HELP: LSS Smoking (literally)

[user 9654]

At rehearsal this morning my LSS started smoking. I had just switched the standby off and started playing some warm up type stuff. I smelled burning and a few seconds later smoke came billowing out of my amp. I killed the power and got a fire extinguisher. It continued to smoke for a while. I pulled the chassis and could find nothing that looked out of the ordinary. The only thing I noticed was the 2nd power tube had been partially knocked out. I was unable to reseat the tube in the socket.

A few months ago the rectifier blew and I got a replacement from the area Mesa tech. He gave me a 5u4GB from EH not a 5y3GB. Could this have been the culprit? Will the warranty still be valid seen as the Mesa tech gave me the incorrect tube?

 

[Platypus]

Take the chassis out and flip it over and look at the power amp section. A stab in the dark guess based on the info given is that you had a power tube failure and smoked a screen grid resistor. I'm surprised the fuse didn't blow... double check the fuse's rating while you're in there to make sure its the right amperage.

 

[boogietone]

I've had that happen to a MKIII once. Power tube blew and took out a screen resistor. Thank God the fuse didn't blow! Whew, that could've been ugly :? !
All kidding aside, a new power tube and a replacement resistor from Mesa, good as new!

 

[user 9654]

Thanks for the replies. I dropped the chassis just to have a look and I didn't see anything smoked. Granted, I have zero technical skills but our acoustic player does and he didn't notice anything. (Although, the last new amp he bought was a 60s Music Man. So he might never have looked at a PC board amp before.) Hopefully its just a screen grid resistor and they can have it back to me in no time.

 

[agefender]

I fried my screen resistor after blowing a tube just the other day. Sounded just like you described. When I had a look everything seemed ok. so I just grabbed the resistors and one just crumbled in my fingers-Still could be a screen resistor.I took them all out and put better ones in?!.

 

[user 9654]

I got a call from the tech today. Three fried resistors. Two weeks lead time from Mesa so I'm ampless for a while.

 

[Nikostrat]

I had the exact same think happen to my LSS. My fault though. Setting up at a gig and pulled the ac cord out and not realizing I had dislodged a power tube enough to short it out. Mine smoked and alas...screen grid fried. My tech in San Diego took care of it and also performed the service bulletin fix on the resistors for the rectifier section. Went ahead and retubed with JJ's btw and the amp has never sounded better.

Good learning experience. Be careful removing the ac cord in back.