Gm40d DEAD

I’m hoping this is an easy fix but my gm40d which I’ve had running with no issues since purchase new when they were just released.

Left the house today and all was well then on returning home power the beast up and there’s nothing. No blue lights, no tsc. At first I thought it was the mains lead fuse but I tested the mains lead on another device and that is ok.

Where is the line fuse? Do I have to take the amp apart to get inside?

As I say it’s been working well with no issues at all until this afternoon.

It’s out of warranty sadly as stupidly I didn’t send the warranty card back in time (because I’m a twat).

Any help and advice? Thanks all.

Seen another thread below that states the main fuse is 250V T800mA and I’ve found that as part of the kettle mains iec connector there’s an in line fuse. The amp cover does not need removed it just clicks out from the iec socket on the back of the amp. One of the problems of being over 50 is that your eyesight goes!

I can confirm that the fuse has blown. I’ll need to source a replacement ASAP.

Ok I can confirm the in line mains fuse was blown. RS components had them in stock. They come in packs of ten part number 563-554. Hopefully I won’t need many more.

Ouch!! Great reminder to bring along some of these fuses (not to mention the usual full set of spare power and preamp tubes) to all the shows I play.

My apologies for missing you for so long George, this is one of the good problems as you have managed to sort it yourself.  A bit of background to go along with this for future members who may need it.

Opening the amp up in any way will void the warranty if it is detected.  This includes replacing the two internal fuses and replacing or swapping out the valves.  There are anti-tamper devices inside which you need to be aware of and there is a recommended procedure for removing the top cover to access the valves which you should stick to so as not to trip them.  Replacing valves is not a dangerous procedure as long as you treat them with respect.  No voltages are exposed with the top cover off, only when a valve is out on the pins of the base.

Fuses have 3 major parameters which are usually stamped into the silver end caps of the glass fuse body.  The most obvious is the current.  Next is the speed.  Finally there is the voltage.  NEVER replace a fuse with a higher value in terms of current!  If it says 400mA then GET a 400mA fuse even though that may require waiting until you can source one.  It is highly risky to put in say a 1A fuse "until I can get the right one".  These values are carefully worked out to give both you and the components inside your amp the maximum protection.  If you put in a higher current fuse you will allow more stressful fault situations to be tolerated and this can create more damage than an original fault.  You could even be risking your continued time on the planet.  The speed is shown as a letter designation at the start of the markings.  "T" shows a Time Delay or Slow Blow type, "F" shows a Fast Blow type.  Always replace like for like.  There are more designations but these two are the general ones we would meet in amps.  The final part is the least important and is usually "250V".  This is the theoretical maximum voltage the fuse should withstand.  In our UK mains voltage we run at 220V so the peaks are above that 250V but the fuses cope and 250V is almost an unread standard value when replacing fuses it is so common.  The additional "L" sometimes found after the current value simply designates a Low Breaking Capacity or general purpose construction.

There are 3 fuses in the system.  The first is the one which George has found built into the mains socket on the back panel.  It is accessible from outside without opening up the amp in any way.  It is effectively your mains fuse and protects the mains wiring and the transformer primary.

Then there are two fuses inside the amp on the main PCB.  One of these is in the HT line between the transformer secondary and the rectifier diodes.  This is commonly the one which blows when there are serious valve problems.  As it sits in the HT line it is dangerous!!!  Seriously consider the implications of this as voltage can hang around there for quite a while and can bite you BADLY!!!!!  (For the record H&K do have drain resistors fitted as is good design practice.)  The second is the LT fuse between the transformer secondary and its diodes.  Both of these fuses are supposed to be replaced by a technician because you need to remove the bottom panel to access them exposing everything inside to your shockable little fingers.  IF YOU HAVE THE RIGHT SKILLSET it is possible to replace them yourself but you NEED TO CONSIDER THIS VERY CAREFULLY AS IT CONCERNS YOUR HEALTH, MAYBE EVEN YOUR LIFE.

