GM stopped working

Hey there new to the forum so idk if I put this in the right spot. But yesterday I was playing a outside show (it was raining then stopped before we started playing ) and I had my GM warming up for roughly 20 minutes before playing. Made it through sound check but about  a minute into our first song my amp had muted it’s self and would come back on  for about 10-15 seconds before muting itself again then would turn on for about 10-15 seconds after a minute of mute . Idk what caused it. But today I turned it on arguably louder then at the show and had no problems. Any ideas what caused it to shut off ?
Thank you in advance

Hi Xnidusx,

I have to admit I don't like the sound of the "raining before we went on" scenario.  HT valve amps and water do NOT mix.  I know you have to run with things when you are gigging, particularly outdoors, but it really is not the ideal conditions for the amp as we all know.  If the amp was muting itself then it would seem to be sensing a "bad" condition and kicking in its protection.  My first guess would be that you had water somewhere, not necessarily in the amp itself, maybe in one of the speaker connectors.  It would be sensed as a "too low impedance" by the sensing circuitry which would kick in the Silent option of the Power Soak.  It could easily dry and clear overnight leaving things fine when you tested today.  Is that possible?

If not then the thing to do is check out the TSC settings with your pick in the slot on the back and if that is ok then just use it.  I'm afraid a fault which clears the next day without sign is not going to be diagnosed unless it appears again.  Without the amp in a fault condition it would be impossible to test for the fault, every suggestion would be just a random guess.  How are your output valves looking on the TSC?

The two middle lights are on but the two outer light are not.  Does that mean I need to replace the tubes ?

The lights are telling you that, (as long as you have 40W selected), the inner pair of output valves has been "benched" by the protection setup and its LEDs lit to show it is not running.  Your amp will work but it is only going to give you a max of 20W with a single pair of valves.

With the amp off standby and playable, use your pick to push into the slot on the rear and run the TSC check.  If you aren't sure, you need to count the flashes on each LED after starting it.  Each LED will tell you the bias setting for the corresponding valve.  Both of the valves should match within 4 flashes for each pair, outer and inner.  The balance between the two pairs is not so important but should not drift too much.  Can you get this info as it is a good indicator of the state of the valves?

Next you could swap the two output valve pairs over.  Swap each of the two outer valves with the one inside it.  When the amp is then run up at 40W do the LED lights, two on two off, swap with the valves?  If they do then any fault they are showing is with the valves themselves.  If they stay where they are even though the valves have changed then any fault is in the circuitry outside of the valve itself.

Are you ok with swapping valves in the GM40D?  There is a procedure to not invalidate the warranty.  You should remove both top screws and the rear middle screw on each end cheek totally.  Ignore the middle front screw as it only holds the silver handle on the end cheek.  Finally LOOSEN not remove the bottom two screws a couple of turns.  The end cheek will flop outwards so the lid can be removed.  This leaves the anti-tamper check intact inside the amp base.  Take care with the LED PCB which sits on top of the Perspex front and its cable which leads down into the base.  These need to be dressed and clipped correctly before replacing the lid.