Grandmeister 36 problem, please help! Significant output drop

My Grandmeister 36 is acting up, and I wanted to ask if anyone here has had a similar issue or might know of anything I might can do.

One night in the middle of a gig my volume completely dropped. Clicked my tuner and saw I was still getting a signal, but I had to put my ear right up to the cabinet to be able to tell any sound was coming out at all. My full-on distortion was coming out too weak to be heard at all over the PA, and was coming out as a very weak clean tone rather than distortion. Power off and back on fixed the issue. All was fine until I played that same venue again months later, and the same exact thing happened a second time. Long story short, there are four venues I play at where the output drops every single time I play there, usually towards the end of our set, and everywhere else the amp works fine. My band mates' amps all work fine at these venues. Power off and back on has always fixed the issue until this past weekend, when no matter how many times I reset the amp, the output kept dropping every six or seven minutes, which completely killed the show. I ran straight to the amp, no pedals no wireless, and the problem continued, so I figure the problem has to be the amp. Got the amp home and into the rehearsal room and played for four hours straight, and could not make it repeat the problem. So if I take the amp to a tech, I can't demonstrate the problem to him, because it's working fine (for now), EXCEPT at these same four venues.

I don't know the first thing about electronics and how amps work. Could it have something to do with the power at these venues? If that was the case, wouldn't it effect the other amps on stage as well? Seems bizarre to me that the amp is completely useless one night and the next night works flawlessly.

I googled the problem and learned that maybe I should replace the preamp tubes, so I have some JJ 12AX7s on the way. Unfortunately, I have no way of knowing if that fixes the problem or not until I go play at one of the four aforementioned venues, because as hard as I've tried at home to make the output drop, it's never done it. Googling the problem, it seemed like other folks experiencing a severe output drop were experiencing it constantly, not intermittently like me. I'm just very confused as to why the amp only screws up at a handful of venues, and then the next night works just fine. Also terrified that it will start screwing up at all the gigs. I absolutely love this amp, so I would much prefer to fix this thing rather than buy a different amp. In fact, I like it so much that I'd probably be dumb enough to buy another one even if this one is completely crapping out on me after only two years.

If anyone has any ideas what the problem could be, or has any insight they'd be willing to share with me, I sure would appreciate it.

I can't remember hearing of a problem like this before but I doubt if it is a fault in your amp.  If it works everywhere else then it is fine.  This is an interaction between the amp and something in the environment in the venues where you are having your problem.  And your friends other amps may be less sensitive to whatever is causing it by their simpler configuration.  The GM is a more sophisticated (some would say complicated) type than most Fender/Marshall types.  There is a lot of computer processing and low voltage logic and switching going on.  That said, that doesn't help you does it?  Let's look at the few facts we have.

The amp works in most places - suggests there is probably no problem with the amp by itself. (But not definitely!)

When it happens it is a lasting condition until you reset it - something inside is being sent into a muting condition but not permanently as a restart resets it. (Maybe something like one or more of the digipots or the power soak is being set to an off condition.)

The first idea has to be the mains quality at these venues.  Are they in or near industrial areas?  If they are then it is possible that heavy equipment kicking in can cause serious spikes or dips on your mains.  It is even possible that the venues themselves have something electrical and heavy on current which starts up late in the evening.  Do they power up electric heaters for example?  If so and it is on the same circuit as your amp's supply that will drop the voltage your amp sees. The TSC (Tube Safety Control) on your output valves senses the raw 22V DC line and presumably takes safety actions if it drops too low for too long. If you are experiencing brown outs it is possible that is kicking in and turning your output stage off.

The second idea is that they may have faulty earthing conditions on their mains supply.  I would recommend EVERYONE who plays through electronic equipment to invest in a safety device such as a Residual Circuit Breaker (Generic RCD).  These are cheap nowadays and will detect a number of earth faults and instantly kick out the supply if it finds one.  They can be bought in the form of a mains plug with the detector built in which can be fitted to the end of a distribution strip to power your own block of gear.  That way everything you use gets protected, and more importantly so do you.  With the high voltages in a valve amp an earth fault could be lethal!  I personally have come across a situation where the wall socket I was using had an earth which was 50V higher than the one a little further round the room used for the PA.  I found out when I went to sing.  This was in a secondary school with pupils using these sockets during the day.  I immediately bought one of these devices on the strength of that.

Third, how are you controlling the amp?  With some sort of MIDI footpedal over a long cable it is just possible that that may be upset by some sort of strong interference if it is faulty maybe regarding its screening or ground line.  Once again, I have personally known situations where someone using communication lines in an absolutely incorrect way (single sided use of RS422 differential lines) works perfectly until some rare random condition occurs and signals get corrupted.  When I investigated the installation, what do you know, there was no problem again.

