GM 40 sound drops

Hi, Waddy from Belgium.

Purchased a new Grandmeister Deluxe 40 a few months ago. Worked fine in the beginning, but when I played it the next 2 gigs I had issues of the sound dropping, awful distortion, and a rather cracking noise on every setting.  Very annoying when playing live, but I did manage to recover the sound by standing it by for about a minute.
Next few repetitions of the band and whoops, same issues, even today again. So now I removed the cover and checked the valves (even tapped them). But I do notice the next couple of things:

1. In 1, 5 or 20 watt soak mode the middle 2 red lights light up at the back (the check-if-the-valves-are-still-alright-controls), however, at full 40 mode none of the red lights work (and yet, the amp even now works well without those red lights signalling).

2. When standing in front of the amp, it seems like the 2 right EL84 valves (Chinese!) light up a little less than the 2 left ones.

3. When standing in front of the amp the left (bigger) transfo is warm, while the right (smaller) one is not warm at all, even after an hour (don't know if this is normal).

I'm considering changing the Chinese valves by better ones, unless someone has any ideas what could be the issue? Thanx in advance!

Hi Waddy.  Welcome to the forum.

The problems you are having are not unusual.  Here are some answers to your questions.

1. This is perfectly correct.  In going from 40W to 1W/5W/20W mode the inner two valves are removed from the circuit.  The inner two LEDS of the TSC on the back light up to show they are not active.

2. It is not unusual to see valves which glow at different levels.  This is usually down to just the visibility of the heater elements in amongst the other valve parts.  You could try swapping the two from the left to the right making sure to keep the outer two in the outer slots and vice versa.  That is swap 1 with 4 and 2 with 3.  See if the different glow moves with the valves.  If it does then there is no problem.

3. The Tx on the left is the mains transformer.  This is working passing current all the time whether you are playing or not.  It does work  a bit harder when you play but it will be warmer than the right hand output transformer in general.

Full marks for observation but there is nothing unusual in what you have seen.  The symptoms you describe are fairly common with new amps and their original valves.  I have spoken to H&K about why they use cheap valves in the past and their answer was that they put in cheap Chinese valves from the factory as they know that nowadays everyone will swap them out almost immediately just because they have their own view as to what is the best valve to use.  It makes no sense to use a more expensive valve putting the sale price up when the buyers are going to ditch them more or less immediately anyway.

As an electronics engineer I can tell you that there is no significant sound difference between valve makes as long as the valves are all new and their specifications match.  Sound differences are down to differences in specification parameters for an individual valve and they are well defined for each valve type, 12AX7, EL84, EL34 etc.  There is good unquestionable engineering evidence out there now to demonstrate this.  However, musicians are notoriously easily led and really want to believe in the existence of "mojo".  That is anything that makes a profound difference to your playing which cannot be analysed scientifically.  ("Magic" in other words!)  In the odd case where it currently exists, mojo is just engineering we have not yet tied down, and valve types are not one of those cases.  In this situation that is, frankly, so much bollocks.  Every aspect of valves and how they do what they do has been known and catalogued for many years and the spec of the 12AX7 or the EL84 dictates that all valve makes must all comply and have the same characteristics within well defined limits to be sold as that valve model.  It also means that all valves which fall within the spec will behave the same in circuit so must sound the same.  No one has ever been able to point out a single parameter of a valve's spec which could be different enough to change the sound for that make of valve.  I would urge you not to be fooled, look for the real concrete evidence for either side of that argument.  If it isn't on show then ask for it!  If the person (me included) can't show it to you then you know what to believe.

Reliability and lifetime within spec is another issue.  New valves are tested on a statistical basis.  Samples are extracted on the line and the number of failures is those tested must be within limits.  There will always be faulty valves which slip through, I have personally had one or two in my time, but these will be minimal.  The cheapest Chinese ones may perhaps not be tested as rigourously, they may be closer to the maximum permitted failure rates in test, so a few more may not be to spec when supplied and they are certainly prone to early demise compared to more expensive ones.  This is not a "sounds different" issue but a reliability one.  I would try these following tests.

