My new GM40 quit working tonight

Hi all,

I've had my GM40 for a little over a week now and when I first plugged it in, it seemed kind of noisy at idle, more so than say a PRS Archon 50 head.  Tonight, however, it totally freaked out.  Approx. 60 seconds after taking it out of standby, the GM40 started making a lot of loud crackling and popping noise through the speaker.  I quickly switched to standby, but the noise continued for another 10-15 seconds or so, so I killed power all together.  After a couple more identical attempts, I unplugged everything and let it sit for an hour.  With only the guitar and speaker plugged in the popping and crackling started again after a few seconds.

Any suggestions?

First thoughts are that it sounds like it could be valve related Autotunz.  Do you have a spare set of valves handy that you can substitute in to check it out?

EDIT: Should also add... Does it still play musically while the noise is going on? If so what do the TSC LEDs on the back tell you when you push your pick in?

Thanks for the quick response! I swapped the three preamp tubes - same problem. I don't have any spare 84's to try

When I turn it on in Standby, there is sometimes a little crackling noise. When I turn off Standby, all is quiet for about 30-45 seconds then it starts popping and crackling LOUD. I'll check to see if music still plays through. It's so loud I'm afraid it's going to blow the speakers, so I always shut it off immediately.

Haven't checked TSC's. I'll do that and report back. Can I test it without the speaker connected?

Thank you!!

I checked the TSC just now.  When pushed, all LED's flash 11 times, then the one on the far right (when viewed from behind the amp) flashes one more time.  Then both middle LED's stay ON. This was done with no speaker connected, so Power Soak was probably kicking in.

Just tried it again with a speaker connected and Power Soak turned off. All LED's flash 11 times, then the one on the far right flashes another 5 times. Then all 4 LEDs stay on. This is all done in Stand By mode, and the speaker is still crackling, just not as loud. IN THE STANDBY MODE!!??!

Another update: With a speaker connected, standby off, guitar plugged in, some popping noise, yes, the guitar plays through.

I pressed the TSC and got the same read out....11 Flashes, followed by 5 flashes on the right hand LED, then all that one stayed on.

I ran the TSC a couple more times, and now it's quiet and all LED's have turned off???

Do I have a power tube starting to fail?

Try swapping the outer pair with the inner pair left to left and right to right. If it is a power valve problem causing the imbalance then the TSC LED flashes should swap round with the valves. That means the extra 5 counts should shift to the inner right valve. If you have a single duff valve in the outer pair which is the pair constantly on for all powers, (the inner pair switches off for anything less than 40W), it should move to the inner pair which gets switched out by the power soak and your lower powers may then be ok, or at least it could sound significantly different backing the idea up.

Are you also able to try taking the signal from the Fx Loop Send into another amp's Loop Return to check the preamp section is ok? Then try the opposite, the other amp's Loop Send into the GM40D's Loop Return. That way we should be able to tell if it is only a power amp problem, only a preamp problem, or both pointing to something wider like maybe PSU.

You could also try swapping V2 with V3. V3 is the PI stage which is part of the power amp. If that valve has gone west then it could cause the noise you are experiencing. Putting in the preamp V2 valve temporarily would give the PI a good valve which would stop the noise from that stage. A noisy V3 now in V2 would still give rise to noise of course but it would be stopped from feeding through from the preamp once the Master Volume is fully down.

I swapped locations as you suggested and now the 2nd LED from the right flashes another 5 to 7 times. (All flash 11)  I had earlier swapped all preamp tubes for new - no change.  The noise continues, although it seems to be less often now.  Last night and early this morning it was almost constant.  Now I have to wait for it to start acting up again....it's intermittent.

Bad tube?  If I try an new tube, do I need to replace 2?  If so, which other one?

Thank you sooooo much for your help!!  I really don't want to have to send it back if it's something relatively simple.

The fact that the LED extra flashes has moved with the valve is a good sign and shows that the matching valve is off spec. That may not be the whole of the problem but it is a good indicator that you need to change out that pair of valves.
Does the noise now go away if you leave the wandering valve in the inner position and play with the power soak dropped to 20W or below? That should remove the inner pair with the bad valve and its partner from the circuit.

You won't need a pair of power valves to test with a replacement, just the one good one will do so you could scavenge one from another amp. But you would be better buying a matched pair if you find you need to replace one, in order to make sure of the balance across the output transformer. Personally I would prefer to buy a pair now to have them on hand as a replacement in the future. That way you don't end up having down time waiting for new ones to arrive when a valve eventually lets go. I am against spending money needlessly on amps and equipment but this is a situation where it is well worth investing a little to be ready for a problem.

