GM40 deluxe - loud humming when 40W mode engaged

Hi All,

bought a GM40 on ebay a few weeks ago, very impressed so far, enjoying playing it. Have the footpedal and wifi, no problems. However...

Decided (for the first time) to engage 40W mode yesterday, immediately on pressing the button, a really loud buzz started coming from the speaker - not just a mains hum, but more like when you put your finger on the guitar lead tip. The amp is still passing signal, but the volume of the buzz is independent of the position of the master volume and won't go away until you go back into one of the other power soak modes or standby.

Before I return the amp to the seller, is there anything you think I could try, or any idea why this might be happening?

As an aside (not sure if connected in some way?) I've noticed the amp passes some signal even in standby mode - is this normal?

I've contacted Hughes and Kettner but no response so far...

Sorry to miss you for a few days, things have been hectic at my end. I realise you may have sent the amp back by now but just for the record here are a few thoughts.

Firstly the simple stuff. Yes, the residual sound is normal. The H&K Standby system is very different and a lot better than the usual long standing method which actually slowly kills your output valves. I have no idea why so many people (I don't mean you) get very up in the air about that little bit of residual sound. Is it any sort of a problem in use? It certainly isn't for me. But in my own case I simply leave any amps I use on and just turn the MV fully down. In the majority of cases that's less stressful than using the standby. I look after my valves and they last me a very long time. I don't subscribe to the theory that the entire amp has to have all its components changed every 18 months or it becomes unreliable.

You won't always get a quick response from H&K if you use their online contact form or email. The support team are often out of the office for days at a time at trade fairs and such. They are quicker at responding to posts on their Facebook page. I would try that. Make sure to get the genuine official H&K page, there are a few H&K groups out there. Search for "Hughes & Kettner Product/Service" and you should find it.

Now the problem. The sound being independent of the MV shows it is being generated after that which we would have suspected as it is triggered by the Power Soak. The first thing to rule out is the output valves. You have two pairs and only one of these pairs is used for each of the 20W, 5W and 1W settings. The second pair kicks in exclusively for the 40W setting. If you have a valve problem in that pair then you will hear that only when you select 40W just as you are experiencing. If you have a spare pair of EL84s then swap them for the inner pair which is the one used only for the 40W setting. If the problem goes away then you have solved it.

If you don't have a spare pair then swap the inner pair for the outer pair. That is 1 <-> 2 and 3 <-> 4. You have now put the suspect pair into use on every setting. Does the problem now appear all the time? If it does then again you have proved it is the valve at fault.

Incidentally, for valve swapping you should know that you DO NOT remove the end caps completely. There is an anti-tamper mechanism, inside on the bottom two screws of each cap. It is not important once the amp is out of warranty but it is still easy to leave it in place. Remove the top two screws and the middle rear screw completely. Leave the middle front screw alone, it only holds the handle on. Loosen the lower two screws only a few turns so the end caps can flop outwards and allow you to take off the lid. Make sure to fit the LED PCB strip, its clips and its cable correctly before replacing it.

My first guess would be a valve. It's simple to check so try that first and tell us what you find.

Hi, many thanks for the detailed response, that’s over and above what I would have expected! I did try swapping out both pairs of power valves (separately) with a pair I have from another amp and am fairly convinced it isn’t them. I also got a response from H&K asking me to send to an amp repair shop in the uk - they suspected output valves and fets. Given the output valves seem to work fine, I’m guessing it must be the fets, wherever they are!

I’m working with the guy who sold me the amp to try and get it repaired, and hopefully it won’t be too expensive (out of warranty, I’m afraid)

Many thanks again

No problem.  Glad you got the reply from H&K.  They are very good with their support despite what some people might say.  Simple politeness rules in dealing with them.  They just don't take well to the more "pushy" types who think that shouting loud right up front will get you more attention.  You don't get the best out of German guys that way.  

With any H&K amp you have to accept that they are hi-tech units, not like the classic Marshalls and Fenders which are very basic and even with errors in their designs.  That does mean the H&K amps are much more reliable, again not what most guitarists choose to believe.  I've spent over forty years designing gear in the electronics industry and I own and play four Marshalls, an original '60s SL100, a JCM800 Splitch, a JVM205H and a recent SC20H.  I can tell you they are all very basic designs, even the JVM is only complex in its switching.  Those classic designs have to be massaged constantly to keep them at their best.  An amp like the GM40D will just go on and on doing what it does with little to do other than change valves at long intervals.  The other side of the coin is that when things do go wrong they aren't easily fixed without specialist gear and knowledge.  My Marshalls can almost be fixed with a hammer, a screwdriver and a cigarette lighter!  (I'm exaggerating of course but you get what I mean.)

The MOSFETs are semiconductors fitted in the cathodes of the output valves.  They are responsible for setting the idling current of the valve and don't take part in the signal amplification.  They are controlled by a digital circuit which monitors their bias current and adjusts it to stay constant as the valve changes with time.  This is the basis of the TSC circuitry and is extremely clever and well designed.  It's why you do not have to set the bias current in your amp.  The MOSFETs are not stressed in normal use so don't go wrong too often and they aren't expensive to buy new.  It isn't too difficult a job to change them if you have a proper bench with decent soldering tools.  Once you get that done the amp should be good to go and I'm sure you will be pleased with it when you can start playing.

Keep us up to date with progress, it's always nice to hear of another happy player.

Thanks bordonbert - it's comforting to hear you think it will be a relatively simple fix! But not one I'd attempt myself. I'll certainly keep you posted, and hopefully this will help anyone who experiences the same problem.