GM40 failure after a few hours of play

I have the GM40 for about 7 months now... play it routinely... great amp sounds... I had to change the tubes about 3 months in.... no biggy..  Was playing last week, about 3 hour practice,  damn thing fizzled out and all tube lights on back were on... turned it off,  just continued through my Archon, turned it on later... no issues... practice yesterday.. same thing.. 2 hour practice.. that does not breed confidence at all in a gigging amp for sure.. I love the way it adds to my Archon wet/dry but I can't live with this... contacted H&K.. any ideas? andy

That's an odd one Alund. To go off with ALL the rear LEDs on then to recover later is very strange. The lights tell you that those valves have been switched off by the protection system. What would report a fault with all four valves then reset itself is a mystery. It surely has to be something common to the bias/protection circuitry for all four valves rather than something which is repeated for each of them going faulty four times over. It can't be just the HT fuse or anything usual. Could it be an occasional short in your speaker cable or in the cabinet itself? The only other fault which I can come up with which might do this is a heater short in one of the valves which would take down the other valves via the change to their heater conditions. I suppose this could also be caused by a preamp valve as they are all in parallel in the heater circuit.

The only thing I can think of to do off the top of my head is to replace the output valves with a known good set and see if it recurs. These could just be any old cheapo temporary borrowed set to check things out. One or even two duff valves should not trigger them all to be turned off unless there is perhaps also something wrong elsewhere. You say you HAD TO change the valves three months in. Do you mean they actually went down on you or you just couldn't stand their sound any longer than that? And did you change them for something esoteric and a bit out of the ordinary in any way? (No "NOS classic Russian military equivalents" I hope?)

Sorry I can't be more help at the moment but if this is a 7 month old new amp I would honestly look into putting it in to be looked at under warranty. That does not sound right nor an easy fix if it isn't just a valve fault.