Assuming that you experience the sound drop on all levels of the power soak we can assume that it can't be an issue with just the inner pair of EL84s as they are out of circuit when at 1W/5W/40W settings.  Swap your outer pair of EL84 valves for your inner pair and test the amp again at a lower power soak setting.  That's swap 1 with 2 and 3 with 4.  If the problem goes away then you have a problem with the output valves which were originally in the outer slots and are now in the inner slots.

Separate to that test (not at the same time), try swapping the 12AX7/ECC83 preamp valves around.  We will not completely solve the problem with this way of testing but if the sound problem changes then we can say it must be down to the valves which we moved.  Mark them 1, 2, 3 with a felt tip pen to show where they were originally situated so you can return to that after the test!  Despite internet gossip this does absolutely no harm to the valves in any way.  Engineers and techs have done this since valves were invented without any problem.  Swap V1 with V2.  If the noise changes significantly in level or character then it is one of these valves causing it.  If there is no change put the valves back in their original sockets.  Now swap V1 and V3.  Once again, if there is no change then put the valves back as they were.  A better way to conduct this test is to temporarily swap in an extra valve which you know to be good, perhaps from another amp.  That is more specific and can identify an individual faulty valve but not everyone has a spare at hand.

It is a good idea to swap out those original valves as soon as you find them becoming irritating, as long as you are able to afford it and that feeling is genuine and not just hankering after something you have never heard but which people rave about.  It has to be said that there are thousands of people out there using the original Chinese valves who prefer their sound to the hyped up NOS Mullards with the mystical qualities at £70 a pop!  My sincere advice is to not listen to the crap talked about "glassy transparent mid ranges" and "elfin gossamer top ends", it is just guitarists wanting to be seen as "gurus" by others in the guitar community.   None of that talk has any evidence to back it up and that would be easy to get on a test bench if it exists.  Just go for something which is reliable, not overly expensive and is common to pick up.  Then, if it sounds good to you, stick with it and don't chase rainbows!  Remember, valves do age - eventually after a long life - and when the sound goes a little off it doesn't mean that that valve make is a bad one because another now sounds better.  It just means that a valve in spec sounds better in a circuit designed for that spec than one which is now not up to spec.

Hi, Bordonbert.

First of all: Thank you very much for your answer. I haven't had the time yet to perform the actions you propose. However, another strange thing happened today while trying to play for a couple of minutes:

When in 20 watt (also in 5 and 1 watt-mode) playing goes fine, until I change channel on my (Original HK) footswitch, then the amp automatically changes to 40 watt-mode! This happens with the Original presets as well as the self-made presets. Really weird, of course, because now I have a sound raise instead of a sound drop, don't want to encounter this in a live setting.

But in the next days I will try your suggestions, who knows I'll discover a bad bulb. I'll keep you informed. Thx again!

That definitely doesn't sound like a valve problem!  I would start with the following procedure before the other tests I suggested to get the amp in a pristine state.

If you don't already have it, get Fredo's control app, set it up to talk to the amp via a midi interface and back up any of your own custom created patches.  If you already have this great, if not and you aren't sure as to how to go about this, come back to us on it and we can advise.  You MUST do this first, or at least take a copy of them manually, otherwise your own patches will be permanently lost.  Now perform a factory reset.  This procedure is simple to do and is outlined in the GM40 manual in section 8.4.  As well as resetting all of the internal software to the factory original state it will reset the 128 patches and the MIDI setup configuration so do take those details down before doing it.  If they are extensive you will be able to upload them back into the amp afterwards using the same app.  It is known that the software can occasionally become corrupted in use and can contain slight glitches which cause effects like the one you are seeing.

You might want to rule out this too.  Have a look at the Manual section 6.2, the second "Heads Up".  It says:

H&K GM40 Manual wrote:Why a Global Power Soak? Many GrandMeister 36 users wanted this mode, which is why GrandMeister Deluxe 40 now comes with this feature on board. Global Power Soak mode has the following advantage when activated via the app or on the amp itself: all Power Soak settings stored in the presets are ignored and the amp works globally for all sounds and presets with your choice of setting, from 40 watts down to 1 watt. This comes in handy, for example, if you want to practice at home at a low volume regardless of the levels programmed in the presets for your live setup.

It may not be the problem but it won't hurt to rule it out before any other tests.  Is it possible that you just have the Power Soak selection set to the Global setting rather than to be applied with each patch?  The manual in that section includes a diagram and details of how to change the Global Settings.