Others may disagree (and frequently do) but I would go for something easy to get, not expensive, and common! The idea that different makes sound very different is a myth. Any differences which do exist are very subtle and don't come into it for a live or bedroom situation, only studio recording could really allow the differences to be heard. They aren't even obvious on the test bench so they won't be obvious at all to your ears just by swapping by hand no matter how good you think your hearing is. Most of us here are perfectly happy with common or garden JJs which you should be able to get anywhere.

I'll try swapping EL84's. In addition to the tube in question, which other one do I swap?

I just checked again before taking off to go buy new tubes and here's the latest status.

Turn the amp on, in Standby mode.  The amp starts making popping and crackling noise while in Standby.  

Turn Standby off and the amp intermittently makes same crackling noise.

TSC check shows all lights off.  Amp still makes noise intermittently.  

Tried switching on Power Soak and there's a loud electrical pop each time you click each level.

Still makes the same noise with Power Soak engaged or disengaged.

Well done with the test.  If the noise is the same with the power soak engaged or not then I think we have no choice other than swapping out a pair of valves.  As the problem moves around with one of the valves it suggests that there is no problem with the TSC or biasing themselves.  The problem really seems to be attached to one of the valves at least primarily.

The power amp valves are arranged in pairs and they need to be kept in those pairs.  If you buy a matched quad of valves then they can go anywhere as they are all the same.  Once you have used them for a while it is best to consider them as pairs and then always keep them together with their partner.  If you replace only a single pair then you must either swap out the inner pair (2 + 3) or the outer pair (1 + 4) in the valve lineup of [1 - 2 - 3 - 4] meaning the pairs are [O - X - X - O].

That is why, when you kick in the power soak and reduce the power, the two inner LEDs (2 + 3) turn on showing that the inner pair has been turned off by the amp halving the power out.  In general it is important for best tone and ease of working for other components to keep the valves driving the two opposite sides of the output transformer supplying equal DC current when idling and equal signal voltage when driven.  That way the DC currents cancel out exactly which we want and the two sides of the signal are kept as equal at the speaker as they are from the guitar.

Once your amp is stable and working it isn't a bad idea to mark them on the glass envelope with a permanent felt tip pen for identification.  There is a load of horsefeathers on the internet about how this is a bad thing to do, it isn't.  Valves are not the same as halogen bulbs!  They are in a plain old hard glass tube and the ink will have no damaging effect on that at all.  I wouldn't want to cover them in fish and chip grease of course, so they are best kept cleanish but a little dirt, dust or ink won't compromise them.  I would use a system such as numbering them as a pair and as an individual kind of "pair num . valve num".  So 1.1 and 1.2 would be for pair 1, valves 1 and 2 say in the outer slots.  The others in the inner slots would be 2.1 and 2.2 of course.  A new pair would come in as 3.1 + 3.2 and be kept together.  That means that in either pair the first number should always be the same.  It's only to be able to identify them individually and to keep them together as pairs.  If you just number them 1, 2, 3, 4 you will be surprised how easily you can forget which went with which once they move around a couple of times.  That would mean that the numbered valves would look like [1.1 - 2.1 - 2.2 - 1.2] when they are fitted in the amp.

You need to swap out your damaged valve with the extra TSC flashes and the one that partnered it which would originally have been the left hand outside when it was on the right outside.  Assuming you have swapped both of them to the inside to test it would mean both inner valves.  I will say try to keep it like that but this isn't too important in your case as the three remaining valves all show the same current via the TSC setup so they are still effectively balanced whichever pair you choose to keep.  Also, keep the third valve for a spare as it still looks good, you never know when it may be useful.

Thank you again, Bordonbert, you've been a huge help!

I'm a little hesitant to run out and spend $50-100 on new tubes for a brand new amp. I'm still in the window where I can simply return it to the dealer and get a new replacement.....

Here's the latest flashing sequence, looking at the amp from the rear. I have move the tubes, but kept them in pairs. The questionable tube is on the far left.


11 + 9 11 11 + 1 11 + 1



Should I try (buy) new tubes or return it?

Thank you!

This thing is driving me mad!!!!

I pulled the top just now and pulled each tube one at a time and reinserted it, making sure it was fully seated.

Turn on results in light crackling with Stand by engaged.

Turn off Stand by and more light crackling, but noticeably less than before.

