My mark III coliseum has burnt out resistors! look and help!

[JimAnsell]

so i finally got my mark III coliseum today.
2 power tubes were smashed, and then when i took the chassis out, i noticed this right away:

i checked them with my meter and they are still giving a reading. around 13-14k ish

then both cathode bias resistors (the resistors on pin 8 of both tubes) on the inner two power tube sockets are fried and not giving me a reading at all.

the fuse was in tact in the amp, and it had a set of sovtek 6l6wxt+ tubes from the tube store, all with the rating of 17. i don't know what that rating means, but i'm assuming that is a matching number they use.

anyone know what value the resistors are supposed to be on pin 8 of those middle tube sockets? also, is it safe to run the amp as it is now?

or am i pretty much forced to have to send this to boogie?

 

[boogietone]

Don't send it to Boogie yet. I had one of those burn out before from a bad power tube. I snipped the legs of the resistor and removed it so I could read the position of the resistor. In my case it was R111. I called Boogie with the info, they identified the resistor value and I was able to order them straight from Boogie. I think those are screen resistors but I'm not 100%. After I received the resistors, I removed the remainder of the snipped legs left in the PCB by desoldering and some tweezers and soldered in the new resistor. I have extras but it appears yours are a different value resistor. Mine wasn't a coliseum. Anyways, after all was said and done the amp worked like a champ!

 

[boogietone]

Don't know what to tell you about pin 8 of the middle sockets. :?

 

[Buddy]

Some heavy current was drawn ... Tech time .
Play it safe .

 

[JimAnsell]

i called mike b, and he told me all the values of resistors that they are supposed to be, and some pointers about doing it myself.
he also told me it would be fine to turn on, which was surprising.
sure enough, not only did i turn it on, but i worked it hard tonight at band practice.

this thing is literally 1987-1989 metallica in a box, with fender twin like cleans.
its a little sharp and kinda harsh on the high frequencies with those EH pre amp tubes...i gotta do something about that.

 

[Boogiebabies]

Those two are 100 Ohm resistors for the heater circuits ground reference.
When a power tube goes thermal they get a good bit of back voltage and pop. Two Radio Shack resistors and your fine. Try to get that carbon off if you can. The two outer 39 Ohm resistors are for the diode bridge for the heaters DC and relay voltage.

You gotta love a good MK III.

Enjoy Jim and don't be shy with the gutshots.
Looks like a Simul !!! Sweet !!!

 

[JOEY B.]

this thing is literally 1987-1989 metallica in a box, with fender twin like cleans.
its a little sharp and kinda harsh on the high frequencies with those EH pre amp tubes...i gotta do something about that.

You will have to use the graphic EQ in a different fashion than a regular simul Mark III. Close your eyes and use the "force" :lol: :lol: :lol: . Something like a GT-12AX7M in V1 will help also .

 

[Buddy]

i called ...

this thing is literally 1987-1989 metallica in a box, with fender twin like cleans.
its a little sharp and kinda harsh on the high frequencies with those EH pre amp tubes...i gotta do something about that.

You can put a Mullard in V1 and V5 And it will smooth thing's out nicely .

 

[Restless Rocks]

Be very careful when you remove the fried filament resistors. Usually when PC board components like that go south at very high temps the solder pads will burn loose from the board and they will break loose from the track when you try and pull the resistor leads out of the hole.

 

[JimAnsell]

well i fixed all the resistors.
holy crap, what a pain in the ass.
i made one HUGE mistake that ****** the whole thing up for me.
mike b said something about clipping out the burnt resistors and then desoldering the traces.
i know from modding pedals that its about a million times easier to just hit the trace with solder and pull one end of said resistor out, then do the other one. THEN hit the joint with the sucker or solder braid and clean it out.
i realized this after i clipped them. i spent hours trying to go the traces free of solder and the last bits of the failed resistor.
it was so frustrating. i have a good weller iron, i used a fine point tip that was nearly new and well tinned every time, and i just couldn't get that braid or the pump to suck up the last bits.
i eventually got it all done, and it did a full on jam test, and everything is working.
the 4 resistors are in, albiet not as neat as i'd like them to be.
the power tube ones were pretty easy.
it really sucks that i had to do this in the first place.
its just a lesson to everyone that buys gear on the net.
ALWAYS pull the chassis and really scout around in there before you fire up the amp.
being gear heads already, i'm sure you all didn't need to be told that,
but its just the way things are these days on ebay.

 

[GIG4FUN]

I am happy this has ended. And, it ends up good; all in all it was bad. But it is good now. This is the same as the mess you had on your hands only with words. I appreciate your frustration and your drive to see it thorough.

Well done. Now this is truly "yours" you gave it life and sweat over it big time.

I had my C+ chassis out and 22 pictures taken and it spied over something fierce before I put power to it. My wife jsut couldn't understand why I was looking the guts over and not jsut turning it on and using it... You are correct, and this gearhead pulls it all down to its vulnerable underbelly before pluggin it into the wall.

You do own a fantastic amp and at a great "dollar amount" but it was a very high cost on other levels.

Thank you for contributing your learnings here.