There are steps you can take to try to get to grips with this.  Unfortunately it will most likely mean getting onto the mains of at least one of the venues, preferably around the time you have had problems.  Your simplest start is to swap out as many cables as you can.  Every single one you use right down to mains cables!

My own feeling is that, as it affects a number of venues, it is less likely to be a mains supply problem but that said....  Can you get someone to carefully measure the mains supply voltage at your socket at one of the venues with a meter?  Maybe even late in the evening when the problem is likely to happen.  Your amp will work down to a quoted -10% supply voltage.  That's usually sufficient but can be not much in the real world as most power companies keep a reserve and only guarantee your voltage within a specified range.  I don't know the USA standards very well but I have a paper showing how complex it is to interpret.  The tightest tolerance seems to be around 5% at the service, i.e. as supplied to the venue, (remember that is the TIGHTEST tolerance, you may be on a circuit intended for less rigourous use if the venue is converted from say commercial).  There will be a drop inside the venue too depending on how good their wiring is inside which is not specified in the specs.  I understand this is usually taken as around 3% over the general network plus 2% over a feeder branch.  So that is about a 10% drop which is considered acceptable.  A little more than that because of poor wiring in the venues could push it over the edge.  And you may just have an amp which is unusually sensitive to this even though it is strictly within the 10% H&K spec!

If you could catch the problem when it occurs then you could try looking at the LEDs on the back to see if they are all lit.  This could point to the amp's TSC circuit kicking in and turning off the output valves.  (That sounds definitely possible.)  Then you could quickly make changes such as varying the Volume control, selecting a different channel, selecting a different power soak setting, in order to see if there is just a single function on the amp which will unlock the muted condition.

I agree with you, an intermittent problem is the worst case to troubleshoot as you have to make it occur before you can do anything about it, and in your case with these venues as the triggering factors you are up against it.  And it could actually be that you really do have a fault in your amp.  In that case it would take a decent tech to explore and diagnose it.  Find one who has an autotransformer controlled supply where he can drop the mains voltage supplied to the amp down to the level where it may cause the drop out.

Can you verify that the 12AX7 tubes solved your issue? I was in panic mode when my sound cut to almost nothing similar issue to yours. Changing out the Pre-amp 12AX7 tubes fixed the issue for me fully. Its seemed a strange thing...as normally I experience different issues with most amps when they go... buzzing, clipping and farting out.. or FX loop issues..diminished tone. The GM36 seems to go almost silent when experiencing tube pre-amp issues, at least mine has.. this time. Both tubes were seemingly in good working condition upon visual inspection, and when tapped on, so again i was surprised it was the pre tubes. Hope you are fixed!

Cheers!

Grim Latimer

Grimmy3-3-3 wrote:Can you verify that the 12AX7 tubes solved your issue?   I was in panic mode when my sound cut to almost nothing similar issue to yours.  Changing out the Pre-amp 12AX7 tubes fixed the issue for me fully.  Its seemed a strange thing...as normally I experience different issues with most amps when they go... buzzing, clipping and farting out.. or FX loop issues..diminished tone. The GM36 seems to go almost silent when experiencing tube pre-amp issues, at least mine has.. this time.    Both tubes were seemingly in good working condition upon visual inspection, and when tapped on, so again i was surprised it was the pre tubes.  Hope you are fixed!

Cheers!

Grim Latimer

Hi

It seems I have similar problem with my GM36. Is that simple work to change those pre-amp tubes? Thanks

Yes it is Carcass. The valve swap is easy and safe on the GM36. There are a couple of things to watch for.

You must not remove the end cheeks completely, that affects an anti tamper strip inside the amp which shows it has been opened and voids any warranty. The method is to remove the top screws and the middle level rear one completely leaving the front middle level one alone as it only holds the silver handle onto the black end cheek. Then comes the key part. Loosen the bottom two screws only a few turns so the end cheek can lean outwards a little. The top cover can easily be taken out then. Don't worry too much about that, there is a lot of spare thread on those bottom ones, just make sure to not screw them further than you need to.

When you remove the top cover you will expose the thin LED PCB strip along the front of the acrylic front panel. Have a look at how that and its metal retaining clips are fitted and make a mental note so you can ensure it is that way when you put the top back on. Also note the cable on the right hand side of the front screen where the power is carried up to the LED strip and be careful not to nip that with the end cheeks as you refit.