The values of these fuses are:

Location	UK	US
External Mains Socket	T800mAL/250V	T1.6AL/250V
Internal PCB FU1 HT	T400mAL/250V	T400mAL/250V
Internal PCB FU2 LT	T800mAL/250V	T800mAL/250V

Finally a word of warning (which few will heed ).  It is fairly common for fuses to blow without any visual indication of the fact at all.  They can snap inside the end cap for example.  You can trust your eyes when it is obviously damaged but not the reverse, if it looks in good condition!  You really need to run a simple continuity test on the fuse to be absolutely sure it is still ok.  This can be as simple as lighting a torch bulb or led through the fuse from a battery though having a cheap resistance meter is better of course.  I have experienced a multimillion pound deep sea survey be held up at a cost of thousands when the fault was simply a fuse which was checked and assumed good.  (Still, that's oilmen for you! )




EDIT: And respect on your use of RS George. Top company here in the UK for parts if you can get to their few retail desks or blag a commercial account with them.

Thanks Bordonbert, as usual an informative and thorough response.

It's probably a good idea to hold spares of these fuses, they cost something like 0.15p each and come in packets of 10.

I will keep my spares in the front pocket of the carry case. If I don't then I am sure, pretty much like the mysterious disappearing guitar picks, they will end up where the odd socks go, never to be found again.

Just for the record my main fuse blew again on 3rd November.

Luckily I have a pack of these.

I had been playing at home and powered the amp down as normal, standby first, then main power, then off at wall.

Went back to it the next day and it was dead.

Not sure if I should be concerned. I’ve no other noticeable power issues at home.

Not that it will make much difference but I’m going to unplug from the wall socket each time at home now.

By main fuse I assume you mean the external one inside the IDE mains socket? Bear in mind that if a fuse blows there is always a reason. That could just be old age but, as this is the second occurence, I would be on your guard for something else. If a second one blows fairly quickly after replacement then it isn't old age. You seem to have a problem which is causing those fuses to blow and you would be wise to track down that problem and have it fixed. If you leave something which regularly blows fuses then it will get worse and ultimately more expensive.

Assuming you are in the UK from your sourcing to RS are you using a T800mAL rated fuse? That's a slow blow, (it has to be). Don't be tempted to put in a higher value "just to be safe". It will be anything but safer. If there is a grumbling fault you will have just given it the ability to work with higher currents than before and so do potentially more damage over time.

Thinking of things which could either blow the fuse at switch off or while it was off, was there a thunderstorm with lightning during the night? If that hits power lines nearby it is possible to get a surge which may cause it but that's really not too likely. Do you use the same power lead all the time? If so try using a different one and try out that lead in a different device. (One which is correctly fused of course.) We could think about what condition your power switch is in. If that is burnt on the contacts inside it can arc at switch on and off and that can blow fuses.

Failing that it could be your home mains setup but that would have to be extraordinarily dangerously faulty to blow a fuse while the unit is off! You would almost certainly see problems with other devices.

Drat. So my mains fuse blew again this week 20/11. I did notice however that it was on switch on. The amp lit up and immediately went off.

Does anyone else have one tube on the gm40d that glows more brightly than normal On power on only? I’ll try and capture a pic and figure out how to post here.

My fender Princeton 68 reissue is going to a respected amp tech soon to fix out an issue, I won’t be using the amp live in December so I might just as well put the gm40d in for investigation as well.

What is the TSC around back saying about the condition of the power tubes? Is that the tube you say glows more bright than others? If so and you can maintain a fuse long enough to run a TCS test do it otherwise if you have a spare set of tubes you can swap the odd glowing one out for that would be a good at home start. Now if you talking about a pre amp tube that flashes on start up then that is completely normal if you were using older vintage Mullard or Amperex ecc83 tubes. However if you using a modern JJ or china pre amp tube than that is absolutely a sign of a tube putting out higher than normal resistance or has a dead short and is likely the problem. Betting if you swap out that offending tube and a good fuse you’ll probably be good to go. Fuse is just doing it’s intended job. Other than that you may need a tech

Hey, thanks for your reply. So the glowing tube isn’t permanent but just blooms on power up. Looking at the amp face on it’s the second in from the left on the back row of four tubes.

