GM36 No Output

Hi there. My Grandmeister 36 has stopped outputting. The tubes were changed back in January and it's been fine up until last night when I switched it on, everything powered up but no output. I tried different leads, speakers and guitars just to be sure and it is definitely the amp. When I put a plectrum into the TSC slot the flash sequence is all four flash once, then from left to right only the first 3 flash 9 times, then the second from the left flashes once. This is my primary amplifier and I need it working as soon as possible!

Assuming that you haven't got either the Fx Loop On without anything in the loop, or the Power Soak set to Silent Mode, we had best look to the valve info you can get.  You would be surprised how often even experienced users make those mistakes.

Have you any LEDs lit when you are in normal playing mode at full 36W?  When you are in Standby they should all light up.  When you flip to Play from Standby they should ALL extinguish, unless you are in 18W mode when one pair will switch off and both their LEDs will light up.  If one stays lit then that valve has a problem and has been shut down.

Ok, onwards to the next step.  If I have this right you have the following.  From Left to Right each valve flashes:

[  9  ]

[ 10  ]

[ 9  ]

[ 1  ]

If that is true then you have a mismatch in the outer pair and a good match in the inner pair with the valve giving 1 flash way off the mark.  You would be best to replace the outer pair with a new matched pair and maybe keep the left valve, (9 flashes), to act as a spare for the inner pair.

In the meantime, can you not play the amp with the power soak in the 18W position?  I can't remember offhand whether it is the outer pair or the inner pair which is switched off in that mode.  (I have seen both advised online.  So much for internet accuracy.)  If you find it is the outer pair used and inners switched off then swap the pairs over, middle two for outer two.  That's a swap of 1<-> 2 and 3 <-> 4.



EDIT:  Wonder if this may help:  TSC LED Issue - See post #20  It seems as though it could maybe point in the same direction as your problem.

Now here is an odd thing. I've just searched for pics of the rear LEDs in 18W mode and can't find any, but I did find 1W mode pics and that is the same. Both clearly show the 1W switch lit up in yellow but one shows the outer pair of LEDs lit and one shows the inner pair lit. That is strange. I wasn't aware of the TSC making any decision as to which it should light up? I'll ask teh H&K guys about this.

Hi there, thanks for the quick response.

I got it back up to the rehearsal room tonight and all of the tube lights stay on in every setting.

I didn't get chance to swap the tubes around, but I'm guessing that won't make any difference anyway. Is it the bias that is going wrong or has a fuse blown somewhere? A week ago it was working perfectly, but when I switched it on on Monday I got a tiny bit of noise and it just fizzed to nothing.

Before I advise on what to do here I would have to ask whether you are experienced in dealing with HT voltages high enough to kill you? This will need the bottom taken off the amp and access gained to the PCBs beneath. That is a High Risk area. If not then hand the amp over to a tech to check and repair. No amp is worth your life. I don't subscribe to the "live fast die young" school of rock playing!

So firstly, are you going to be looking inside yourself? Are you competent and experienced enough to do that? This is NOT a sprawling handwired 1960s amp which could be fixed by a chimp with sealing wax, string and a big screwdriver! (No, I'm not implying all techs are chimps, they are extremely skilled and knowledgeable people when they are properly qualified. I'm only saying that old style amps are incredibly simple in comparison to a high-tech modern type like the GM36. You must accept that.)

Oh no I won't be carrying out any of the work on this, I'd just like to be able to take it to someone who can fix it quickly, and if I have a guide on what is wrong with it they'll be able to sort is asap.

Good to see you being practical about this. I would look for a tech who has experience with the H&K if you can find one. These amps are very different to the average guitar amp which they spend most of their time on.

The tech will need to check the HT fuse first. That's labelled on the PCB as FU1 and it's a T400mA, that's a Slow Blow or Time Delay type. If that isn't the fault he will know what to check out and will have known good valves to substitute to rule out your own as the culprit. There is nothing at this stage which we can recommend which he would not know as a matter of course really.

Good luck, let us know how this progresses.

Thought I'd just give a quick update now that I've got it back from H&K and it's all sorted.

There was a problem within the power supply which took out the standby circuit, corrupted the valves and took out the mosfets.

Quite a nasty problem really, but the guys at JHS Instruments who are H&K's UK distributor did a fantastic job and only charged me for two days service despite having it on their desk for five days.

Thanks for the update, it's always good to know how things turned out and what was needed to fix it. That certainly was something you could not have dealt with yourself. The TSC bias control circuitry is a great feature to have but if it should get damaged there is nothing to do but to replace the relevant bits. Good to hear that the service agents at JHS are on the ball too. It's always good to know there is a decent backline when it is really needed.

So it's back to rocking out then with the amp in full flow! Let us know how it performs in a week or two.