Removing the valves is pretty easy once you have the clips and cover off them. Just grip them firmly and 'vibrate' them as you pull upwards. Don't wiggle them from side to side so much you bend the contact pins. There is no problem with handling valves with clean hands! The idea that you must wear gloves or swab them down with solvent afterwards is a daft internet myth that has grown over recent years. Everyone says it so everyone "knows" it must be true! Valves are not halogen bulbs. There is no coating to become damaged. They are just a hard glass cylinder which does not care about the odd fingerprint or even being labelled with felt pen marker for identification. They all already have printing on them anyway. (I guess that must be an ink made of magical pixie dust under a leprechaun moon down in Fairy Hollow.) Valve engineers and technicians have been doing that for 80 years without any issues and the idea that the same valves that we use are going to be treated so fussily when worked on in the field by the military with their hands is a joke. They got us through WW2 without crapping out all over the place didn't they?

Carcass77 wrote:

Grimmy3-3-3 wrote:Can you verify that the 12AX7 tubes solved your issue?   I was in panic mode when my sound cut to almost nothing similar issue to yours.  Changing out the Pre-amp 12AX7 tubes fixed the issue for me fully.  Its seemed a strange thing...as normally I experience different issues with most amps when they go... buzzing, clipping and farting out.. or FX loop issues..diminished tone. The GM36 seems to go almost silent when experiencing tube pre-amp issues, at least mine has.. this time.    Both tubes were seemingly in good working condition upon visual inspection, and when tapped on, so again i was surprised it was the pre tubes.  Hope you are fixed!

Cheers!

Grim Latimer

Hi

It seems I have similar problem with my GM36. Is that simple work to change those pre-amp tubes? Thanks

I had this trouble on Tuesday this week and just replaced the 3 Preamp Tubes. Worked like a Charm. So now Im a memvber of the Forum and Thanks to all those who have taken the time to share expertise. I will be an avid reader here!

bordonbert wrote:Yes it is Carcass.  The valve swap is easy and safe on the GM36.  There are a couple of things to watch for.

You must not remove the end cheeks completely, that affects an anti tamper strip inside the amp which shows it has been opened and voids any warranty.  The method is to remove the top screws and the middle level rear one completely leaving the front middle level one alone as it only holds the silver handle onto the black end cheek.  Then comes the key part.  Loosen the bottom two screws only a few turns so the end cheek can lean outwards a little.  The top cover can easily be taken out then.  Don't worry too much about that, there is a lot of spare thread on those bottom ones, just make sure to not screw them further than you need to.

When you remove the top cover you will expose the thin LED PCB strip along the front of the acrylic front panel.  Have a look at how that and its metal retaining clips are fitted and make a mental note so you can ensure it is that way when you put the top back on.  Also note the cable on the right hand side of the front screen where the power is carried up to the LED strip and be careful not to nip that with the end cheeks as you refit.

Removing the valves is pretty easy once you have the clips and cover off them.  Just grip them firmly and 'vibrate' them as you pull upwards.  Don't wiggle them from side to side so much you bend the contact pins.  There is no problem with handling valves with clean hands!  The idea that you must wear gloves or swab them down with solvent afterwards is a daft internet myth that has grown over recent years.  Everyone says it so everyone "knows" it must be true!  Valves are not halogen bulbs.  There is no coating to become damaged.  They are just a hard glass cylinder which does not care about the odd fingerprint or even being labelled with felt pen marker for identification.  They all already have printing on them anyway.  (I guess that must be an ink made of magical pixie dust under a leprechaun moon down in Fairy Hollow.)  Valve engineers and technicians have been doing that for 80 years without any issues and the idea that the same valves that we use are going to be treated so fussily when worked on in the field by the military with their hands is a joke.  They got us through WW2 without crapping out all over the place didn't they?

Cyclecrazy wrote:

Carcass77 wrote:

Grimmy3-3-3 wrote:Can you verify that the 12AX7 tubes solved your issue?   I was in panic mode when my sound cut to almost nothing similar issue to yours.  Changing out the Pre-amp 12AX7 tubes fixed the issue for me fully.  Its seemed a strange thing...as normally I experience different issues with most amps when they go... buzzing, clipping and farting out.. or FX loop issues..diminished tone. The GM36 seems to go almost silent when experiencing tube pre-amp issues, at least mine has.. this time.    Both tubes were seemingly in good working condition upon visual inspection, and when tapped on, so again i was surprised it was the pre tubes.  Hope you are fixed!

Cheers!

Grim Latimer

Hi

It seems I have similar problem with my GM36. Is that simple work to change those pre-amp tubes? Thanks

I had this trouble on Tuesday this week and just replaced the 3 Pre-amp Tubes.  Worked like a Charm.  So now Im a memvber of the Forum and Thanks to all those who have taken the time to share expertise.  I will be an avid reader here!

So, it's working now and I'm happy. I've noticed that one of 3 nearest to front panel tubes is lit with 1 light instead of 2 like two others. So, I just ordered and replaced that one. But, be aware, you can't see what's going on with the right valve as it's covered, so you need to remove the amp's side panels and then unscrew the cover of the pre-amp valve. Anyway, thanks so much to all here for the help.