mark iv, loud pop, what to do?

[aliebert]

So I've read the posts on blown fuses but I wasn't sure if i should revive an old thread or start a new one...specifically, a fuse blew, and there was a definite unpleasant burning smell...I'm going to get some new fuses and see if I can identify the problem, and I opened the chassis and took a look and don't see anything that looks obviously burnt- i guess my quesiton is if there was internal damage, would I actually be able to see it?

also, the old fuse in the amp has '250v' printed on it, while the mark iv manual says the fuse should be for 100-117 volts- could this be an issue as well?


Here's what happened:

-bought a used 1991 mark iv A combo, with some pretty old tubes. Decided to retube it because they sounded worn.

-bought a pair of brand new Mesa 6L6s and pair of Mesa EL34s at guitar center, and 4 12ax7s.

-put thenew tubes in, let the amp sit on standby a few minutes

-turned the power on, and immediately heard a loud pop, smelled burning/electrical smell, and the power went off.

-unplugged, pulled out the fuse, and observed it was blown.

 

[dct]

specifically, a fuse blew, and there was a definite unpleasant burning smell...

Hmmm. That's never a good sign. If a fuse blows, it could be as simple as a power tube going bad. That happened to me and the fuse blew. The smell means that something burnt or got too hot. Not good news. To be safe, a trip to a tech would be in order.

I'm going to get some new fuses and see if I can identify the problem, and I opened the chassis and took a look and don't see anything that looks obviously burnt- i guess my quesiton is if there was internal damage, would I actually be able to see it?

Not necessarily.

also, the old fuse in the amp has '250v' printed on it, while the mark iv manual says the fuse should be for 100-117 volts- could this be an issue as well?

That just means that the fuse is rated for 250V as the maximum voltage it's designed for. That's fine.

-bought a pair of brand new Mesa 6L6s and pair of Mesa EL34s at guitar center, and 4 12ax7s.

The manual says that you can use EL34s in the 2 outer sockets. Is that where you put them?
Also, the Mark IV has 5 12AX7s. Did replace only 4?

-turned the power on, and immediately heard a loud pop, smelled burning/electrical smell, and the power went off.

Since all these signs happened as soon as you switched it from standby to run, it'll probably need servicing. There are occasions where components can be defective but they will appear to be fine.
Not what you wanted to hear, I'm sure.

 

[Harry the Horder]

Try replacing the fuse,remove all the output valves (6L6 and EL34) hit the standbye switch if the fuse blows you have a short somewhere if it doesn't then it could be your BIOS is set to low and is drawing to much current! (see forum on watford valves site)

 

[aliebert]

Thanks everybody for the tips! Sorry I didn't reply sooner. The issue is resolved now so I thought I'd say what happened so someone else can learn from it if they search.

Bottom line was, even though I bought 4 brand new power tubes, and 5 brand new preamp tubes, at least 3 of them were dead on arrival. I got some new fuses, and systematically swapped tubes to find the problem. It was confusing at first because after switching out the new power tubes for the old but working ones, there was some bad noise and hum and crackle; turned out that that the new EL34s were bad, and so was one of the new preamp tubes. Also, when doing the final test where I determined the new power tubes were bad, the fuse almost blew again before i went back to standby, and the smell came back- seems like the smell was coming from the bad power tubes. Now the amp sounds better than ever, no noise...

One lesson learned, if you're looking to retube because the tubes are worn, probably best to swap out only the power tubes and change preamp tubes only as they noticably fail.