table and still handing over a wad of cash for our toys.  "Boutique" pedals at £150+ which are just cheap clones of older £30 ones?  As Yoda once said to me, "Strong the power of "No" is, but weak is the power to say, hmm!"  Well, that's just me moaning again.  As I'm so fond of saying, "They 'aint gonna listen to me!"

It only takes a semi decent cabled MIDI interface and you can do your patch backups immediately.  The cheap Chinese jobs on eBay for a few pounds are no good.  Any one you go for MUST support "Sysex" data transfer.  That is a system which uses larger user defined size data blocks than the older standard MIDI so the data buffers in the equipment must be larger to hold it.  The cheaper interfaces have only standard sized buffers and won't work.

Anyway, glad you're getting the sounds you wanted now.  Enjoy!

Meister BB! I like it!

I see you are modest in triumph, the mark of a true Meister.
Besides, sound scientific methodology is far more often getting to the solution by a matter of ruling out everything that isn't, rather than guessing what it is right off the bat.

Indeed, the wifi gizmo thing at 170€ is a bit steep, I just need to find something that will serve the purpose for more reasonable money. Thing is, I have no ipad and will be using my macbook pro, I have found the controller app for desktop, so I'm sure there must be some kind of cable adaptor from amp midi to mac I could use that doesn't need to be wifi.. that's the next trek into the jungle...

As for controller, as I said I'm tempted by the Musicom Lab MkV (over the Crocodile Tail because it has a display for preset names). I would use it to control the GM, my Ocean Machine, and a few analog pedals (Floyd!). It will probably be a few months before I have the whole setup rigged, cabled and running, I'll make sure to open a thread with my wanderings and results, and pictures of course!

Thanks guys, and be well.