Testing TSC now shows

11 + 1 11 11 11 + 1

There's not much popping and crackling at the moment, but the amp continues to make strange sounds through the speaker. High pitched, screechy sounds, fluttering sounds, random electrical type pops and static like noises. Sometimes loud, sometimes soft.

I really don't like the way this problem is changing on you, especially the change in TSC readings.  If it is brand new when you got it it could be that one of the valves has had some swarf inside which has needed to be burned off before it settles down.  I have known that to happen before.

Is this a new amp still under warranty?  If it is I would seriously arrange to have someone take a look inside for you.  They would be able to substitute for the valves as well to rule that out instantly.  Shouldn't cost you anything if it is still a warranty job.



EDIT: Just seen I had two posts to read not just the previous one. It IS a warranty job. If the dealer isn't too far from you I would go there and show him the problem. They can then advise you as to what would be the best course of action. This may be something that clears itself up by using it, or it could respond to a change of valves, but there is always the niggling doubt that it could be something more significant which would let go fully in the future. If you have registered the issue with your dealer and demonstrated it to him your case is stronger.

Even better than under warranty, I can still return it to the dealer for a replacement.  It's not a local dealer, however, I'd have to ship it.

I've had the amp for less than 2 weeks.  It worked fine (although always a little noisy) until a couple of nights ago.

The TSC values have changed again and it continues to make strange random noises either in Standby or full operation.  Pushing the Power Soak buttons causes a HUGE screeching noise until deactivated.

I think I'll be shipping it back today....

Thanks!

Yes, I would. The screeching when the Power Soak is engaged is definitely not good and doesn't sound at all valve related to me. It's a shame to have to ship it back and spend the time waiting but I think it would be best for your own peace of mind. This has got beyond a valve replacement now.

Boxed and ready to ship. I can't thank you enough for all your suggestions and tips. What a valuable asset you are to this forum!

We'll chat again.....

I received my replacement amp yesterday and so far so good.  This amp is much quieter and no crazy noises.  Thank you again for your assistance in troubleshooting.

I do have one question....

Every time I turn the amp on, it automatically turns on the delay and built-in FX (chorus, phaser, etc.) whether I have the foot switch plugged in or not.  

If the foot switch is not plugged in, is there a way to turn off the delay and FX without having to plug in the foot switch?  I can't find one.  

When I do plug in the foot switch, the lights are not illuminated as though the delay and FX are not engaged, however, they are engaged. I then have to cycle it on and back off in order to actually turn it off.

Glad the new model is behaving better.  There was obviously something fundamentally wrong in the first one which went beyond just valve swapping.

The FSM432 is a great footpedal (if priced in the upper end of the market as is most H&K gear in my opinion).  It is important to appreciate that settings you change from the footswitch are stored in the amp not in the footswitch itself.  The FSM stores nothing except its own settings.  The amp will remember the last settings you kicked into action from the FSM.  As I remember it the amp will also remember the settings applied to each individual channel providing there is no other system overriding that such as...  With the FSM in Stompbox mode the effects settings are universal to all channels (according to the manual).  When the GM36 was working without a footswitch it remembered any settings on a per channel basis so I would check that with your GM40D.  Just let it run up without the FSM connected and then manually turn off the effects on a particular channel.  If this aspect is the same the effects should kick back in when you manually change channels, and drop out again when you switch back to the first one.  Each channel remembers the settings for its own effects.  I could be wrong on this aspect, the manual is not absolutely clear on this to my mind, but it seems logical to me that it would be the same and is worth checking out.

To manually access the effects settings you simply press the Fx Access button and it should start flashing.  The controls are now switched to manipulate the effects themselves.  Turn the Intensity fully down and they should be all off except Reverb which has its own permanent control.  Press the Fx Access button again and you are back in normal mode and the effects should be cancelled for that channel, hopefully the other channels' effects settings will not have been affected.

Can anyone else who owns a GM40D help with this aspect?  Am I talking bollocks here?  (What, again? )


EDIT:  Should add that your light problem is not unusual if you are plugging the footswitch in "on the fly".  With most digital control systems the accurate setting up of other connected units is done at boot up.  If the footswitch is connected when the amp is run up it will initialise everything to include it.  If it isn't connected right from the start it sets up without it and plugging it in will not necessarilly initialise everything correctly, if at all.  I have the same problem with my Marshall JVM.  It isn't unusual at all.  This is not a M$ Plug 'n Play system!  It is a very basic entry level microprocessor which is chosen to perform only the tasks that we need it to and there is no scope for refinements like hot swapping of attached gear.