GM40 fault? - Everything's gone very quiet

Hi

I've had my GM40 for just over a year now (just out of warranty and I didn't register for the 3 year warranty - doh ), and it seems to have suddenly developed a fault.

I hadn't changed any settings, but a couple of days ago, when I turned it on it [seemed to be] completely silent. But after a bit of experimentation I found that I was getting some signal out - if I turned the master volume to max and power to 40W! I was testing this by playing back some stored music on a looper in the effects loop. I could hear the music - quietly - through the cab and also through the Red Box out. But at more normal levels both were near enough silent.

So the problem is after the effects loop stage and before the Red Box output.

Does anyone have any ideas what might have caused this or what I could do about it?

Thanks!

Forgot to mention - the LEDs show no problems with the tubes.

Hi Steve, sorry to hear about your problem.  The Redbox output is taken from the very last point in the circuit just after the output transformer.  This is before the power soak switching so we can more or less assume it isn't the PS either, (barring some sort of short).   You have pointed out that the output valves seem to be ok according to the TSC but have you tried swapping out the V3 PI valve?  Just swap it for V2 and test in the loop as you have done.  See if that fixes things.

If not then the Fx Loop sits before the Preamp Line Out with the uController unit sitting between them.  As well as to the Preamp Line Out it feeds the preamp+loop signal to the active Master Volume control stage then to a variable gain buffer to set relative power amp input levels for the Clean and Ultra channels, then on to the power amp front end proper.  Can you check whether you are getting a normal signal out of that Preamp Line Out socket?  If you are then we absolutely know the power amp is getting that signal through to its front end stages.

One other thing that could be related, (it's a slim possibility), is that the uController has a Mute function which it applies via the Noise Gate.  It is possible some feature like this or even one of the level controls has become permanently engaged at low level due to software corruption.  If the valve is not at fault you could try applying a factory reset of the software, the procedure is in the manual.  Make sure to back up any custom patches you have created before you do this as they will also be wiped back to the factory originals.  Hope you got that!

Incidentally the H&K Standby system isn't as is commonly found.  Many of these work on the output valves by removing their HT while leaving the heaters on and that is not good for them, (see cathode poisoning).  In fact it is a more important killer of valves than leaving them on so the Master Volume is still your best Standby.  Valves don't "wear out" by leaving them powered up.  H&K's system gets around that problem by controlling the current in the output valves via the TSC.  That way they can leave them running hot and ready to go, with HT still applied and minimise their current to a trickle level to prevent poisoning.

Hi borbonbert,

Thanks very much for your help.

I tested the preamp line out and it seems to be working correctly. Then I swapped (what I assume to be) the V2 and V3 valves - the two glass 12AX7Bs. After that there was no signal from the preamp line out (or red box / cab). So, unless I've screwed something up in swapping the valves (I don't think I have), I assume one of these two valves is faulty and I've moved the fault up stream - before the preamp line out.

Does that sound right to you?

By the way, I also tried a factory reset (before switching valves) - which didn't change anything.

Cheers,
Steve

Hahaha!  Oh dear, my apologies Steve, I should have explained why I was asking you to do this a little more clearly.  The problem you may have is that the power amp uses four output valves which are driven by a Phase Inverter pair of triodes which is the job that the V3 12AX7 is doing.  It is part of the power amp and comes after the Preamp Line Out.  If V3 goes down then the whole power amp is screwed as V3 feeds all four output valves with their signal.

Your first test of the Preamp Line Out showed that the first two valves V1 and V2 are working fine and they are the same (99%) as the V3 Phase Inverter valve.  In order to check out the PI valve you just need to swap it with the V2 valve temporarily.  That way you are looking to see if you are putting a good valve in place of a faulty one.  If the power amp comes back to life we know the original V3 is faulty.

However, if it is faulty then putting it in the V2 slot temporarily will stop the preamp working just as you have found.  To prove it is that valve once and for all, leave the known good valve from V2 in the V3 slot and feed a signal back in through the Loop Return as you did in your original testing using your looper.  Don't worry for now that the preamp has stopped outputting, that is a good sign and does show the original V3 PI valve is most likely faulty.