Not sure if I do have something wrong but occasionally I am getting the global mute activated. Doesn’t show on the app but the yellow lamp is active at the rear. When this happens, as would be normal all tsc lights are on.

I had been dicking about with global power soak settings on the app but I have since unchecked the apply global power soak settings.

Shortly after getting the amplifier now, gosh is it three years already? change the stock tubes for JJ’s.

I’ve had no issues other than the very intermittent issue above. It might be time for a service.

Really good advice from Dave there.  I've seen the inconsistent "bloom" (good description) in valves before.  As long as the glow is coming from the heater filament and not the anode plate and they all settle down to look pretty much the same it hasn't been a problem in my case.  As you say your odd valve is one of the back ones when looking from the front it's a power amp valve (EL84).  That would certainly be capable of blowing fuses if it has a fault.  One way to check this oddity out is to swap the inner pair for the outer pair.  If the "bloom" moves with the valve then the cause of the odd warm up is inside your EL84.  If it stays where it is and shows with the new valve then it's something in the valve support circuitry.  You could also try tapping it gently with a chopstick to see if it is microphonic (which would be a secondary symptom in this case) or if it triggers the fault.  (It's not crucial here but it is important to build good habits working inside amps.  NO METAL OR PENCILS TO TAP WITH!  There may be situations where you could shock yourself or drop them and cause more damage.  The upper valve cavity in the GM40D is not one of those situations but bear it in mind.)

I think your error of the amp switching to Mute mode is most likely a response to the problem and not a problem itself.  The amp responds to a number of situations where it senses a fault and kicks in the power soak to try to safeguard it and the attached speaker.

If you do replace the output valve then make sure to do it as a matched pair, your inner ones as they stand at the moment.  You don't need to buy a matched quad really, just a pair will do for now, though at 3 years old it may be time to swap them all out if you have used it a lot and don't mind laying out the cash.  You could then keep the outer pair as working spares just in case.  As you may know, I'm personally not a fan of swapping out components based on some internet idea of how long they might live in the worst case.  Lifespan is very long and follows a Normal or Bell curve.  How far up the short end of that do you want to go?  There will be some valves that last only a few months, most last many years and a few seem to go on for ever.  Modern components are ridiculously more reliable than people believe.  It's far better to change things when they show signs of going faulty than in pensioning them off at some preconceived (guessed) time.  Like most guitarists I always keep a pair of output valves and a couple of preamp valves as spares so I can immediately swap them out one at a time to investigate a problem and prove or rule out the valves as faulty.

I reckon Dave has nailed it when he suggests it is most likely the valves.  You could get your hands on a known good pair of EL84s (a friend's amp maybe?) and a 12AX7 then swap each individual preamp valve out in turn with the good one then swap out the output pair.  The problem is it's difficult to know with this sort of fault when you have fixed it.  How long do you test that the fuse doesn't blow before assuming it was that valve?  Anyway, the idea of replacing the valves at this stage is a sound piece of advice if you don't mind spending the money but you should still try to identify a faulty one.  Unless you are prepared to just throw money at your setup willy nilly you will want to keep any which are still working for spares!  And your choice of JJs is fine.  Don't wind yourself up chasing rainbows without real good cause!

Tremendous support and advice from you both above. I’ll certainly try a swap out. I have the originals, though to make sure I’m clear if the valve that I have which blooms at power up is the second in from the left at the back, which two should I look to replace 2 and 1? 2 and 3, or 2 and 4?

The valves in the H&K amps and most others are grouped in matched pairs.  The outer pair, 1 & 4, work together and the inner pair, 2 & 3, also work together.  If your suspect valve is one of the taller EL84s and has one on one side and two on the opposite side it is one of the "inner pair" 2 & 3.  Swap out both of those.  Similarly if it had three on one side and none on the other it is one of the "outer pair".  If you are swapping pairs then you swap the two on the left with each other followed by the two on the right with each other, 1<->2 and 4<->3.  That keeps the matched pairs together in the inner and outer slots.

One thing which sometimes complicates this is when the valves are numbered in the schematic diagram differently to that 1-4 labelling.  Once you see the logic of how it works that becomes obvious.