Electrical lightning storm in my tubes.

[plan-x]

In the one (6l6) closest to the rectifier tube. Blew the fuse. Replaced it with another tube, turned it on, no problem, flipped it out of standby and the fireworks were on again. So I suspect a screen grid resistor. And so does my amp tech after I phoned him. As well as possible tube socket damage after I described the amount of arcing going on.

 

[Charles Reeder]

Hey plan-X:

It does sorta sound like the screen resistor(s) is(are) the problem. If so; and you caught it in time, then about the only thing you will need to do is replace the bad tube(s) and the screen resistor that burned...and...(very important in my opinion) it's mate. If you were playing in 50-watt mode; the screen resistor on the tube that is a mate to the one that put on the 'light-show' should be replaced as well. If in 100-watt mode.. use a multimeter to check the screen resitors for the other pair of tubes as well. Better safe than sorry.

It would be a good idea to buy a few extra screen resistors (they're cheap enough). Not only might you need some again later...but it is important to match the resistors by their ohmage values just as you match the tubes that will go into those sockets.

When I replaced the 2 screen resistors on my Lonestar that were on the sockets in 50-watt mode...I checked the values of the other pair (used in 100-watt mode) and noticed that although they were not 'truly' 470 ohms as specified...(important) they were both 'perfectly' matched to each other. So, you want to have enough resistors on hand , to cull through them with a multimeter and pick perfect matches. Note: the 2 screen resistors for 50-watt mode DO NOT HAVE TO MATCH the ones used on the other tubes...but they should match each other.

From past contact with you; I feel you are quite capable of doing this replacement your self. You will need to loosen and remove all the nuts on all the jacks and control pots on the back of the chassis, so that they can be withdrawn into the amp. You will have to remove the thin 'sub'-circuit board that butts up to the inside back of the chassis as well. This will give you access to the screen resistors for replacement.

Of course all dire warnings of high-voltage and eminent demise apply!

If you are lucky (like me) just replacing the resitors and tubes affected will be all that's needed to get your amp back into action.
If that doesn't do it...then it's 'tech-time'.

An observation of mine. Mesa opts for 'fixed-bias' supposedly to cut down on repairs etc.; yet they don't do what a 'Major' competitor does to prevent screen resistors (and other stuff) from blowing when a tube goes bad. Ever notice that second fuse on the back of some Marshalls labeled 'H.T.'? It is meant to blow in the event of a bad tube. Some were located inside the amp. My Rivera M30 has a fuse located inside the chassis that blew when a tube went bad. I replaced the tube and the fuse and was back in action. Wish Mesa would do likewise.

Charles

 

[plan-x]

Thanks Charles. Where did you aquire those resistors? What wattage rating? What type? ie, carbon, metal, etc.

 

[ja22y]

As Charles said, most likely the grid resistors. I don't have the schematic in front of me now but perhaps it might have the wattage of those resistors. Typically they are at least 1W, metal oxide. Get the ones with 2% tolerance so you have more chance of finding a match. For parts go to mouser.com

Check this clip out, you might get a kick, hope yours was not as bad:
http://www.youtube.com/watch?v=qr4b5i8awEk

 

[plan-x]

As Charles said, most likely the grid resistors. I don't have the schematic in front of me now but perhaps it might have the wattage of those resistors. Typically they are at least 1W, metal oxide. Get the ones with 2% tolerance so you have more chance of finding a match. For parts go to mouser.com

Check this clip out, you might get a kick, hope yours was not as bad:
http://www.youtube.com/watch?v=qr4b5i8awEk

Thanks. And yes, it was that bad.