With this "swap testing" it is always a good idea to mark your valves and replace them in their original slots after testing is finished.  Then just swap the one which you find faulty.  A small marking with a permanent marker will NOT harm the valve at all in any way, that is an internet myth and needs to be dispelled.  It comes from the fact that halogen bulbs for a car must not be touched with your fingers, but they are totally different as they have a special coating which is damaged by grease and dirt when they heat up.  Valves live in a plain old high temperature glass bulb which is not coated in any way.  They already have writing printed on them by the manufacturer for Pete's sake, (no it's not magic ink!   ).  It only serves to keep out the air from the vacuum inside.  There is nothing to damage with a little marker so V1, V2, V3 on their tops as you fit them is good.  Engineers have been doing this to valves since the dawn of the valve age without any problems.

Sometimes they may be selected for a particular parameter such as low noise or balance across both of their triodes, and those characteristics may suit a particular role they play.  For example, when you replace the PI valve it may pay you to get one specially selected for balance as it suits the way the PI works.  Most valve suppliers can do this for an extra £1 or so.

Hi borbonbert,

Thanks again for a very clear and helpful response.

With the 'faulty' (V3) value swapped with the V2 valve, a signal fed into the effects loop return does result in a normal output from the power amp. So that proves the original V3 valve is faulty. I'm so relieved!!! So I've ordered some replacements and I know what to do next time! Thanks so much.

Cheers,
Steve

Excellent result Steve.  As the tech savvy guys here would advise, 90% of all valve amp problems come down to the valves themselves and the solution in those cases is simple.  There are always the other 10% to sort out though and a little logical diagnostic work is always a good thing to tie down where the problem really lies.  Most people would say just swap out the valves first and if that doesn't fix it move on to looking for something deeper.  That usually means doing dumb things like swapping out every capacitor because they "wear out", (they don't!), or resoldering every joint on a PCB because "modern boards are riddled with dry joints", (they certainly aren't!)  Swapping out for a known good valve is fine if you have a spare valve hanging around but can often be done with a bit of common sense as you have, by swapping within your faulty amp to see how that changes the problem.  If not you have to pay for it and I don't sit well with the idea of spending money unnecessarily, mine or anyone else's, but today's world says that isn't the right way to think so, what the Hell do I know?

Using a bit of logic to find ways of either proving or disproving that something is so before you put your hand in your pocket is not hard, it just requires clear thinking.  That's where the problem lies today!  The people who sell us the stuff have made us believe that just paying up as we fiddle around is the logical way to do things.  It isn't!  At least not to someone who actually knows the field well.  A good tech would never think that way.  They would apply their own specialist knowledge of the unit as a technician as to its most common faults and a logical set of tests for exploring where the fault most likely lies, then using what they find they only need to dig in a small area of the circuit to fix it.  Charging for new valves which didn't solve the problem, a new block of caps swapped out at random which didn't solve the problem, then their time for resoldering the whole amp is something I have known people be charged for.  To my mind at least, that is a rip off and the sign of a very poor technician.  An engineer generally has no experience of a wide range of specific units, that is the superior experience of the technician.  But he has to work with a logic that is above that needed by the tech as the job of the tech is simply to fix THAT amp and he does not need to pick out the details of how and why every component is there.  Whereas the engineer has to understand THAT MODEL of amp in its entirety to be able to improve, modify or redesign it.  It's not higher or lower ranking, it's just a different mission statement and way to achieving that.

Congrats to you on starting that way even though you may not have any background in this.  Your Loop test with your looper signal was a good bit of diagnostic work and was logical in this way.  It certainly homed in on the power amp.  And the Redbox test too was helpful.  Always keep that level of analysis in your head when you are working with your gear, it's the savvy way to do things.  It's so depressing to just get post after post saying "So my amp has just died, so what could it be?"  (I didn't add the usual obligatory "dood" in there this time.  I don't think I need to nowadays.  )  And then when people like me give simple steps to take to investigate we get a reply which clearly shows the poster has not read our advice and hasn't performed the tests.  Where do you go in situations like that?

Anyway, well done to you, problem solved.  Now get on and rip up the room with your GM40D!  That's what they are really for.

(EDIT: And just for the record I would add here that this was not really a problem with the GM40D in any way, this was a problem with a valve in a GM40D. That's a different category. The amp did not really develop a fault, a replaceable "service" item just went down as it would in any valve amp.)

Just had a similar problem Good enough reason to carry a spare set of tubes and know how to change them (easy with the GMD40), and luckily I had spare 12AX7 to swap in!